Pupil sheet · Physical and chemical changes
Heat it, then test it — questions
Name ______________________ Date ____________
Try every question before you use a nudge. Write in full sentences where you are asked to explain.
1. identify
Solid coconut oil is white and soft. A spoonful is warmed gently in a dish until it melts to a clear liquid. Then the dish is left in a cool room, and the oil sets to a white solid again. A pupil tests a piece from the start and a piece from the end: both can be scratched with a fingernail, both float on water and neither dissolves in it. Which answer identifies the change and gives the right reason?
Circle one answer.
- Chemical: the liquid looked different from the solid
- Physical: the solid at the end has the same properties
- Physical: it can be changed back by cooling it
- Chemical: it changed state when it was warmed
Nudge (only if you are stuck): Is the solid at the end a different substance from the one at the start?
2. deduce
A grey zinc granule is dropped into dilute hydrochloric acid in a test tube. Match each observation to what it tells you.
Write the number of the matching option.
- Shows that a new substance was made
- A clue only
- 1. The mixture fizzes: ______
- 2. The test tube gets slightly warm: ______
- 3. The zinc granule slowly gets smaller: ______
- 4. A lighted splint at the mouth of the tube gives a squeaky pop, the test for hydrogen: ______
Nudge (only if you are stuck): Could each observation also happen in a physical change?
3. identify / explain
(a) Barium chloride solution and sodium sulfate solution are both colourless. A pupil mixes them in a beaker and notes three things: no bubbles form, the beaker stays at room temperature, and the mixture turns cloudy white at once as a solid appears. Identify the observation that is a clue that a chemical change may have happened. (b) A hot, strong solution of potassium nitrate is cooled. White crystals of potassium nitrate appear in it. Explain why this is a physical change. (c) Use (b) to explain why a solid appearing is only a clue, not proof, that a new substance was made.
(a) The clue
(b) Why the crystals are a physical change
(c) Clue, not proof
Nudge (only if you are stuck): For (b), where was the potassium nitrate before the crystals appeared?
4. identify
A pinch of black manganese(IV) oxide powder is added to some hydrogen peroxide solution in a test tube. The four observations below are made. A glowing splint does not relight in air. Which observation proves that a new substance was made?
Circle one answer.
- A glowing splint held in the tube relights
- The mixture fizzes as soon as the powder goes in
- The outside of the test tube becomes warm
- Black powder is still in the tube at the end
Nudge (only if you are stuck): For each observation, ask: could boiling or dissolving show it too?
5. identify
Each change below shows a clue: a gas, a colour change or a temperature change. In which of them is a new substance made? Select all that are correct.
Tick every correct statement.
- Red food colouring turns a glass of water pink
- A bottle of fizzy water is opened and bubbles rise
- A slice of apple turns brown in the air
- A wet T-shirt on a line feels cold as it dries
- Yeast gives off a gas that makes bread dough rise
- A sparkler burns and gets very hot
Nudge (only if you are stuck): For each one, ask what is there at the end that was not there at the start.
6. deduce
Three solids, A, B and C, are each heated, then left to cool. A test is then done on each cooled solid. The table shows the results. Which solids made a new substance?
| Solid | While heating | After cooling | Test on the cooled solid |
|---|---|---|---|
| A | a grey strip of tin melts to a shiny liquid | a grey lump | it can be hammered flat without breaking, as the strip could |
| B | pink-brown copper foil turns black on its surface | foil with a black coating | the coating dissolves in dilute acid, giving a blue solution; copper does not react with it |
| C | a red-brown powder turns almost black | red-brown powder again | it does not dissolve in water, as before |
Circle one answer.
- B only
- A and B
- A, B and C
- B and C
Nudge (only if you are stuck): In each row, does the test show the cooled solid behaving like the solid at the start?
7. identify / suggest / explain
A raw egg is put in a glass of vinegar. Bubbles of gas form on the shell at once. After a day the hard shell has gone, and the egg is held only by a soft, rubbery skin. The shell is mainly calcium carbonate. (a) Identify the observation that is a clue to a chemical change. (b) Suggest the name of the gas in the bubbles. (c) A pupil stirs Epsom salt crystals into a glass of water, and they also disappear from view. When the water is left to evaporate, the same crystals are left. Explain why the crystals disappearing is a physical change but the shell disappearing is a chemical change.
(a) The clue
(b) The gas
(c) Epsom salt and eggshell
Nudge (only if you are stuck): For (c), ask which change makes a substance that was not there at the start.