Life, earth, space, physical science, and chemistry. Each lesson below is also in the interactive study guide, where you can take a quiz to check yourself.
Lesson 1 Life Science: The Human Body
Your body is a team of organ systems working together every second, whether you are running a two-mile or catching your breath afterward. The ASVAB General Science section expects you to know what each system does and how they connect. Learn the big jobs first, and the details fall into place.
The Circulatory and Respiratory Systems
The circulatory system moves blood through the body using the heart as a pump and blood vessels as the roadways. Arteries carry blood away from the heart, veins carry it back, and tiny capillaries are where oxygen and nutrients pass into your cells. The respiratory system brings in oxygen and removes carbon dioxide, with the lungs doing the actual gas exchange in tiny air sacs called alveoli. These two systems work as partners: the lungs load oxygen into the blood, and the heart pushes that oxygen-rich blood out to the muscles and organs. When you exercise, both speed up together to keep your cells supplied.
The Digestive System
The digestive system breaks food down into nutrients your body can absorb and use for energy. It starts in the mouth, where chewing and saliva begin the process, then travels down the esophagus to the stomach, where acid and enzymes continue breaking food apart. The small intestine does most of the nutrient absorption, passing those nutrients into the bloodstream. The large intestine absorbs water and packages the leftover waste. Organs like the liver and pancreas help by producing bile and enzymes that make digestion possible.
The Nervous and Endocrine Systems
The nervous system is the body's fast communication network, using the brain, spinal cord, and nerves to send electrical signals in fractions of a second. It controls everything from your reflexes to your thoughts and lets you react instantly to your surroundings. The endocrine system is the slower chemical messenger system, using glands to release hormones into the blood. Glands like the thyroid, pancreas, and adrenal glands control long-term processes such as growth, metabolism, and stress response. Together, fast nerves and slow hormones keep the whole body coordinated and balanced.
The Skeletal and Muscular Systems
The skeletal system is the body's frame, made of 206 bones in the adult body that provide structure, protect organs, and give muscles something to pull against. Bones also store minerals like calcium and produce blood cells inside their marrow. The muscular system creates movement, and it comes in three types: skeletal muscle that you control voluntarily, smooth muscle that lines your organs and works automatically, and cardiac muscle found only in the heart. Muscles attach to bones by tough cords called tendons and work by pulling, never pushing. This is why muscles often work in pairs, with one contracting while its partner relaxes.
The Immune System
The immune system is the body's defense force against germs like bacteria and viruses. Its first line of defense is physical barriers such as your skin and the mucus in your nose and throat. If germs get past those barriers, white blood cells hunt down and destroy the invaders. The body can also build antibodies, which are special proteins that recognize a specific germ and help fight it off faster the next time. This memory is the reason vaccines work and why you usually only get certain illnesses, like chickenpox, once.
Blood Basics and Nutrition
Blood is made of red blood cells that carry oxygen using a protein called hemoglobin, white blood cells that fight infection, platelets that help blood clot, and plasma, the liquid that carries everything. Blood comes in types A, B, AB, and O, and type O negative is the universal donor because it can be given to almost anyone in an emergency. Good nutrition fuels all of this: carbohydrates give quick energy, proteins build and repair tissue, and fats store energy and protect organs. Vitamins and minerals are needed in small amounts but matter a lot, since shortages cause specific problems such as scurvy from too little vitamin C or anemia from too little iron.
Worked example
Which organ system is responsible for exchanging oxygen and carbon dioxide with the outside air?
- The question asks about gas exchange with the air, so focus on which system deals with air moving in and out of the body.
- The circulatory system moves blood, and the digestive system handles food, so rule those out.
- The lungs take in oxygen and release carbon dioxide, and the lungs belong to the respiratory system.
- The correct answer is the respiratory system.
Tip: Link the key word to the system: air and breathing means respiratory, blood and heart means circulatory.
Worked example
A person's diet is very low in vitamin C. Which condition are they most likely to develop?
- Recall that each vitamin deficiency causes a specific problem.
- Vitamin C is needed to keep gums, skin, and connective tissue healthy.
- A long-term lack of vitamin C causes scurvy, which shows up as bleeding gums and slow-healing wounds.
- The correct answer is scurvy.
Tip: Memorize a few classic deficiency pairs: vitamin C and scurvy, vitamin D and rickets, iron and anemia.
Worked example
Which type of blood cell is mainly responsible for carrying oxygen to the body's tissues?
- Break the blood down into its parts: red cells, white cells, platelets, and plasma.
- White blood cells fight infection and platelets help clotting, so neither carries oxygen.
- Red blood cells contain hemoglobin, the protein that binds oxygen.
- The correct answer is red blood cells.
Tip: Remember red for oxygen, white for defense, and platelets for plugging leaks.
Key takeaway: Each organ system has one big job, and knowing those jobs, plus how blood and nutrition support them, covers most General Science body questions. Focus on the pairs that work together, such as the heart and lungs or nerves and hormones, and memorize a handful of classic vitamin and mineral deficiencies. Match the key word in the question to the right system and the answer usually stands out.
Lesson 2 Life Science: Cells, Genetics, and Ecology
Life science, also called biology, is the study of living things. On the ASVAB General Science section you will see questions about the smallest units of life (cells), how living things get energy, how traits are inherited, how organisms are grouped, and how they depend on one another. Do not worry if biology felt overwhelming in school. This chapter breaks it into plain, memorable pieces. If you learn a handful of key terms and a few simple rules, you can answer most life science questions with confidence.
The Cell and Its Parts
The cell is the basic unit of life. Every living thing is made of one or more cells. Three parts show up on tests again and again. The cell membrane is the thin outer layer that surrounds the cell and controls what goes in and out, like a security gate. The nucleus is the control center that holds the cell's DNA and directs its activities, like the brain of the cell. The mitochondria are the powerhouses that break down food to release energy the cell can use. A handy phrase to remember is that the mitochondria are the powerhouse of the cell. Other parts include cytoplasm, the jelly-like fluid that fills the cell, and ribosomes, which build proteins.
