Physical vs Chemical Changes
Distinguish between physical changes (reversible, no new substances formed) and chemical changes (new substances formed, often irreversible), using conservation of mass to understand both types
What a learner can do afterwards
- Classifies given changes as physical or chemical with justification
- Explains what conservation of mass means and why mass is conserved in chemical reactions
- Names observable signs that a chemical reaction has occurred (colour change, gas produced, temperature change, precipitate)
- Explains why dissolving is a physical change but burning is a chemical change
1 · Read
Some changes just change how something looks. Other changes turn a substance into something brand new. A physical change keeps the same substance. A chemical change makes a new substance. In a chemical change the atoms rearrange into new substances. No atoms are made or lost, so the total mass stays the same. If a gas drifts away, or oxygen from the air joins in, you have to count it too.
Dissolve sugar in your tea and it seems to disappear, but it's still sugar. Let the water evaporate and you'd get the sugar back. That's a physical change. Now burn toast in the toaster. The bread reacts with oxygen and turns into ash, smoke, and gases. You can never turn that ash back into bread. That's a chemical change.
Look for these signs that a chemical change might be happening: a colour change, gas bubbles, a sudden temperature change, or a new solid forming in a liquid (called a precipitate). One sign is a good clue, though scientists often check for more than one before they're sure.
Physical changes rearrange the same substance, but chemical changes create new substances, and mass is conserved either way.
Readings 'Two kinds of change' and 'Counting mass in a chemical change' are original to this dataset.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.