Physical and chemical changes · Grade 8
When materials change, some changes can be undone and some can’t. Melting, freezing and dissolving are physical changes: no new substance is made, and they are reversible. Burning, rusting and cooking are chemical changes: new substances are made, and they are usually irreversible.
Key facts
- Physical changes (reversible): melting, freezing, evaporating, condensing, dissolving and mixing. The substance stays the same, just in a different form.
- Chemical changes (usually irreversible): burning, rusting, cooking, milk going sour, and vinegar fizzing with bicarbonate of soda. New substances are made.
- Signs of a chemical change: bubbles of gas, a colour change, a temperature change, light or a flame, a new solid forming, or a new smell.
- Burning needs fuel, oxygen and heat: the fire triangle. Take one away and the fire goes out.
- Rusting: iron + oxygen + water → rust (hydrated iron oxide). Iron needs both air and water to rust.
- Mass is conserved in every change. If a gas escapes (or joins from the air), the mass measured in an open container changes.
Worked examples
Example 1
A beaker of vinegar and bicarbonate of soda has a mass of 152.0 g before they react and 149.8 g after the fizzing stops. Why is it lighter, and what mass of gas escaped?
Answer: carbon dioxide gas escaped into the air; 2.2 g