Year 7 Science — Physical and chemical changes
Decide whether a change has made a new substance or merely rearranged an old one, say what combustion needs, and name the slow changes that turn rock into soil.
Name: ________________________
Before you start
Look closely at a burning candle. Two different kinds of change are going on at once.
- 1. The flame
- 2. The wick
- 3. The pool of melted wax
- 4. The solid wax of the candle
The wax melting into a pool, being drawn up the wick and turning to vapour are physical changes — it is still wax. Inside the flame, that vapour burns and becomes something else. That part is a chemical change.
1.Blowing exhaled air through lime water turns it milky. What does that milkiness tell you was in the exhaled air?
- a) Water vapour
- b) Nitrogen
- c) Carbon dioxide
- d) Oxygen
2.Sort each change by whether a new substance was formed.
Groups: Physical change · Chemical change
- Adding baking soda to vinegar
- Melting an ice cube
- Cooking food
- Burning a magnesium ribbon
- Rusting of an iron nail
- Crushing chalk into powder
- Folding paper into a shape
3.A different question about the same changes: sort each one by whether you can get back what you started with.
Groups: Can be reversed · Cannot be reversed
- Making popcorn from corn
- Chopping vegetables
- Melting an ice cube
- Boiling water
4.Water is boiled in an open pan and the steam is caught and cooled back into water. Which pair of words fits this change?
- a) Physical, and it can be reversed
- b) Chemical, and it can be reversed
- c) Physical, and it cannot be reversed
- d) Chemical, and it cannot be reversed
5.A black basalt rock exposed to damp air for a long time grows a red layer on its surface. What kind of change is that?
- a) A physical change — the surface has been worn smooth by rain
- b) Neither — the rock was red underneath a black coating from the start
- c) A physical change — the red is dust that has settled and stuck
- d) A chemical change — iron in the rock has turned into iron oxide
6.Match each process to what it does.
- Combustion
- Weathering
- Erosion
- carries broken rock away by wind or flowing water
- a substance reacts with oxygen, giving out heat or light
- breaks rock down where it lies, and eventually makes soil
7.Follow the wax of a burning candle, starting from the step where the flame's heat melts the wax at the top of the candle.
- The flame's heat melts the wax at the top of the candle
- The liquid wax is drawn up the wick
- The heat turns that wax into vapour
- The wax vapour burns, and the flame is what you see
8.Which statement describes a physical change?
- a) The change is impossible to reverse
- b) A chemical reaction takes place and gives out heat
- c) The state of a substance may change, and no new substance is formed
- d) A substance with different properties is formed
Answer key — Year 7 Science — Physical and chemical changes
- 1. c) Carbon dioxide
- 2. Melting an ice cube → Physical change; Crushing chalk into powder → Physical change; Folding paper into a shape → Physical change; Rusting of an iron nail → Chemical change; Burning a magnesium ribbon → Chemical change; Adding baking soda to vinegar → Chemical change; Cooking food → Chemical change
- 3. Melting an ice cube → Can be reversed; Boiling water → Can be reversed; Chopping vegetables → Cannot be reversed; Making popcorn from corn → Cannot be reversed
- 4. a) Physical, and it can be reversed
- 5. d) A chemical change — iron in the rock has turned into iron oxide
- 6. Combustion → a substance reacts with oxygen, giving out heat or light; Weathering → breaks rock down where it lies, and eventually makes soil; Erosion → carries broken rock away by wind or flowing water
- 7. 1. The flame's heat melts the wax at the top of the candle 2. The liquid wax is drawn up the wick 3. The heat turns that wax into vapour 4. The wax vapour burns, and the flame is what you see
- 8. c) The state of a substance may change, and no new substance is formed