Overview
The ASVAB General Science subtest covers fundamental concepts across biology, chemistry, physics, and earth science. This guide synthesizes key flashcard content into focused summaries with definitions, relationships, and exam strategies. Mastering these concepts will help you maximize your AFQT score and qualify for specialized military occupational specialties (MOS).
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Biology
Cell Structure & Function
The cell is the basic unit of life, with specialized organelles performing distinct functions.
• Mitochondria – The "powerhouse of the cell"; produces ATP (adenosine triphosphate) through cellular respiration (glucose + oxygen → ATP + CO₂ + water)
• Chloroplasts – Site of photosynthesis in plant cells; converts sunlight, water, and CO₂ into glucose and oxygen
• Ribosomes – Site of protein synthesis; translates mRNA into chains of amino acids that fold into functional proteins
• Nucleus – Contains DNA and directs cell activity
#### Key Terms
• ATP – The primary energy currency of the cell
• Photosynthesis – 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂
• mRNA (messenger RNA) – Carries genetic instructions from DNA to ribosomes
> Watch Out For: Photosynthesis occurs in chloroplasts (plants only), while cellular respiration occurs in mitochondria (all cells). Don't confuse the two processes — they are essentially the reverse of each other.
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Genetics & DNA
• DNA contains four nitrogenous bases: Adenine (A), Thymine (T), Guanine (G), and Cytosine (C)
- Base pairing rule: A pairs with T; G pairs with C
• Dominant allele – Expressed even when only one copy is present
• Recessive allele – Only expressed when two copies are present (homozygous recessive)
• Homozygous – Two identical alleles (AA or aa)
• Heterozygous – Two different alleles (Aa)
#### Cell Division Comparison
| Feature | Mitosis | Meiosis |
|---|---|---|
| Daughter cells produced | 2 | 4 |
| Genetic identity | Identical | Unique |
| Chromosome number | Full (diploid) | Half (haploid) |
| Purpose | Growth/repair | Sexual reproduction |
#### Key Terms
• Allele – A variant form of a gene
• Phenotype – The observable traits of an organism
• Genotype – The genetic makeup of an organism
> Watch Out For: Mitosis = 2 identical cells. Meiosis = 4 unique cells. A common trick question asks about the genetic uniqueness of the daughter cells, not just the number.
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Human Body Systems
#### Digestive System
• Large intestine – Primarily absorbs water and electrolytes; compacts waste into feces
• Small intestine – Primary site of nutrient absorption (don't confuse with the large intestine)
#### Endocrine System
• Pancreas – Produces insulin via beta cells in the islets of Langerhans
• Insulin enables body cells to absorb glucose, lowering blood sugar levels
#### Circulatory System — Blood Types
| Blood Type | Antigens Present | Can Donate To | Can Receive From |
|---|---|---|---|
| A | A antigen | A, AB | A, O |
| B | B antigen | B, AB | B, O |
| AB | A and B antigens | AB only | All types |
| O negative | None | All types | O negative only |
• O negative (O-) = Universal donor (no A, B, or Rh antigens)
• AB positive (AB+) = Universal recipient
> Watch Out For: Universal donor is O negative; universal recipient is AB positive. These are frequently swapped on tests.
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Chemistry
Atomic Structure
• Atomic number – The number of protons in an atom's nucleus; uniquely identifies an element and determines its place on the periodic table
• Mass number – Total number of protons + neutrons
• Isotopes – Atoms of the same element with the same number of protons but different numbers of neutrons (e.g., Carbon-12 vs. Carbon-14)
#### Key Terms
• Proton – Positively charged particle in the nucleus
• Neutron – Neutral particle in the nucleus
• Electron – Negatively charged particle orbiting the nucleus
> Watch Out For: Isotopes have the same atomic number but different mass numbers. Changing the number of neutrons does NOT change the element.
