Pupil sheet · Physical and chemical changes
Physical and chemical changes — final check
Name ______________________ Date ____________
Work alone, with the lesson closed. Answer every question; there are no hints.
1. deduce / explain
Calcium carbonate is a white solid. A lump of it is heated very strongly for ten minutes. The gas given off turns limewater milky. When the lump is cold, it still looks white. (a) Identify the gas, and explain why the limewater result proves that a new substance was made. (b) A few drops of water are added to the cold lump. It hisses, gives off steam and crumbles into a powder. Drops of water on a lump of calcium carbonate do nothing. Explain what this shows about the solid left after heating.
(a) The gas and why it is proof
(b) What the water shows
2. identify
Which row identifies the change correctly and gives the evidence that decides it?
| Row | Change | Kind | Evidence |
|---|---|---|---|
| A | Chocolate melts in a warm room and sets again with a dull white surface | chemical | it looks different at the end |
| B | A silver bracelet slowly turns black; the black layer rubs off as a dull powder | chemical | it has a property silver does not have |
| C | Runny honey left in a cold cupboard turns cloudy and grainy | chemical | a solid appears in it |
| D | Gold is melted and poured into a ring-shaped mould | physical | it could be melted down again |
Circle one answer.
- A
- B
- C
- D
3. identify / explain / describe / suggest
A strip of shiny magnesium ribbon conducts electricity. (a) The strip is cut into short pieces. Each piece is still shiny and still conducts. Identify the kind of change, and explain your answer. (b) One piece is held in a flame. It burns with a bright white light and leaves a white powder. The powder does not conduct electricity. Identify the kind of change, and give the evidence that proves it. (c) Describe what happens to the magnesium atoms when the ribbon burns. (d) Suggest one other test, not conducting electricity, that would show that the white powder is not magnesium. Give the result you would expect for each.
(a) Cutting
(b) Burning
(c) The atoms
(d) Another test
4. determine / explain
Propane is the gas in many camping stoves. It burns in oxygen: C3H8 + 5O2 → 3CO2 + 4H2O. Determine the number of carbon, hydrogen and oxygen atoms on each side of the equation. Give each answer as a whole number. Then explain, in terms of particles, why burning propane is a chemical change, and whether any atoms are made or destroyed.
Carbon atoms on the left: ______________ unit ________
Hydrogen atoms on the left: ______________ unit ________
Oxygen atoms on the left: ______________ unit ________
Carbon atoms on the right: ______________ unit ________
Hydrogen atoms on the right: ______________ unit ________
Oxygen atoms on the right: ______________ unit ________
Why burning propane is a chemical change, and whether any atoms are made or destroyed.