What the ASVAB is actually testing
Physical science on the GS subtest covers two main areas: matter and its properties (states, density, the periodic table, atomic structure) and basic chemistry (chemical vs. physical changes, simple reactions, conservation of mass). You won't be asked to balance complex equations — you will be asked to identify what's happening in a reaction and interpret atomic data from a periodic table entry.
Matter and its states
Matter exists in four states; the ASVAB focuses on three:
- Solid — fixed shape and volume, particles tightly packed
- Liquid — fixed volume, no fixed shape, particles slide past each other
- Gas — no fixed shape or volume, particles move freely
Adding heat causes transitions: solid → liquid (melting), liquid → gas (evaporation). Removing heat reverses them. Sublimation is when a solid converts directly to gas (dry ice is the classic example).
Atomic structure and the periodic table
An atom's nucleus holds protons and neutrons. Electrons orbit outside in shells. What to know:
- The atomic number (always the smaller number on a periodic table entry) = number of protons = the element's identity
- The mass number = protons + neutrons
- Neutral atoms have equal protons and electrons; ions have gained or lost electrons
The periodic table is organized so that elements in the same column (group) behave similarly. Metals on the left donate electrons; nonmetals on the right tend to accept them. Noble gases (far right, Group 18) are chemically inert.
Chemical vs. physical changes
This distinction shows up on almost every GS test:
| Change | Signs | Example |
|---|---|---|
| Physical | Same substance, different form | Ice melting, salt dissolving |
| Chemical | New substance(s) formed | Wood burning, iron rusting |
Key indicators of a chemical change: color change, gas produced, heat released or absorbed, or a precipitate forms.
Conservation of mass
In any chemical reaction, the total mass of the reactants equals the total mass of the products. Nothing is created or destroyed — atoms are rearranged. If a question gives you reactant masses and asks for a product mass, add them up.
Study approach
Focus on being able to read a periodic table entry and pull out atomic number, mass number, and number of electrons. Then lock in the physical vs. chemical change distinction — that's a reliable point on test day.