Chemical change — Science definition

Defined in 2 courses:

Where it is taught

In use

When a substance genuinely reacts and becomes one or more new substances with new properties, that's a chemical change.

Physical vs. Chemical Properties

Questions using chemical change

Problem 1

Which statement correctly uses chemical change in Chemical Changes: Evidence & Conservation?
  1. One illustrative result establishes every real condition and needs no further evidence.
  2. Any bubbles or color change conclusively prove a chemical reaction.
  3. A mass change inside an open container means matter was destroyed.
  4. chemical change: A change that forms substances with different chemical identities.
Answer and explanation

chemical change: A change that forms substances with different chemical identities.

A change that forms substances with different chemical identities. Two A atoms and two B atoms are represented on each side. Their combinations change from A₂ and B₂ to two AB particles. The closed-system mass remains 60 g. This model directly states a rearrangement; a color change alone would not supply the same evidence of substance identity.

Problem 2

Which observation is the best evidence that a chemical change has happened?
  1. a change of shape
  2. a change in temperature only
  3. the formation of a new substance with new properties
  4. a change of state
Answer and explanation

the formation of a new substance with new properties

The clearest sign of a chemical change is a genuinely new substance forming, with properties different from what you started with.

Problem 3

Explain why dissolving salt in water is a physical change, not a chemical change.
Answer and explanation

The salt doesn't become a new substance — it's still salt, just spread out among the water molecules. If you evaporated the water, you'd get the same salt back.

Problem 4

Which observation is the strongest evidence that a chemical change occurred?
  1. The sample got smaller
  2. A new substance with different properties formed
  3. The sample changed shape
  4. The sample was warmed
Answer and explanation

A new substance with different properties formed

A new substance is the defining outcome; size, shape and warming can all accompany physical changes.

Taught alongside

Elsewhere

Browse all courses