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Chemical Bonds
• Covalent bond – Electrons are shared between atoms; typically forms between two nonmetals
- Polar covalent bond – Electrons shared unequally (e.g., water, H₂O)
- Nonpolar covalent bond – Electrons shared equally (e.g., O₂)
• Ionic bond – Electrons are transferred from one atom to another; forms between a metal and nonmetal
#### Water (H₂O)
• Formula: H₂O (2 hydrogen atoms + 1 oxygen atom)
• Bond type: Polar covalent (oxygen pulls electrons more strongly)
• This polarity gives water its unique properties (surface tension, solvent ability, etc.)
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Acids, Bases & pH
• pH scale ranges from 0–14
- Below 7 = Acidic (more H⁺ ions)
- pH 7 = Neutral (pure water)
- Above 7 = Basic/Alkaline (more OH⁻ ions)
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Chemical Reactions
• Law of Conservation of Mass – Matter is neither created nor destroyed in a chemical reaction; mass of reactants = mass of products
• Exothermic reaction – Releases energy/heat (e.g., combustion, burning)
• Endothermic reaction – Absorbs energy/heat (e.g., photosynthesis, dissolving baking soda in water)
#### Key Terms
• Reactants – Substances entering a chemical reaction
• Products – Substances formed by a chemical reaction
> Watch Out For: Exothermic = energy exits the reaction (releases heat). Endothermic = energy enters the reaction (absorbs heat). The prefix is your clue!
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Physics
Motion & Forces
• Newton's Second Law: F = ma
- Force (F) = Mass (m) × Acceleration (a)
- Greater mass or greater acceleration requires proportionally greater force
- Unit of force: Newton (N)
#### Speed vs. Velocity
| Property | Speed | Velocity |
|---|---|---|
| Type | Scalar | Vector |
| Includes direction? | No | Yes |
| Example | 60 mph | 60 mph north |
> Watch Out For: Speed is scalar (magnitude only); velocity is a vector (magnitude + direction). This distinction is a classic test question.
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Energy & Waves
• Law of Conservation of Energy – Energy cannot be created or destroyed; it can only be converted from one form to another (total energy in a closed system remains constant)
- Example: Potential energy → Kinetic energy as a ball falls
#### Electromagnetic Waves (Visible Light)
• Color is determined by wavelength (and frequency)
• Visible spectrum order (longest to shortest wavelength): Red, Orange, Yellow, Green, Blue, Indigo, Violet
- Memory aid: ROY G BIV
- Red = longest wavelength, lowest frequency
- Violet = shortest wavelength, highest frequency
#### Mechanical vs. Electromagnetic Waves
| Wave Type | Requires Medium? | Example |
|---|---|---|
| Mechanical | Yes | Sound, water waves |
| Electromagnetic | No | Light, radio waves, X-rays |
> Watch Out For: Sound waves are mechanical — they cannot travel through the vacuum of space. Light is electromagnetic and can travel through a vacuum.
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Electricity
• Ohm (Ω) – SI unit of electrical resistance
• Defined as the resistance that allows 1 ampere of current when 1 volt is applied
• Ohm's Law: V = IR (Voltage = Current × Resistance)
#### Key Terms
• Volt (V) – Unit of electrical potential/voltage
• Ampere (A) – Unit of electrical current
• Ohm (Ω) – Unit of electrical resistance
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Earth Science
Geology & Rock Cycle
• Three rock types:
1. Igneous – Forms when magma (underground) or lava (surface) cools and solidifies (e.g., granite, basalt)
2. Sedimentary – Forms from compacted/cemented sediment layers (e.g., sandstone, limestone)
3. Metamorphic – Forms when existing rock is changed by heat and pressure (e.g., marble, slate)
#### Key Terms
• Magma – Molten rock below Earth's surface
• Lava – Molten rock at Earth's surface
• Richter Scale – Measures magnitude (energy released) by earthquakes; each whole-number increase = ~31.6× more energy released
> Watch Out For: The Richter scale measures earthquake magnitude, not damage or depth. Each whole number is not a simple 10× increase — it's approximately 31.6× more energy.