Plant Cells vs Animal Cells
Plant and animal cells share the basics: both have a membrane, a nucleus, and mitochondria. But plant cells have three extra features that animal cells do not have. First, a cell wall, a stiff outer layer outside the membrane that gives the plant structure and support. Second, chloroplasts, the green structures that capture sunlight to make food through photosynthesis. Third, a large central vacuole that stores water and helps the plant stay firm. Remember it this way: only plants make their own food, so only plant cells have chloroplasts and a rigid cell wall.
Photosynthesis and Respiration
These two processes are opposites, and together they power life on Earth. Photosynthesis happens in plants (in the chloroplasts). Plants take in carbon dioxide and water, use energy from sunlight, and produce sugar (food) and oxygen. In short: sunlight plus carbon dioxide plus water yields sugar plus oxygen. Cellular respiration happens in nearly all living cells (mainly in the mitochondria). It does the reverse: it takes in sugar and oxygen and releases energy, carbon dioxide, and water. Plants do both, while animals only do respiration. This is why plants give off the oxygen we breathe, and why animals and plants both need to break down food for energy.
DNA, Genes, and Inherited Traits
DNA is the molecule inside the nucleus that carries the instructions for building and running a living thing. DNA is shaped like a twisted ladder called a double helix. A gene is a segment of DNA that codes for one specific trait, such as eye color. Chromosomes are the tightly packed structures that hold the DNA; humans have 46 (23 pairs). Offspring inherit one copy of each gene from each parent. Traits can be dominant or recessive. A dominant trait shows up whenever at least one dominant gene is present. A recessive trait shows up only when both copies are the recessive version. Uppercase letters stand for dominant genes and lowercase for recessive, so a person with Bb shows the dominant trait, while only bb shows the recessive trait.
Classifying Living Things
Scientists organize life into groups from broadest to most specific. The order, from largest group to smallest, is: kingdom, phylum, class, order, family, genus, species. A species is the most specific group, meaning organisms that can breed and produce fertile offspring. Every organism gets a two-part scientific name from its genus and species (for example, humans are Homo sapiens). One common test split is between vertebrates, animals with a backbone (such as fish, amphibians, reptiles, birds, and mammals), and invertebrates, animals without a backbone (such as insects, worms, spiders, and jellyfish). Most animals on Earth are actually invertebrates.
Ecology: Energy and Relationships in Nature
Ecology is the study of how living things interact with each other and their environment. A habitat is the place where an organism lives. Energy flows through an ecosystem in a food chain. Producers, usually plants, make their own food using sunlight and form the base. Consumers eat other organisms: herbivores eat plants, carnivores eat animals, and omnivores eat both. Decomposers, such as bacteria and fungi, break down dead material and return nutrients to the soil. A food web is many food chains linked together, showing that most animals eat more than one thing. Symbiosis means two species living closely together. There are three types: mutualism, where both benefit; commensalism, where one benefits and the other is unaffected; and parasitism, where one benefits and the other is harmed.
Worked example
Which cell structure is responsible for producing energy for the cell?
- The question asks which part supplies energy.
- Recall the roles: the nucleus is the control center, the membrane controls what enters and exits, and the mitochondria break down food to release energy.
- The key phrase is that the mitochondria are the powerhouse of the cell.
- So the mitochondria are the energy producer.
Tip: Match each part to a one-word job: nucleus equals control, membrane equals gate, mitochondria equals power. Then pick the one about energy.
Worked example
A plant takes in carbon dioxide and water and, using sunlight, produces sugar and oxygen. What is this process called?
- Notice the clues: sunlight is used, and oxygen is given off.
- Respiration does the opposite (it uses oxygen and releases carbon dioxide), so it does not fit.
- Making food from sunlight, carbon dioxide, and water is the definition of photosynthesis.
- The answer is photosynthesis.
Tip: If you see sunlight and oxygen being produced, think photosynthesis. If you see oxygen being used up and energy released, think respiration.
Worked example
Brown eye color (B) is dominant over blue eye color (b). A person has the gene pair Bb. What eye color will this person have?
- A dominant trait appears whenever at least one dominant gene (the uppercase letter) is present.
- The person has one B (brown, dominant) and one b (blue, recessive).
- Because a dominant gene is present, the dominant trait shows.
- So the person will have brown eyes. Only a bb pair would give blue eyes.
Tip: One uppercase letter is enough to show the dominant trait. The recessive trait needs two lowercase letters.
Key takeaway: Cells are the basic unit of life: the nucleus controls the cell, the membrane guards it, and the mitochondria power it. Plant cells add a cell wall, chloroplasts, and a large vacuole. Photosynthesis makes food using sunlight and releases oxygen, while respiration breaks food down to release energy. DNA holds the instructions, genes code for traits, and dominant traits show with just one dominant gene while recessive traits need two. Life is classified from kingdom down to species, and animals split into vertebrates (with a backbone) and invertebrates (without). In ecosystems, energy flows from producers to consumers to decomposers, and symbiosis describes close relationships that may help both, help one, or harm one. Learn these key terms and you can reason your way through most life science questions.
Lesson 3 Earth Science
Earth Science is one of the most predictable topics on the ASVAB General Science section, which is great news for you. The test loves to ask about Earth's layers, the three rock types, the water cycle, and weather fronts - and these facts do not change. Master the vocabulary in this chapter and you will pick up easy points on test day. Take it one concept at a time, and use the memory tricks along the way.
