ASVAB General Science study guide

The four-step method for a fact you cannot recall, the confusable pairs this subtest is built on, and the core facts of life, earth and space, and physical science.

What this subtest actually asks of you

General Science is 15 of the 135 scored questions on the computer ASVAB — 11% of the scored test. It is the one topic here that is not part of your AFQT score, so it does not change the number that decides whether you can enlist — but it does feed the line scores that decide which jobs you qualify for (how the ASVAB is scored).

The official subtest description covers three domains — life science, earth and space science, and physical science — and the questions are recall and recognition: vocabulary, structures, processes, and one-step physical reasoning. There is nothing to compute beyond a single step.

Two consequences, and they shape how to study it:

It is broad rather than deep, so coverage wins. Fifteen questions spread across three domains is roughly five each, so a gap in one domain costs you real points. Learn the vocabulary and the core process of each, and do not try to master any of them.
Study each fact beside its near neighbor. Learn the process that runs the other way, the term that sounds alike, the pair that differs by a single step — because that pair is where a well-built wrong answer comes from, and deciding which side of the pair the question is asking about is most of the work here.

The four-step method for a fact you cannot recall

This is the honest procedure for the moment you read a question and realize you do not know the answer. It is what turns a guess from a coin flip into a decent bet.

1. Name the domain before anything else. "This is about rocks" → earth science → which rock type forms from cooled magma? Naming the domain narrows the field to a dozen possibilities instead of the whole subject.

2. Ask what would have to be true. A physical or biological claim carries a consequence. If something "absorbs light and heats up", then the thing next to it must be cooler — and an option whose consequence contradicts that is out. This is one step of reasoning, and it is what this subtest's few reasoning questions are actually testing.

3. Eliminate the swapped pair and the absolute. If two choices are each other's opposite (conduction and convection, weather and climate, mitosis and meiosis), then the question is testing that distinction and everything else is a distractor. Options using all, none, always or never are usually wrong, because few things in nature are absolute.

4. Fall back on scale and unit. For anything numerical, the wrong answers are usually off by a factor or a scale: meters against millimeters, seconds against years, a million against a billion. Knowing the size of the answer narrows four choices fast.

The confusables

Each line is a boundary this subtest is built on. Two choices from one row means the question is about that boundary.

Question	The boundary
How does mass differ from weight?	how much matter vs how hard gravity pulls it
How does weather differ from climate?	what is happening now vs the long-run pattern
How does rotation differ from revolution?	spinning on an axis vs traveling around a body
How do conduction, convection and radiation move heat?	through contact vs by moving fluid vs by electromagnetic waves
How does mitosis differ from meiosis?	body cells dividing vs sex cells dividing
How does a mixture differ from a compound?	combined but separable vs chemically bonded
How does kinetic energy differ from potential energy?	energy of motion vs stored energy
How does a cell wall differ from a cell membrane?	rigid support (plants) vs the outer boundary (all cells)
How do igneous, sedimentary and metamorphic rock form?	cooled from magma vs laid down in layers vs changed by heat and pressure
How does a hypothesis differ from a theory?	a testable proposal vs an explanation supported by much evidence
How does a hypothesis differ from a prediction?	the proposal vs what it says will happen
How does a planet differ from a moon?	orbits a star vs orbits a planet
How does a bacterium differ from a virus?	a living single-celled organism an antibiotic can kill vs a thing that must use a host cell to copy itself
How does a vitamin differ from a mineral?	made by living things (or by your skin, from sunlight) vs an element that comes from the soil
How do carbohydrates, proteins and fats differ?	the quickest fuel vs the building material vs the long-term store
How does atomic number differ from atomic mass?	protons alone vs protons plus neutrons
How does kilo differ from milli?	a thousand times a unit vs a thousandth of a unit

The facts this subtest asks for

A checklist of names is not enough: a question that asks which cell part releases energy is not answered by a list that mentions mitochondria in passing. So every idea below gets a row of its own — what it is and what it does — and those tables are the fact sheet the questions are asked from and the flashcards are drawn from.