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Earth's Atmosphere
Earth's atmospheric layers from lowest to highest altitude:
1. Troposphere – Weather occurs here; where we live
2. Stratosphere – Contains the ozone layer (~12–50 km); absorbs ultraviolet (UV) radiation
3. Mesosphere
4. Thermosphere
5. Exosphere
> Watch Out For: The ozone layer is in the stratosphere, not the troposphere. Don't confuse the layer where weather occurs (troposphere) with where UV protection occurs (stratosphere).
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Earth's Systems & Astronomy
#### The Water Cycle
• Evaporation – Solar energy converts liquid water → water vapor (oceans, lakes, rivers → atmosphere)
• Condensation – Water vapor cools and forms clouds
• Precipitation – Water falls as rain, snow, sleet, or hail
• Runoff/Infiltration – Water returns to bodies of water or groundwater
#### Earth's Seasons
• Caused by Earth's 23.5-degree axial tilt relative to its orbital plane
• NOT caused by Earth's distance from the Sun
• When the Northern Hemisphere tilts toward the Sun → Summer; tilted away → Winter
#### Solar System Classification
| Body | Orbits Sun | Spherical Shape | Cleared Orbit? |
|---|---|---|---|
| Planet | Yes | Yes | Yes |
| Dwarf Planet | Yes | Yes | No |
| Moon | No (orbits planet) | Usually | N/A |
• Dwarf planets: Pluto, Ceres, Eris, Makemake, Haumea
> Watch Out For: Earth's seasons are caused by axial tilt, NOT distance from the Sun. In fact, Earth is slightly closer to the Sun during Northern Hemisphere winter!
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Quick Review Checklist
Use this checklist to confirm you've mastered all critical concepts before test day:
Biology
• [ ] Mitochondria = ATP production via cellular respiration
• [ ] Chloroplasts = photosynthesis (sunlight + CO₂ + H₂O → glucose + O₂)
• [ ] Ribosomes = protein synthesis (mRNA → amino acid chains)
• [ ] DNA bases: A-T and G-C pairings
• [ ] Mitosis = 2 identical cells; Meiosis = 4 unique cells (half chromosomes)
• [ ] O negative = universal blood donor; AB positive = universal recipient
• [ ] Pancreas produces insulin (beta cells, islets of Langerhans)
• [ ] Large intestine = water/electrolyte absorption
• [ ] Recessive allele requires TWO copies to be expressed
Chemistry
• [ ] Atomic number = number of protons (identifies the element)
• [ ] Isotopes = same protons, different neutrons
• [ ] Covalent bond = electron sharing; Ionic bond = electron transfer
• [ ] H₂O = polar covalent bonds
• [ ] pH 7 = neutral; below 7 = acidic; above 7 = basic
• [ ] Law of conservation of mass: reactants = products
• [ ] Exothermic releases heat; endothermic absorbs heat
Physics
• [ ] F = ma (Newton's Second Law)
• [ ] Speed = scalar; Velocity = vector (includes direction)
• [ ] Ohm (Ω) = unit of electrical resistance
• [ ] Mechanical waves require a medium (sound cannot travel in space)
• [ ] Visible light: ROY G BIV (red = longest wavelength; violet = shortest)
• [ ] Law of conservation of energy: energy cannot be created or destroyed
Earth Science
• [ ] Three rock types: igneous, sedimentary, metamorphic
• [ ] Igneous = cooled magma/lava
• [ ] Ozone layer is in the stratosphere
• [ ] Seasons caused by 23.5° axial tilt, NOT distance from Sun
• [ ] Richter scale measures earthquake magnitude (~31.6× per whole number)
• [ ] Evaporation = liquid water → water vapor via solar energy
• [ ] Dwarf planet = orbits Sun, spherical, but has NOT cleared its orbital neighborhood
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Good luck on your ASVAB! Focus on understanding relationships between concepts rather than memorizing facts in isolation — the test rewards applied knowledge.