Earth's Layers
The Earth is built like an egg with four main layers. The crust is the thin, rocky outer shell where we live - it is thinnest under the oceans (about 5 to 10 km) and thickest under continents (up to about 70 km). Below the crust is the mantle, the thickest layer, made of hot, slowly flowing solid rock. Movement in the mantle drives the motion of the plates above it. Next comes the outer core, a layer of liquid iron and nickel - its motion generates Earth's magnetic field. At the center is the inner core, a solid ball of iron and nickel. Even though the inner core is the hottest layer, it stays solid because of the crushing pressure at the center of the planet. Memory trick: from outside in, think Crust, Mantle, Outer core, Inner core - and remember outer is liquid, inner is solid.
Plate Tectonics: Earthquakes, Volcanoes, and Mountains
Earth's crust is broken into large slabs called tectonic plates that ride on the slowly moving mantle. Where plates meet, big things happen. At convergent boundaries, plates push together - this builds mountains (the Himalayas formed this way) and can force one plate under another in a process called subduction, which fuels volcanoes. At divergent boundaries, plates pull apart and new crust forms, as at the Mid-Atlantic Ridge. At transform boundaries, plates slide past each other, like the San Andreas Fault in California - this grinding motion causes earthquakes. Earthquake strength is measured with the Richter scale (or the modern moment magnitude scale), and each whole number step means about 10 times more ground shaking. The Ring of Fire around the Pacific Ocean is famous for earthquakes and volcanoes because so many plate boundaries meet there.
The Rock Cycle: Igneous, Sedimentary, Metamorphic
All rocks fall into three families, and the ASVAB expects you to know how each forms. Igneous rock forms when melted rock cools and hardens - think of the word ignite, meaning fire. Granite forms from magma cooling slowly underground; basalt and obsidian form from lava cooling quickly at the surface. Sedimentary rock forms when layers of sediment (sand, mud, shells, pebbles) are pressed and cemented together over time - sandstone, limestone, and shale are examples, and this is the only rock type where you find fossils. Metamorphic rock forms when existing rock is changed by intense heat and pressure without fully melting - the word means changed form. Limestone becomes marble, and shale becomes slate. The rock cycle connects all three: any rock can melt into magma, break down into sediment, or be squeezed and heated into metamorphic rock.
Weathering vs. Erosion
These two terms are easy to confuse, and the ASVAB knows it. Weathering is the breaking down of rock in place - the rock crumbles but does not go anywhere. Erosion is the carrying away of those broken pieces by wind, water, ice, or gravity. Memory trick: weathering breaks, erosion takes. Weathering comes in two types. Physical (mechanical) weathering breaks rock without changing its chemistry - for example, water freezes in a crack, expands, and splits the rock (frost wedging). Chemical weathering changes the rock's actual makeup - acid rain dissolving limestone, or iron in rock rusting. After erosion moves the material, deposition drops it in a new place, which is how deltas form at river mouths and how sedimentary rock gets its raw material.
The Water Cycle
The water cycle moves water endlessly between the oceans, the air, and the land, powered by the sun. Evaporation: the sun heats liquid water and turns it into water vapor that rises into the air. (Plants also release vapor through their leaves - that is called transpiration.) Condensation: as the vapor rises and cools, it turns back into tiny liquid droplets, forming clouds. Precipitation: when the droplets grow heavy enough, they fall as rain, snow, sleet, or hail. The water then collects in rivers, lakes, oceans, and underground (as groundwater), and runoff carries surface water back to the sea to start the cycle again. Quick check: evaporation is liquid to gas, condensation is gas to liquid, precipitation is water falling from clouds.
Weather: Air Pressure, Fronts, Humidity, and Clouds
Weather is driven by air pressure, moisture, and moving air masses. Air pressure is the weight of the atmosphere pressing down, measured with a barometer. High pressure usually brings clear, calm weather; low pressure brings clouds and storms. Falling barometer readings signal a storm is coming. Humidity is the amount of water vapor in the air; relative humidity compares the vapor present to the maximum the air could hold at that temperature - warm air can hold more moisture than cold air. A front is the boundary where two air masses meet. A cold front pushes under warm air and forces it up quickly, causing brief heavy storms followed by cooler, clearer weather. A warm front slides gently over cold air, bringing steady, lighter rain and warmer temperatures. Know the main cloud types: cirrus are thin, wispy, high-altitude ice clouds that often signal changing weather; cumulus are puffy, cotton-ball clouds of fair weather; stratus are flat gray layers that bring drizzle; and any cloud with nimbus in its name (nimbostratus, cumulonimbus) is a rain cloud - cumulonimbus is the towering thunderstorm cloud.
Layers of the Atmosphere
The atmosphere has layers stacked from the ground up: troposphere, stratosphere, mesosphere, thermosphere, and exosphere. The troposphere is the lowest layer, where we live and where essentially all weather happens; temperature drops as you go up. The stratosphere sits above it and contains the ozone layer, which absorbs harmful ultraviolet radiation from the sun - jet airliners often cruise near the bottom of this calm layer. The mesosphere is where most meteors burn up, and it is the coldest layer. The thermosphere is extremely thin but very hot, and it is where auroras occur and where many satellites and the space station orbit. The exosphere is the outermost fringe fading into space. Memory trick for the order going up: The Smart Marine Trains Everyday - Troposphere, Stratosphere, Mesosphere, Thermosphere, Exosphere. Bonus fact the test likes: air is about 78 percent nitrogen and 21 percent oxygen.
Worked example
ASVAB-style question: Which layer of the Earth is liquid? (A) crust (B) mantle (C) outer core (D) inner core
- Recall the four layers from outside in: crust, mantle, outer core, inner core.
- The crust is solid rock - that is the ground under your feet, so rule out A.
- The mantle is hot rock that flows very slowly, but it is considered solid, so rule out B.
- The inner core is solid because extreme pressure keeps the iron and nickel from melting, so rule out D.
- The outer core is molten (liquid) iron and nickel, and its motion creates Earth's magnetic field. The answer is C.