Life science

Cells and what their parts do.

Question	What it does	The one fact to know
What does the nucleus do?	holds the cell's DNA and directs its activities	the control center of the cell
What do the mitochondria do?	release the energy from food	a hard-working cell such as a muscle cell is packed with them
What do the ribosomes do?	build proteins	they are where protein is assembled
What does the cytoplasm do?	the jelly the cell's parts sit in	it holds the parts in place
What does the cell membrane do?	the outer boundary	it controls what passes in and out of the cell
What does the cell wall do?	a rigid layer outside the membrane	plants and bacteria have one and animal cells do not

Photosynthesis and respiration are opposites.

Question	What it takes in and gives out	The one fact to know
What does photosynthesis take in and give out?	takes in carbon dioxide and water, uses light, and makes sugar — releasing oxygen	it removes carbon dioxide from the air
What does respiration take in and give out?	uses oxygen and sugar to release energy, giving off carbon dioxide and water	it returns that carbon dioxide
How are photosynthesis and respiration related?	one reverses the other	a question offering both is asking which side of the exchange it describes

Food chains, and what the animals are called.

Question	What it means	The one fact to know
What are producers?	plants, which make their own food	every food chain starts here
What are consumers?	everything that eats something else	every animal in the chain is one
What are herbivores?	eat plants only	rabbits and cows — and there are more of them than carnivores
What are carnivores?	eat animals	lions and hawks — fewer, because energy is lost at each step
What are omnivores?	eat both	humans are one
What are decomposers?	break down dead material	bacteria and fungi, which return nutrients to the soil
Why are food chains short?	energy is lost at every step	that is why large predators are few

Classification.

Question	What it means	The one fact to know
What is the order of the groups?	kingdom → phylum → class → order → family → genus → species	it runs from the broadest group to the narrowest
What is a species?	the smallest group	two animals are the same species when they can breed and produce fertile young

Body systems — and the two words to keep apart inside them.

Question	What it is or does	The one fact to know
What does the circulatory system do?	the heart plus the vessels	its job is to move blood, and with it oxygen and nutrients
What do the arteries do?	carry blood away from the heart	away — the A is for away
What do the veins do?	bring blood back to the heart	in — the IN in veIN is the direction it runs
What do the capillaries do?	where exchange happens	the finest of the three, so exchange can happen through the wall
What does the respiratory system do?	the lungs and gas exchange	it takes in oxygen and gives off carbon dioxide
What does the digestive system do?	runs from mouth to intestine	nutrients are absorbed here
What does the nervous system do?	the brain, spinal cord and nerves	it carries signals around the body
What does the neuron do?	the working cell of the nervous system	the cell that carries the signal
What do the skeletal and muscular systems do?	give support and movement	bones support, muscles pull

Plants.

Question	What it does	The one fact to know
What do the roots do?	absorb water and minerals and anchor the plant	the minerals come in through the root hairs
What does the stem do?	transports	it carries water up and sugar down
What do the leaves do?	photosynthesize	chlorophyll is the pigment that captures the light
What do the flowers do?	reproduce	the stamens and pistils are the parts that make the seed

Genetics.

Question	What it means	The one fact to know
What is DNA?	the material genes are made of	a gene is a length of it
What is a dominant trait?	appears when only one copy of the gene is present	it masks the other copy
What is a recessive trait?	appears only when both copies are recessive	with one copy, a dominant gene masks it
What is a carrier?	has one recessive copy but shows the dominant trait	the recessive gene can pass on without ever showing

Nutrition: the six things the body has to be given.

