Glossary
chemical change
A chemical change makes a new substance — rusting, burning, cooking and reactions with an acid. The evidence is a change that is hard to reverse: a new color, a gas, a solid or heat.
A chemical change produces a new substance. Iron and oxygen make rust; wood and oxygen make ash and gases; vinegar and baking soda make carbon dioxide. The atoms are rearranged into different molecules, so the products have properties the starting materials never had, and the change is normally hard to undo — you cannot un-burn a match or un-rust a nail.
The signs worth looking for are a color change that is more than mixing, bubbles of gas given off, a solid that settles out of a liquid, and heat, light or sound released. Burning, rusting, tarnishing, fermenting, digesting, cooking an egg and running an engine all qualify.
The boundary this subtest actually tests is with a physical change, which alters a substance's form but not its identity: ice melting, sugar dissolving, paper tearing and glass shattering are physical, because the material is unchanged afterwards. Ask one question of any example — is there a new substance? If there is, the change is chemical; if the same material has only been rearranged, it is physical.
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Related terms
- physical change — A physical change alters a substance's size, shape or state — melting, freezing, dissolving, tearing — without making a new substance. The material is the same before and after.
- periodic table — The periodic table arranges the elements by atomic number in rows called periods and columns called groups; elements in the same group behave alike because they share outer electrons.
- pH scale — The pH scale runs from 0 to 14 and measures how acidic or basic a solution is: below 7 is acidic, exactly 7 is neutral, and above 7 is basic, or alkaline.
Source: Official ASVAB — subtest descriptions (what each subtest asks) and the CAT-ASVAB scored-question counts (accessed 2026-09)