Tip: Remember the pair: OUTer core is liquid (think of liquid pouring OUT), inner core is solid. This exact fact shows up often.
Worked example
ASVAB-style question: A rock formed from layers of compressed sand and shell fragments, containing fossils, is most likely which type? (A) igneous (B) sedimentary (C) metamorphic (D) volcanic
- Break down the clues: layers, compressed sediment, and fossils.
- Igneous rock forms from cooled melted rock - fossils would be destroyed by that heat, so rule out A (and D, since volcanic rock is a kind of igneous rock).
- Metamorphic rock forms under intense heat and pressure that usually destroys fossils and erases layering, so rule out C.
- Sedimentary rock forms exactly this way: sediments pile up in layers and get cemented together, trapping fossils. The answer is B.
Tip: Fossils are found only in sedimentary rock. If a question mentions fossils or layers, sedimentary is almost always the answer.
Worked example
ASVAB-style question: The barometer has been falling steadily all afternoon. What weather should you expect? (A) clear, calm skies (B) an approaching storm (C) lower humidity (D) no change
- A barometer measures air pressure.
- Falling pressure means a low-pressure system is moving in.
- Low pressure lets air rise; rising air cools, water vapor condenses, and clouds and precipitation form.
- So falling pressure signals clouds and storms, while rising or high pressure signals fair weather. The answer is B.
Tip: Link them as opposites: high pressure = happy weather, low pressure = lousy weather. A falling barometer means trouble is on the way.
Key takeaway: You now have the core Earth Science facts the ASVAB tests most. Earth has four layers - crust, mantle, liquid outer core, solid inner core. Tectonic plates collide (mountains, volcanoes), separate (new crust), and slide (earthquakes). The three rock types are igneous (cooled melt), sedimentary (compressed layers, home of fossils), and metamorphic (changed by heat and pressure). Weathering breaks rock in place; erosion carries it away. The water cycle runs evaporation, condensation, precipitation. Falling pressure and cold fronts mean storms; cumulonimbus clouds mean thunder. And the atmosphere stacks up as troposphere, stratosphere, mesosphere, thermosphere, exosphere. Drill the quiz below until these feel automatic - these are some of the easiest points you can bank on test day.
Lesson 4 Space Science
Space science questions show up regularly on the General Science section, and the good news is that most of them test a small set of facts you can lock down fast: the order of the planets, why Earth has seasons, what causes moon phases and eclipses, and the difference between things like asteroids and comets. In this chapter we will tour the solar system from the Sun outward, then zoom in on Earth to understand day and night, the year, seasons, and tides, and finish by zooming way out to stars and galaxies. Think of it as a map of the neighborhood, then the whole city.
The Solar System: Planet Order and the Two Planet Families
Our solar system has eight planets orbiting the Sun. In order from the Sun outward they are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. A classic memory trick: My Very Educated Mother Just Served Us Noodles. The planets split into two families. The four inner planets (Mercury, Venus, Earth, Mars) are the terrestrial or rocky planets: small, dense, with solid rocky surfaces. The four outer planets (Jupiter, Saturn, Uranus, Neptune) are the gas giants (Uranus and Neptune are often called ice giants): huge, made mostly of gas like hydrogen and helium, with no solid surface to stand on, and all four have ring systems (Saturn's are just the most visible). Between Mars and Jupiter lies the asteroid belt. A few traits worth memorizing: Mercury is the smallest planet and closest to the Sun; Venus is the hottest planet because its thick carbon dioxide atmosphere traps heat (a runaway greenhouse effect); Mars is the red planet, colored by iron oxide (rust) in its soil; Jupiter is the largest planet; Saturn is famous for its bright rings. Pluto was reclassified as a dwarf planet in 2006.
The Sun Is a Star
The Sun is an ordinary star, and it looks huge and bright only because it is so close to us, about 93 million miles away. It contains more than 99 percent of all the mass in the solar system, and its gravity is what holds the planets in their orbits. The Sun is a giant ball of hot gas, mostly hydrogen and some helium. Its energy comes from nuclear fusion in its core: hydrogen atoms are fused together into helium, releasing enormous amounts of energy that travels to Earth as light and heat. The Sun is not burning like a fire, fusion is a nuclear process, not a chemical one. Sunlight takes a little over 8 minutes to reach Earth. Compared to other stars, the Sun is average in size and temperature, a middle-aged yellow star about 4.6 billion years old.
Moons, Asteroids, Comets, Meteors, and Meteorites
A moon (or natural satellite) is a body that orbits a planet. Earth has one moon; Mars has two small ones; Jupiter and Saturn each have dozens. Now the space rocks, which the test loves to ask about. Asteroids are rocky, metallic objects, most of which orbit the Sun in the asteroid belt between Mars and Jupiter. Comets are chunks of ice, dust, and rock, often described as dirty snowballs, that travel in long, stretched-out orbits; when a comet gets close to the Sun, some of its ice turns to gas and forms a glowing tail that always points away from the Sun because of the solar wind. Meteoroid, meteor, and meteorite describe the same object at three stages: a meteoroid is a small piece of rock or metal traveling through space; when it enters Earth's atmosphere and burns up from friction, the streak of light is called a meteor (a shooting star); if a piece survives the trip and lands on the ground, that surviving rock is a meteorite. Memory hook: it is only a meteorITE once it hITs the ground.
Rotation vs. Revolution: Day, Night, and the Year
Two different motions of Earth explain two different cycles, and the ASVAB expects you to keep them straight. Rotation is Earth spinning on its own axis, like a top. One complete rotation takes about 24 hours, and this is what causes day and night: the side of Earth facing the Sun has daytime while the side facing away has night. Because Earth rotates from west to east, the Sun appears to rise in the east and set in the west. Revolution is Earth traveling in its orbit around the Sun. One complete revolution takes about 365.25 days, which is one year. That extra quarter day is why we add a leap day (February 29) every four years. Quick check: rotation = spin = day; revolution = orbit = year.
