Heat it, then test it — answer key

1. PQ1 · Practice

Model answer. The oil melted and set again. The piece from the end is as soft as the piece from the start, it floats and it does not dissolve, just as before: the same properties, so it is still coconut oil. No new substance was made, so it is a physical change. "It can be changed back" gives the right kind of change for the wrong reason: some chemical changes can be turned back too.

A full answer includes

• I chose: physical, because the solid at the end has the same properties.
• I did not choose "it can be changed back by cooling it": undoing a change does not show which kind it is.

Also accept

• Physical: the solid at the end has the same properties

Watch for

• “It is physical because it can be changed back by cooling.” — It is physical, but not for that reason: some chemical changes can be turned back too. The reason is that the solid at the end has the same properties, so no new substance was made.
• “The clear liquid looks different, so it is a new substance.” — The clear liquid is the same oil in another state. When it sets, the tests show the same properties as before.
• “Anything that changes state when it is warmed is a chemical change.” — The oil changed state when it was warmed, but it is still coconut oil. Heating can cause a physical or a chemical change.

2. PQ2 · Practice

Options as numbered on the pupil sheet: 1 — Shows that a new substance was made; 2 — A clue only.

Model answer. Fizzing (1) shows only that a gas comes off: a boiling liquid bubbles too. A temperature change (2) is a clue: some substances make water warmer or colder just by dissolving in it. A granule getting smaller (3) is a clue: a dissolving solid gets smaller too. The squeaky pop (4) identifies hydrogen, a gas that was not in the tube at the start, so a new substance was made.

A full answer includes

• I matched 1 (the fizzing) to a clue only.
• I matched 2 (the tube gets warm) to a clue only.
• I matched 3 (the granule gets smaller) to a clue only.
• I matched 4 (the squeaky pop) to shows that a new substance was made.
• I did not match the shrinking granule to proof: a solid that dissolves gets smaller too.

Also accept

• 1, 2 and 3: a clue only; 4: shows that a new substance was made

Watch for

• “Bubbles show that a new substance was made.” — Fizzing shows that a gas comes off, not which gas. The squeaky-pop test identifies it.
• “A temperature change proves a chemical change.” — Some substances make water warmer or colder just by dissolving in it. A temperature change is a clue, not proof.
• “A solid that gets smaller has turned into a new substance.” — A solid that dissolves gets smaller too, and it is still the same substance. Getting smaller is a clue only.

3. PQ3 · Practice

Model answer. (a) A solid forming when two solutions are mixed is a clue to a chemical change. The other two are not clues: they only show that no gas and no temperature change were seen. (b) The potassium nitrate was there all along, dissolved in the water. As the solution cools, less of it can stay dissolved, so some comes out as crystals: the same substance, now as a solid. No new substance, so it is a physical change. (c) A solid appeared in (b) with no new substance made, so a solid appearing cannot be proof on its own. Proof needs a substance at the end that was not there at the start: a test that identifies it, or a property that the starting substances did not have.

A full answer includes

• (a) I identified the white solid that forms when the two solutions are mixed.
• (a) I did not choose the missing bubbles or the steady temperature: nothing happening is not a clue.
• (b) I wrote that the crystals are the same potassium nitrate that was dissolved.
• (b) I wrote that no new substance was made.
• (b) I did not write that the crystals are new because they are a solid.
• (c) I wrote that a solid can appear in a physical change too.
• (c) I wrote that proof needs a substance at the end that was not there at the start.

Also accept

• (a) A solid forms when the solutions are mixed. (b) The crystals are the same potassium nitrate, so no new substance. (c) A solid can appear in a physical change too.

Watch for

• “Any solid that appears in a liquid is a new substance.” — The potassium nitrate was dissolved in the water before the crystals appeared. A solid can come out of a solution with no new substance made.
• “A clue is the same as proof.” — A clue says that a chemical change may have happened. Proof shows a substance at the end that was not there at the start.

4. PQ4 · Practice

Model answer. The glowing splint relights: that is the test for oxygen. A glowing splint does not relight in air, so the gas is not just air. It is oxygen made from the hydrogen peroxide, a substance that was not there at the start. The fizzing and the warmth are clues only: a boiling liquid bubbles too, and some solids warm water just by dissolving. The black powder at the end is the manganese(IV) oxide that was added, so it shows nothing new.

