Chapter 3 · Matter and Energy
3.6Changes in Matter: Physical and Chemical Changes
10 min · two checks
Predict
When water boils, has a chemical change occurred?
The idea
- Classify changes as physical or chemical.
- Name evidence that often signals a chemical change, and a trap in that evidence.
In a physical change the substances stay themselves. Cutting, dissolving, melting, boiling, and bending rearrange or separate particles. Salt dissolved in water is still sodium chloride and water; evaporate the water and the salt remains. In a chemical change, atoms are reorganized into different substances. Burning wood produces carbon dioxide, water, and ash that will not turn back into wood by cooling.
Clues of a chemical change include a new color that is not just dilution, a gas produced from two liquids, a solid forming when two solutions meet, and a lasting temperature change from the process itself. None of these is proof by itself. Boiling produces a gas and is physical. Dissolving a colored crystal spreads color and is often physical. The decisive question is whether you have new substances.
Keep these
- Physical change: same substances, new form or mixture.
- Chemical change: new substances.
- Bubbles, color, or heat are clues, not automatic proof.