Class 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.Sand that a river carried settles on the riverbed, and over thousands of years it becomes rock. What does the chapter say about undoing changes like that?
- a) They can be reversed by grinding the rock back to sand
- b) They cannot be reversed
- c) They can be reversed, but just in a laboratory
- d) They reverse themselves each rainy season
2.Blowing exhaled air through lime water turns it milky. What does that milkiness tell you was in the exhaled air?
- a) Carbon dioxide
- b) Oxygen
- c) Water vapour
- d) Nitrogen
3.Match each process to what it does.
- Combustion
- Weathering
- Erosion
- breaks rock down where it lies, and eventually makes soil
- a substance reacts with oxygen, giving out heat or light
- carries broken rock away by wind or flowing water
4.Which statement describes a physical change?
- a) The change is impossible to reverse
- b) A substance with different properties is formed
- c) The state of a substance may change, and no new substance is formed
- d) A chemical reaction takes place and gives out heat
5.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
- Chopping vegetables
- Boiling water
- Making popcorn from corn
- Melting an ice cube
6.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) Chemical, and it can be reversed
- b) Physical, and it can be reversed
- c) Chemical, and it cannot be reversed
- d) Physical, and it cannot be reversed
7.Sort each change by whether a new substance was formed.
Groups: Physical change · Chemical change
- Adding baking soda to vinegar
- Cooking food
- Burning a magnesium ribbon
- Folding paper into a shape
- Crushing chalk into powder
- Melting an ice cube
- Rusting of an iron nail
8.Someone's clothes catch fire and a woollen blanket is wrapped around them. Why does that put the fire out?
- a) It soaks up the fuel that is feeding the flames
- b) It cuts off the supply of air, so the burning has no oxygen
- c) It reflects the heat back onto the fire and smothers it with light
- d) It cools the clothes below their ignition temperature at once
Answer key — Class 7 Science — Physical and chemical changes
- 1. b) They cannot be reversed
- 2. a) Carbon dioxide
- 3. 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
- 4. c) The state of a substance may change, and no new substance is formed
- 5. Melting an ice cube → Can be reversed; Boiling water → Can be reversed; Chopping vegetables → Cannot be reversed; Making popcorn from corn → Cannot be reversed
- 6. b) Physical, and it can be reversed
- 7. 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
- 8. b) It cuts off the supply of air, so the burning has no oxygen