Question	What it does	The one fact to know
What do the carbohydrates do?	the body's main and quickest source of energy	starches and sugars, broken down into glucose
What does the protein do?	builds and repairs tissue	muscle, skin, blood and the enzymes that run the body's reactions
What do the fats do?	store energy for later	they cushion the organs and carry the fat-soluble vitamins
What do the vitamins do?	needed in small amounts, each with its own job	a shortage shows up as one specific illness, which is what the question is describing
What does vitamin A do?	for vision	the one the eye needs
What does vitamin C do?	for healing and fighting infection	the one a cold remedy claims
What does vitamin D do?	for strong bones	the one the body makes itself, because sunlight on the skin starts the reaction
What do the minerals do?	come from the soil rather than from living things	an element, so no living thing can make one
What does calcium do?	the main building material of bone and teeth	bones and teeth are made of it
What does iron do?	what lets the blood carry oxygen	the mineral the blood needs
What does water do for the body?	carries everything else	most of the body is made of it
What makes a vitamin different from a mineral?	a vitamin is something the body cannot make enough of	a mineral is an element the body cannot make at all, which is why both have to come from food

Bacteria and viruses, and why the difference decides the treatment.

Question	What it is	The one fact to know
What are bacteria?	living single-celled organisms	they eat, grow and reproduce on their own by dividing, and plenty are useful — the ones in yogurt and in the gut
What are viruses?	not cells, and far smaller	a virus carries no copying machinery of its own, so it has to take over a living host cell and use that cell to make more of itself
What are antibiotics?	kill bacteria	they do nothing to a virus, which is why a cold cannot be treated with one
What is a vaccine?	prevents a viral illness	it teaches the immune system to recognize the virus before it meets it

Earth and space science

The three rock types, and how each forms.

Question	How it forms	The one fact to know
How does igneous rock form?	cools and hardens from magma or lava	granite, basalt
How does sedimentary rock form?	laid down in layers of sediment pressed together	sandstone, limestone — and it is where fossils are found
How does metamorphic rock form?	existing rock changed by heat and pressure	without melting; marble, slate

Inside the Earth.

Question	What it is	The one fact to know
What is the inner core?	the center of the planet	solid
What is the outer core?	the layer around the inner core	liquid
What is the mantle?	the layer between the core and the crust	the thickest of the four
What is the crust?	the thin rocky layer we live on	the only layer broken into the moving plates
What is plate tectonics?	what moves the crust	earthquakes happen along faults and volcanoes sit at plate edges

The water cycle.

Question	What it does	The one fact to know
What happens during evaporation?	lifts water into the air as vapor	it is the step the Sun drives
What happens during condensation?	turns that vapor back into liquid droplets	clouds are made of them
What happens during precipitation?	falls as rain or snow	it is how the water comes back down
What happens during runoff and infiltration?	return the water to the ground	one runs off over the surface and one soaks in

The atmosphere.

Question	What it is	The one fact to know
What is the most abundant gas?	nitrogen, at about 78% by volume	oxygen is 21%, and trace gases make up the rest
What are greenhouse gases?	carbon dioxide, methane and water vapor	nitrogen is not one of them
What is the troposphere?	the lowest layer of the atmosphere	weather happens here
What is the stratosphere?	the layer above the troposphere	it holds the ozone layer
What are the mesosphere and thermosphere?	the two layers above the stratosphere — mesosphere, then thermosphere	the four run troposphere, stratosphere, mesosphere, thermosphere

Seasons come from the tilt of Earth's axis.

Question	What it says	The one fact to know
What causes the seasons?	the tilt of Earth's axis, not its distance from the Sun	the distance answer is the trap this question is built on
What does the tilt do?	each hemisphere receives sunlight more directly for part of the year	it is why the two hemispheres have opposite seasons

The Moon.

Question	What it is	The one fact to know
What is a new moon?	the phase that is invisible from Earth	because it sits between us and the Sun
What is the first quarter?	half lit and growing	the lit half is on its way to full
What is a full moon?	fully lit	it is on the opposite side of Earth from the Sun
What is the last quarter?	half lit and shrinking	the mirror image of the first quarter
What are the tides?	mainly the Moon's gravitational pull	the Moon pulls the water toward it

The planets.