Seasons, Moon Phases, and Eclipses
Seasons happen because Earth's axis is tilted about 23.5 degrees, not because Earth gets closer to or farther from the Sun. As Earth revolves around the Sun, the tilt stays pointed the same direction, so different hemispheres lean toward the Sun at different times of year. When the Northern Hemisphere is tilted toward the Sun, it gets more direct sunlight and longer days, that is northern summer, and at the same time the Southern Hemisphere has winter. Six months later the situation reverses. Moon phases happen because the Moon orbits Earth about once a month, and we see different amounts of its sunlit half. The cycle runs new moon (Moon between Earth and Sun, dark to us), waxing crescent, first quarter, waxing gibbous, full moon (Earth between Moon and Sun, fully lit face), then waning gibbous, last quarter, waning crescent, and back to new. Waxing means growing brighter, waning means shrinking. Eclipses are about shadows. A solar eclipse happens when the Moon passes between the Sun and Earth and the Moon's shadow falls on Earth, blocking the Sun; it can only occur at a new moon. A lunar eclipse happens when Earth passes between the Sun and the Moon, and Earth's shadow falls on the Moon; it can only occur at a full moon, and the Moon often turns a reddish color.
Tides, Stars, and Galaxies
Tides are the regular rise and fall of ocean water, caused mainly by the gravitational pull of the Moon (the Sun helps, but the Moon is the bigger factor because it is so much closer). Most coastlines see about two high tides and two low tides each day. When the Sun, Moon, and Earth line up (at new moon and full moon), their pulls combine and we get extra-large spring tides; when they form a right angle (at the quarter moons), we get weaker neap tides. Zooming out: a star is a giant ball of hot gas producing energy by nuclear fusion. Stars come in different colors that indicate temperature, blue-white stars are the hottest and red stars are the coolest, with yellow stars like our Sun in the middle. The distances between stars are so vast that we measure them in light-years, the distance light travels in one year (about 6 trillion miles). A light-year measures distance, not time. Stars group together by gravity into galaxies, enormous collections of billions of stars plus gas and dust. Our galaxy is the Milky Way, a spiral galaxy, and our Sun is just one of its hundreds of billions of stars. The universe contains billions of galaxies.
Worked example
ASVAB-style question: Which of the following causes Earth to experience seasons? (A) Earth's changing distance from the Sun (B) The tilt of Earth's axis (C) Earth's rotation on its axis (D) The phases of the Moon
- Eliminate the common trap first. Choice A sounds logical, but Earth's distance from the Sun barely changes, and in fact Earth is slightly CLOSER to the Sun during Northern Hemisphere winter, so distance cannot be the cause.
- Check choice C. Rotation is Earth spinning once every 24 hours, that explains day and night, not seasons.
- Check choice D. Moon phases come from the Moon's orbit around Earth and have nothing to do with Earth's temperature or seasons.
- That leaves choice B. Earth's axis is tilted about 23.5 degrees, so as Earth revolves around the Sun, each hemisphere leans toward the Sun for part of the year (summer, more direct light, longer days) and away for part of the year (winter).
- The answer is B.
Tip: Distance from the Sun is the number one wrong answer the test hopes you will pick. Seasons = tilt, every time.
Worked example
ASVAB-style question: A rock from space streaks through Earth's atmosphere, and a fragment of it is later found on the ground. The fragment found on the ground is called a: (A) meteor (B) meteoroid (C) meteorite (D) comet
- Recall the three-stage naming rule for the same object.
- In space, the small rock is a meteoroid, so B describes it before it arrives, not on the ground.
- While burning through the atmosphere, the streak of light is a meteor, so A describes the flash, not the rock you can pick up.
- A comet is a completely different object, an icy body with a tail that orbits the Sun, so D is out.
- Once a surviving piece lands on Earth, it is a meteorite. Memory hook: it is a meteorITE once it hITs the ground. The answer is C.
Tip: Meteoroid in space, meteor in the air, meteorite on the ground. Say it three times and this question becomes a free point.
Worked example
ASVAB-style question: During a lunar eclipse, which body is in the middle of the lineup? (A) The Moon (B) The Sun (C) Earth (D) None, they do not line up
- An eclipse is a shadow event, so first ask: whose shadow is falling on what?
- LUNAR eclipse means the Moon is the thing being eclipsed, something is shading the Moon.
- The only body big enough and positioned to cast a shadow on the Moon is Earth, so the order must be Sun, then Earth, then Moon, with Earth in the middle.
- Double-check with the moon phase rule: a lunar eclipse can only happen at a full moon, when the Moon is on the opposite side of Earth from the Sun, which matches Earth being in the middle.
- The answer is C.
Tip: Name the eclipse after the victim: in a LUNAR eclipse the Moon goes dark (Earth in the middle); in a SOLAR eclipse the Sun is blocked (Moon in the middle).
Key takeaway: The eight planets in order are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune: four small rocky inner planets, then the asteroid belt, then four gas giants. The Sun is an average star powered by nuclear fusion of hydrogen into helium. Asteroids are rocky, comets are icy with tails that point away from the Sun, and the same space rock is a meteoroid in space, a meteor burning in the atmosphere, and a meteorite on the ground. Earth's rotation (24 hours) causes day and night; its revolution around the Sun (365.25 days) defines the year; the 23.5 degree axial tilt, not distance, causes the seasons. Moon phases come from seeing different amounts of the Moon's sunlit side; a solar eclipse needs a new moon with the Moon in the middle, and a lunar eclipse needs a full moon with Earth in the middle. Tides are driven mainly by the Moon's gravity. Star color indicates temperature (blue hottest, red coolest), distances are measured in light-years, and our Sun is one of hundreds of billions of stars in the Milky Way galaxy. Nail these facts and space science becomes some of the easiest points on the test.