A full answer includes

• I chose: a glowing splint held in the tube relights.
• I did not choose the fizzing: bubbles show that there is a gas, not which gas.
• I did not choose the black powder at the end: it is the powder that was added.

Also accept

• A glowing splint held in the tube relights

Watch for

• “Bubbles show that a new substance was made.” — Fizzing shows that a gas comes off, not which gas. The glowing-splint test identifies it as oxygen.
• “A temperature change proves a chemical change.” — Some substances warm water just by dissolving in it. Getting warm is a clue, not proof.
• “A solid left at the end is a new substance.” — This black powder is the manganese(IV) oxide that was put in. A solid at the end shows a new substance only if it has properties that nothing at the start had.

5. PQ5 · Practice

Model answer. The apple turning brown, the sparkler burning and the yeast giving off a gas all make new substances. The other three show a clue but make nothing new: the gas in fizzy water was dissolved in it and only comes out; the food colouring spreads out among the water particles and is still the same dye; the water in the T-shirt evaporates, which cools the cloth, and is still water, now as a vapour.

A full answer includes

• I chose the apple turning brown.
• I chose the sparkler burning.
• I chose the yeast making the dough rise.
• I did not choose the opened bottle of fizzy water: that gas was dissolved in it already.
• I did not choose the food colouring: the dye only spreads out.
• I did not choose the drying T-shirt: the water only evaporates.

Also accept

• The apple, the sparkler and the yeast

Watch for

• “Bubbles show that a new substance was made.” — The gas in fizzy water was dissolved in it before the bottle was opened. It only comes out of the water: no new substance.
• “A colour change proves a chemical change.” — The food colouring changes the colour of the water, but the dye particles only spread out. It is still the same dye.
• “A temperature change proves a chemical change.” — The T-shirt cools because the water takes energy from it as it evaporates. The water is still water, now as a vapour.

6. PQ6 · Practice

Model answer. B made a new substance: copper does not react with dilute acid, but the black coating dissolves in it and gives a blue solution, so the coating has a property that copper does not have. It is copper(II) oxide, made from the copper and oxygen in the air. A melted and set again as a lump, and it can still be hammered flat without breaking, as the strip could: it is still tin. Melting and setting are changes of state, and the lump is only a new shape. C changed colour while hot, but the colour came back when it cooled and it still does not dissolve in water: the same substance. So only B made a new substance.

A full answer includes

• I chose B only.
• I did not choose C: its colour came back when it cooled.
• I did not choose A: a new shape is not a new substance.

Also accept

• B only

Watch for

• “C made a new substance, because it changed colour.” — C went back to red-brown when it cooled, and it still does not dissolve in water. A colour that comes back on cooling is only a clue.
• “A made a new substance, because it has a new shape.” — A melted and set as a lump, but it can still be hammered flat, as the tin strip could. A new shape is a physical change.
• “Every solid that changes when it is heated makes a new substance.” — A melted and set again, and C changed back. Only B has a substance at the end with a property that the start did not have.

7. PT1 · Apply

Model answer. (a) Bubbles of gas form on the shell: a gas given off is a clue. (b) An acid on a carbonate gives off carbon dioxide, so the gas is carbon dioxide. (c) The Epsom salt dissolves: its particles spread out among the water particles and are still Epsom salt, which is why the same crystals are left when the water evaporates. No new substance, so it is physical. The shell reacts with the acid in the vinegar and makes carbon dioxide and other new substances, so it is a chemical change, even though both solids disappear from view.

A full answer includes

• (a) I identified the bubbles of gas forming on the shell.
• (b) I suggested carbon dioxide.
• (c) I wrote that the Epsom salt is still Epsom salt, spread out among the water particles.
• (c) I wrote that no new substance is made when it dissolves.
• (c) I wrote that the shell reacts with the vinegar and makes a new substance, such as the gas.
• (c) I did not write that the shell only dissolved, like the crystals.

Also accept

• (a) Bubbles of gas. (b) Carbon dioxide. (c) The Epsom salt is still Epsom salt, spread out; the shell reacts and makes new substances, such as carbon dioxide.

Watch for

• “A solid that disappears in a liquid has only dissolved.” — A solid disappears from view when it dissolves and also when it reacts. Ask whether a new substance, such as a gas, is made.
• “Something that disappears no longer exists.” — The Epsom salt is still there, spread out among the water particles: the crystals are left again when the water evaporates.