Physical Science
States of matter, the periodic table, and basic chemical reactions are the chemistry-side of GS, understand the patterns and the memorization becomes minimal.
Formula Reference
- Density: D = m/v (mass divided by volume, units g/cm³ or kg/m³)
- States of matter in order of energy: solid → liquid → gas → plasma
- Atomic structure: protons + neutrons in nucleus; electrons in shells around it
- Atomic number = number of protons; mass number = protons + neutrons
- Periodic table periods (rows) = energy levels; groups (columns) = similar properties
- Law of Conservation of Mass: mass of reactants = mass of products in any reaction
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
Same particles, different spacing and energy. Adding heat gives particles energy to spread out: solid → liquid → gas.
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.
Common Pitfalls
- ⚠Confusing atomic number (protons only) with mass number (protons + neutrons)
- ⚠Thinking electrons have significant mass, they're negligible compared to protons and neutrons
- ⚠Mixing up physical change (no new substance, e.g. melting ice) with chemical change (new substance formed, e.g. burning wood)
- ⚠Assuming all metals are on the left, metalloids sit along a staircase in the middle of the periodic table
Worked Examples
Q1: An element has 17 protons and 18 neutrons. What is its mass number?
Answer: Mass number = protons + neutrons = 17 + 18 = 35. (This is chlorine-35.)
Q2: A sample of liquid has a mass of 50 g and a volume of 25 cm³. What is its density?
Answer: D = m/v = 50 g ÷ 25 cm³ = 2.0 g/cm³
Q3: Ice melting in a glass is best described as a: (A) chemical change (B) physical change (C) nuclear reaction (D) combustion reaction
Answer: Melting changes the state but not the chemical composition of water, no new substance forms. Answer: B