Lesson 5 Physical Science: Motion, Energy, and Waves
Physics is not about memorizing scary formulas. It is about describing how the everyday world works: why a truck is harder to stop than a bicycle, why a metal bench feels colder than a wooden one, and why you see lightning before you hear thunder. The ASVAB tests these ideas at a basic level. If you can picture the situation, you can usually reason your way to the answer. In this chapter we cover motion, forces, energy, heat, waves, sound, light, and density. Take it one section at a time. You have got this.
Motion Basics: Speed, Velocity, and Acceleration
Speed tells you how fast something moves: speed equals distance divided by time. A car that covers 120 miles in 2 hours has a speed of 60 miles per hour. Velocity is speed plus a direction, such as 60 miles per hour heading north. Because velocity includes direction, it is called a vector; plain speed is not. Acceleration is any change in velocity over time. That means speeding up, slowing down, or even turning at a constant speed all count as acceleration, because in each case the velocity is changing. On the ASVAB, remember these three definitions and the formula speed equals distance divided by time. If you know any two of distance, time, and speed, you can find the third.
Newton's Three Laws of Motion
First law (inertia): an object at rest stays at rest, and an object in motion keeps moving in a straight line at constant speed, unless an outside force acts on it. This is why you lurch forward when a vehicle brakes suddenly. Second law: force equals mass times acceleration, written F equals m x a. The bigger the mass, the more force it takes to accelerate it, which is why a loaded truck needs more braking force than an empty one. Force is measured in newtons; one newton accelerates a 1 kilogram mass at 1 meter per second squared. Third law: for every action there is an equal and opposite reaction. When a rifle fires, the bullet goes forward and the rifle kicks back against your shoulder with equal force. Rockets work the same way: hot gas pushes down and out, and the rocket is pushed up.
Energy Forms and Conservation
Energy is the ability to do work, and it comes in forms: kinetic energy (energy of motion), potential energy (stored energy, such as a rock held above the ground or a stretched spring), chemical energy (in fuel and food), thermal energy (heat), electrical energy, and nuclear energy. The law of conservation of energy says energy cannot be created or destroyed, only changed from one form to another. A swinging pendulum is the classic example: at the top of its swing it has maximum potential energy and zero kinetic energy; at the bottom it has maximum kinetic energy and minimum potential energy. The energy trades back and forth, but the total stays the same (some is slowly lost to air friction as heat, which is still energy, just a different form). Kinetic energy grows quickly with speed: doubling an object's speed makes its kinetic energy four times as large.
Heat, Temperature Scales, and States of Matter
Heat moves in three ways. Conduction is heat transfer by direct contact, as when a metal spoon left in hot soup gets hot along its whole length; metals are good conductors. Convection is heat carried by moving fluids (liquids or gases): warm air rises and cool air sinks, creating circulating currents, which is how a room heater warms the whole room. Radiation is heat transfer by electromagnetic waves and needs no matter at all; this is how the sun warms the Earth across empty space. Temperature is measured on three scales. On Fahrenheit, water freezes at 32 degrees and boils at 212 degrees. On Celsius, water freezes at 0 degrees and boils at 100 degrees. Kelvin is the SI scale used in science: 0 kelvin is absolute zero, the coldest possible temperature, and water freezes at about 273 kelvin. Matter commonly exists as solid, liquid, or gas. Melting (solid to liquid), evaporation or boiling (liquid to gas), condensation (gas to liquid), and freezing (liquid to solid) are phase changes. Sublimation is a solid turning directly into a gas, as dry ice does. During a phase change, the temperature of the substance stays constant while heat is absorbed or released.
Waves, Sound, and Light
A wave carries energy from place to place. Wavelength is the distance from one crest to the next. Frequency is how many waves pass a point each second, measured in hertz. Amplitude is the height of the wave, which relates to its energy: for sound, bigger amplitude means louder; for light, brighter. For a given wave speed, frequency and wavelength trade off: higher frequency means shorter wavelength, because wave speed equals frequency x wavelength. Sound is a mechanical wave, which means it needs a medium (matter) to travel through; sound cannot travel through the vacuum of space. Sound travels fastest in solids, slower in liquids, and slowest in gases, because particles that are packed closer together pass the vibration along faster. In air, sound moves at roughly 343 meters per second, far slower than light, which is why you see lightning before you hear the thunder. Light is an electromagnetic wave and needs no medium; it travels through space at about 300,000 kilometers per second. Reflection is light bouncing off a surface, as in a mirror. Refraction is light bending as it passes from one material into another, which is why a straw in a glass of water looks bent at the surface. Visible light is one small slice of the electromagnetic spectrum, which runs from low-frequency radio waves through microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays at the highest frequencies.
Density, Buoyancy, and SI Units
Density is mass divided by volume, often given in grams per cubic centimeter. Water has a density of 1 gram per cubic centimeter, and that number is your yardstick: an object less dense than the fluid it is placed in floats, and an object more dense sinks. This is buoyancy. Archimedes' principle says the upward buoyant force on an object equals the weight of the fluid the object displaces (pushes aside). A steel ship floats even though steel is denser than water because the ship's hull encloses a lot of air, making the ship's overall density less than water's. Finally, know the basic SI (metric) units, because science questions use them: length is measured in meters, mass in kilograms, time in seconds, temperature in kelvin, force in newtons, energy in joules, power in watts, and frequency in hertz. Note that mass and weight are different: mass is the amount of matter in an object and never changes, while weight is the force of gravity on that mass, so an astronaut weighs less on the moon but has the same mass.
Worked example
A convoy travels 180 miles in 4 hours. What is its average speed?
- Recall the formula: speed equals distance divided by time.
- Plug in the numbers: 180 miles divided by 4 hours.
- Divide: 180 divided by 4 equals 45.
- The average speed is 45 miles per hour.
