The idea
In a physical change, the substance is still the same substance afterwards. In a chemical change, new substances exist that did not before.
Evidence of a chemical change
No single sign is proof. Two or more together is convincing.
- A colour change that will not reverse
- A gas produced (bubbles in a solution that was not boiling)
- A precipitate — a solid appearing from two clear liquids
- Heat or light given off, or a noticeable chill
- A new odour
The classic trap
Boiling water bubbles vigorously and produces a gas. It is a physical change: those bubbles are water. Dissolving sugar looks dramatic and is also physical — evaporate the water and the sugar is still sugar.
The question is never “did something happen?” It is “is it still the same substance?”
Conservation of mass
Mass is conserved in any change, physical or chemical. Burn steel wool in a sealed container and the total mass is unchanged — burn it in the open and it appears to gain mass, because oxygen from the air joined it.
Test this yourself in Physical and Chemical Change Stations.
Curriculum connection
C2.1
investigate properties, changes, and interactions of matter that are important for the dynamic equilibrium of ecosystems and their sustainability
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C2.6
investigate and describe physical and chemical properties of elements and compounds, including those that make up common household products
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