Question	What it is	The one fact to know
What is the order from the Sun?	Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune	the first four are rocky and the next four are gas and ice giants
What is Jupiter?	the largest planet	larger than all the other planets put together
What is Saturn?	the planet with the prominent rings	the one you recognize from its rings
What keeps the planets in orbit?	gravity	the Sun's pull, which is also why they do not drift away

Physical science

States of matter.

Question	What it is	The one fact to know
What is a solid?	a fixed shape and a fixed volume	the particles are locked in place
What is a liquid?	a fixed volume, but it takes the shape of its container	the particles slide past each other
What is a gas?	neither a fixed shape nor a fixed volume	the particles fly apart and fill the container
What happens when a substance changes state?	melting, boiling, evaporating, condensing, freezing	it is a physical change, so the substance is the same

Atoms and elements.

Question	What it is	The one fact to know
What is an atom?	protons and neutrons in the nucleus, with electrons around it	the smallest piece of an element
What is a proton?	sits in the nucleus	it carries a positive charge
What is a neutron?	sits in the nucleus	it carries no charge
What is an electron?	orbits the nucleus	it carries a negative charge
What do you call the number of protons?	the element's identity	which is why it is the atomic number

Reading the periodic table.

Question	What it is	The one fact to know
What is the atomic number?	the number of protons in the nucleus	it is what makes an element that element
What is the atomic mass?	protons plus neutrons	very close to twice the atomic number for the light elements
What does an element's symbol tell you?	one or two letters	not always the English name — Na is sodium, Fe is iron, K is potassium, Au is gold, C is carbon
What is a period?	a horizontal row	the rows and the columns carry information in different directions
What is a group?	a vertical column	elements in one group behave alike, because they have the same number of electrons in the outer shell
What are metals and non-metals?	metals on the left, non-metals on the right	the boundary is the staircase line on the periodic table
What are the noble gases?	helium, neon and argon, in the far-right column	their outer shells are full, so they barely react with anything

Metric prefixes, and moving the decimal point instead of doing arithmetic.

Question	What it means	The one fact to know
What are the base units?	the meter for length, the gram for mass, the liter for volume	one system, one base unit each
What does kilo mean?	1,000 of the unit	one kilometer is 1,000 meters and one kilogram is 1,000 grams
What does centi mean?	one-hundredth of the unit	one meter is 100 centimeters
What does milli mean?	one-thousandth of the unit	one meter is 1,000 millimeters and one liter is 1,000 milliliters
What does micro mean?	one-millionth of the unit	the smallest of the prefixes here
How do you convert to a smaller unit?	multiply	2.5 kilometers = 2,500 meters
How do you convert to a bigger unit?	divide	500 grams = 0.5 kilograms
How do you check a unit conversion?	carry the units through and check the size: an answer that comes out smaller when you moved to a smaller unit has the direction wrong	if the answer came out smaller when you moved to a smaller unit, the direction is wrong

Mixtures and compounds.

Question	What it is	The one fact to know
What is a mixture?	two or more substances together that are not chemically combined — air, salt water, sand mixed with iron filings	it keeps the properties of its parts, which is why it can be separated by physical means
What is a compound?	chemically bonded — water (H2O), carbon dioxide (CO2), salt (NaCl)	breaking one apart takes a chemical change

Chemical versus physical change.

Question	What it does	The one fact to know
What does a chemical change do?	makes a new substance	rusting, burning, cooking
What does a physical change do?	changes form, not identity	melting, dissolving, boiling

Newton's three laws.

Question	What it says	The one fact to know
What does the first law say?	an object keeps doing what it was doing unless a force acts on it	that is inertia
What does the second law say?	force = mass × acceleration	it also gives you mass = force ÷ acceleration
What does the third law say?	every action has an equal and opposite reaction	the two forces act on different objects

Energy, work and the two quantities people mix up.