Tip: The same triangle of relationships works in every direction: distance equals speed x time, and time equals distance divided by speed. Learn one formula and you own all three.
Worked example
What force is needed to accelerate a 12 kilogram ammunition crate at 3 meters per second squared across a frictionless surface?
- Use Newton's second law: force equals mass x acceleration.
- Plug in the values: 12 kilograms x 3 meters per second squared.
- Multiply: 12 x 3 equals 36.
- The required force is 36 newtons.
Tip: Units are your friend. Kilograms x meters per second squared gives newtons automatically, so if your units come out right, your setup is probably right too.
Worked example
A metal block has a mass of 54 grams and a volume of 20 cubic centimeters. Will it float or sink in water?
- Density equals mass divided by volume.
- Plug in: 54 grams divided by 20 cubic centimeters.
- Divide: 54 divided by 20 equals 2.7, so the density is 2.7 grams per cubic centimeter.
- Compare with water, which has a density of 1 gram per cubic centimeter.
- Since 2.7 is greater than 1, the block is denser than water, so it sinks.
Tip: For float-or-sink questions, you rarely need the exact number. Just compare the object's density to the fluid's density: less dense floats, more dense sinks.
Key takeaway: You now have the core physics toolkit for the ASVAB. Speed is distance divided by time, velocity adds direction, and acceleration is any change in velocity. Newton's laws explain inertia, force equals mass x acceleration, and action-reaction pairs. Energy changes form but is never created or destroyed. Heat moves by conduction (contact), convection (moving fluids), and radiation (waves through space). Water freezes at 32 Fahrenheit, 0 Celsius, and about 273 kelvin, and boils at 212 Fahrenheit and 100 Celsius. Waves are described by frequency, wavelength, and amplitude; sound needs a medium and travels fastest in solids, while light needs no medium and travels far faster. Reflection bounces light, refraction bends it. Density is mass divided by volume, and objects float when they are less dense than the fluid around them. Review the SI units, work the practice questions below, and trust the process: every one of these ideas gets easier each time you see it.
Lesson 6 Chemistry Basics
Chemistry can sound intimidating, but the General Science section only expects the basics - and you already know more chemistry than you think. Every time you watch iron rust, dissolve sugar in coffee, or light a campfire, you are watching chemistry in action. In this chapter we will break matter down to its smallest pieces (atoms), learn how those pieces combine into elements, compounds, and mixtures, get comfortable reading the periodic table, and cover the reactions, acids, bases, and gases that show up again and again on the test. Take it one concept at a time - these ideas build on each other, and by the end they will feel like common sense.
Atoms: Protons, Neutrons, and Electrons
The atom is the basic building block of all matter. Every atom has a tiny, dense center called the nucleus, which contains protons (positive charge) and neutrons (no charge). Whizzing around the nucleus are electrons (negative charge), which are far lighter than protons and neutrons. Two numbers describe every atom. The atomic number is the number of protons - it is the atom's identity card. Change the number of protons and you have a different element entirely. The mass number is protons plus neutrons (electrons are so light they barely count toward mass). So carbon, with atomic number 6 and mass number 12, has 6 protons and 12 - 6 = 6 neutrons. In a neutral atom, the number of electrons equals the number of protons, so the charges cancel out. Atoms of the same element with different numbers of neutrons are called isotopes - same atomic number, different mass number.
Elements, Compounds, and Mixtures
An element is a pure substance made of only one kind of atom - gold (Au), oxygen (O), and iron (Fe) are elements. You cannot break an element into anything simpler by ordinary chemical means. A compound forms when two or more different elements chemically bond in a fixed ratio - water (H2O), table salt (NaCl), and carbon dioxide (CO2) are compounds. A compound's properties are usually completely different from its ingredients: sodium is a metal that reacts violently with water, and chlorine is a poisonous gas, yet together they make ordinary table salt. A mixture is different - it is two or more substances physically combined, not chemically bonded, and the parts keep their own properties. Salt water, air, and trail mix are mixtures, and you can separate them by physical means like evaporation or filtering. Quick test: if separating it requires a chemical reaction, it is a compound; if you can separate it physically, it is a mixture.
Reading the Periodic Table
The periodic table organizes all known elements by increasing atomic number. Vertical columns are called groups (or families) - elements in the same group have similar chemical behavior. For example, Group 1 holds the very reactive alkali metals like sodium (Na) and potassium (K), and Group 18 on the far right holds the noble gases like helium (He) and neon (Ne), which almost never react at all. Horizontal rows are called periods. Metals sit on the left and center of the table - they are typically shiny, solid at room temperature (mercury is the famous liquid exception), and good conductors of heat and electricity. Nonmetals sit in the upper right - they are poor conductors and are often gases or brittle solids. A staircase line separates the two, with a few in-between elements called metalloids (like silicon) along it. Memorize the common symbols: H hydrogen, He helium, C carbon, N nitrogen, O oxygen, Na sodium, K potassium, Fe iron, Cu copper, Au gold, Ag silver, Pb lead, Ca calcium, Cl chlorine. Watch out for the tricky ones that come from Latin names: Fe (iron), Au (gold), Ag (silver), Na (sodium), K (potassium), Pb (lead).
Chemical vs Physical Changes
A physical change alters the form or state of a substance without creating anything new. Melting ice, boiling water, dissolving sugar, tearing paper, and bending metal are all physical changes - the substance is still the same stuff, and many physical changes are easy to reverse. A chemical change (chemical reaction) produces one or more brand-new substances with different properties. Burning wood, rusting iron, digesting food, and baking a cake are chemical changes. Clues that a chemical change happened include: a gas is produced (bubbles), light or heat is given off, a permanent color change, or a solid (precipitate) forms out of a liquid. Chemical changes are usually hard or impossible to reverse - you cannot un-burn a log. On the test, ask yourself one question: did a new substance form? If yes, it is chemical. If it just changed shape, size, or state, it is physical.
Acids, Bases, and the pH Scale
Acids and bases are measured on the pH scale, which runs from 0 to 14. A pH below 7 is acidic, exactly 7 is neutral, and above 7 is basic (also called alkaline). The farther from 7, the stronger the acid or base. Pure water is neutral at pH 7. Everyday acids include lemon juice and vinegar (around pH 2 to 3), stomach acid (hydrochloric acid, around pH 1 to 2), and soda. Everyday bases include baking soda (around pH 9), soap, ammonia, and drain cleaner (very basic, pH 13 to 14). Litmus paper is the classic test: acids turn blue litmus red, and bases turn red litmus blue. A memory trick - acids taste sour (think citrus), while bases feel slippery (think soap). When an acid and a base react with each other, they neutralize, producing water and a salt. That is why an antacid tablet (a base) calms excess stomach acid.
Everyday Reactions: Combustion, Oxidation, and Rusting
Two reactions show up constantly on the ASVAB. Combustion is rapid burning: a fuel combines with oxygen and releases heat and light. Burning a fuel like wood, gasoline, or propane with plenty of oxygen produces carbon dioxide (CO2) and water (H2O). Fire needs three things - fuel, oxygen, and heat (the fire triangle) - and removing any one of them puts the fire out. Oxidation is the broader family of reactions where a substance combines with oxygen (more precisely, loses electrons). Combustion is fast oxidation; rusting is slow oxidation. Rust forms when iron reacts with oxygen in the presence of water or moisture, producing iron oxide - the flaky reddish-brown coating on old steel. Both water and oxygen are needed, which is why painting, oiling, or galvanizing (coating with zinc) protects metal: the coating keeps oxygen and moisture away from the iron.
Solutions, Solubility, and Common Gases
A solution is a mixture where one substance dissolves evenly into another. The solute is what gets dissolved (like salt), and the solvent does the dissolving (like water). Water dissolves so many things it is called the universal solvent. Solubility is how much solute can dissolve in a given amount of solvent. For most solids, solubility increases as temperature rises - hot tea dissolves sugar faster and holds more of it than iced tea. Gases behave the opposite way: gases dissolve better in cold liquids, which is why a warm soda goes flat faster than a cold one. A solution holding all the solute it can is saturated. Now the common gases. Nitrogen (N2) makes up about 78 percent of the air, oxygen (O2) about 21 percent. Oxygen supports combustion and is what our bodies need from every breath. Carbon dioxide (CO2) is produced by respiration and combustion, is used by plants for photosynthesis, and is heavier than air, which makes it useful in fire extinguishers - it smothers flames by displacing oxygen. Hydrogen (H) is the lightest and most abundant element in the universe and is highly flammable, which is why blimps switched to non-flammable helium.
Worked example
An atom has an atomic number of 11 and a mass number of 23. How many protons, neutrons, and electrons does the neutral atom have?
- The atomic number equals the number of protons, so the atom has 11 protons.
- The mass number is protons plus neutrons. Subtract to find neutrons: 23 - 11 = 12 neutrons.
- In a neutral atom, electrons equal protons, so there are 11 electrons.
- Answer: 11 protons, 12 neutrons, 11 electrons. Bonus: atomic number 11 is sodium (Na).
Tip: Protons = atomic number. Neutrons = mass number minus atomic number. Electrons = protons (when the atom is neutral). Master this three-step pattern and every atom question becomes simple subtraction.
Worked example
A liquid turns blue litmus paper red. Which of these could it be: ammonia, pure water, vinegar, or soap solution?
- Recall the litmus rule: acids turn blue litmus red; bases turn red litmus blue.
- The liquid turned blue litmus red, so it must be an acid with a pH below 7.
- Check each option: ammonia and soap are bases (pH above 7), pure water is neutral (pH 7), and vinegar is an acid (about pH 2 to 3).
- Answer: vinegar.
Tip: Memory hook: Blue to Red means aciD is ahead. Acids are sour things like vinegar and citrus; bases are slippery things like soap and ammonia.
Worked example
A steel fence left outside develops a reddish-brown, flaky coating over several months. Is this a chemical or physical change, and what caused it?
- Ask the key question: did a new substance form? The reddish-brown flaky coating is rust (iron oxide), which is a new substance with different properties than steel.
- Because a new substance formed, this is a chemical change.
- Identify the cause: iron in the steel slowly combined with oxygen from the air in the presence of moisture - a slow oxidation reaction.
- Answer: it is a chemical change caused by iron reacting with oxygen and water to form iron oxide (rust).
Tip: Rusting, burning, and digesting are always chemical changes. Melting, freezing, dissolving, and bending are physical. When in doubt, ask: could I get the original substance back easily? If not, it is probably chemical.
Key takeaway: You now have the chemistry toolkit the ASVAB expects. Atoms are built from protons and neutrons in the nucleus with electrons around them; the atomic number counts protons and the mass number counts protons plus neutrons. Elements are one kind of atom, compounds are elements chemically bonded in fixed ratios, and mixtures are physical combinations you can separate physically. On the periodic table, columns are groups with similar behavior, rows are periods, metals sit left, and nonmetals sit upper right - and remember the Latin-based symbols like Fe, Au, Ag, Na, K, and Pb. Physical changes keep the same substance; chemical changes make new ones. The pH scale runs 0 to 14 with 7 neutral - acids turn blue litmus red, bases turn red litmus blue. Combustion is fast oxidation and rusting is slow oxidation, both needing oxygen. In solutions, the solute dissolves in the solvent, solids dissolve better hot while gases dissolve better cold. And in the air around you: about 78 percent nitrogen, 21 percent oxygen, with CO2 feeding plants and smothering fires, and hydrogen as the lightest, most flammable element. Review the tricky symbols and the pH landmarks one more time, then test yourself with the practice questions - you are ready for this.