Question	What it is	The one fact to know
What is kinetic energy?	the energy of motion	it depends on the mass and the square of the speed
What is potential energy?	stored energy	a stretched spring, a raised weight
What are thermal, chemical and electrical energy?	the other forms	energy is conserved: it changes form but is never created or destroyed
What is average speed?	distance ÷ time	the formula the question is asking you to recall
What is weight?	mass × gravity	a 10 kg mass weighs about 100 newtons on Earth
Which stays the same everywhere: mass or weight?	mass is how much matter, weight is how hard gravity pulls it	the pair the question is built to confuse

Simple machines.

Question	What it is	The one fact to know
What do most simple machines do?	trade distance for force	the same work, spread over more distance
What is a lever?	a bar on a pivot	the classic first-class example
What is a pulley?	a wheel with a rope	one fixed pulley changes the direction of the pull
What is a wheel and axle?	a wheel turning a smaller axle	the steering-wheel idea
What is an inclined plane?	a ramp	it raises a load over a longer distance, so it needs less force
What is a wedge?	two inclined planes back to back	it is what an axe or a knife is
What is a screw?	an inclined plane wrapped around a cylinder	it is a ramp in a spiral

Electricity.

Question	What it is	The one fact to know
What are conductors?	copper, aluminum, salt water	they let charge flow
What are insulators?	rubber, glass, plastic, dry wood	they do not
What are semiconductors?	silicon and its relatives	they conduct only under the right conditions
What is a series circuit?	one path	the same current through everything, resistances add, and one break stops the lot
What is a parallel circuit?	separate paths	each branch gets the full supply voltage, the total resistance is lower than the smallest branch, and the other branches keep working
What is resistance at the same voltage?	more resistance means less current	the current decreases
What is Ohm's law?	voltage, current and resistance tied together	it has a full worked treatment in the Electronics Information guide

Magnetism.

Question	What it is	The one fact to know
What are magnetic poles?	like poles repel and unlike poles attract	the same rule as the Electronics guide
What is an electromagnet?	current through a coil makes one	it is a magnet only while the current flows
What is a motor?	converts electricity into motion	the machine a generator runs in reverse
What is a generator?	converts motion into electricity — a motor run backwards	the reverse of a motor, which is why the pair is a common trap

Waves and light.

Question	What it is	The one fact to know
What is sound?	a wave that needs a medium	no air, no sound
What is light?	a wave that does not	and that is why the two are always offered against each other
What is reflection?	light bouncing off a surface	it stays in the same material
What is refraction?	light bending as it crosses into a new material	it happens because light changes speed, which is why a stick looks bent in water

How heat moves.

Question	How it travels	The one fact to know
How does conduction travel?	through direct contact	the objects have to touch
How does convection travel?	by a moving fluid	warm air rising and cool air sinking
How does radiation travel?	as waves	it is how heat crosses empty space, which is how the Sun reaches us

Acids and bases.

Question	What it is	The one fact to know
What is the pH scale?	runs 0 to 14	below 7 acidic, exactly 7 neutral, above 7 basic
How does an acid differ from a base?	the two ends of the scale	7 is neither, which is what separates them

Where energy comes from.

Question	What it is	The one fact to know
What are fossil fuels?	coal, oil and natural gas	finite, and they will run out
What are renewables?	wind, solar, hydro and geothermal	replenished continuously

How to practice this

Work the free General Science practice set with the four steps in front of you — naming the domain and eliminating the swapped pair is the whole skill.
Keep a running "swapped pair" list from the questions you get wrong. That is more useful here than a list of topics, because the pairs are what the questions are made of.
Cover the tables above and say a sentence about each row out loud. Recognition feels like knowledge and is not; saying it is.
Read the explanation on every question, including the ones you got right.

How to study for the ASVAB has the rest of the method, and what the pass adds.

Where to go next

Next topic — Arithmetic Reasoning
The exam itself — how the ASVAB is scored, how many questions, and what makes the ASVAB hard.

General Science carries 15 of the 135 scored questions on the computer test — the published subtest table's count — and the AFQT score is not computed from it: that score comes from Arithmetic Reasoning, Mathematics Knowledge, Word Knowledge and Paragraph Comprehension.

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