Same particles, or new ones? — answer key

1. PQ7 · Practice

Model answer. Liquid nitrogen and nitrogen gas are both made of N₂ molecules. When the liquid boils, the molecules move far apart and move faster, but each one is still an N₂ molecule. No new substance: a physical change.

A full answer includes

• I chose: they stay N₂ molecules but move far apart.
• I did not choose "they get bigger": the molecules stay the same size; only the spaces between them grow.

Also accept

• They stay N₂ molecules but move far apart

Watch for

• “The molecules break up into nitrogen atoms.” — Boiling only separates the molecules from each other. Each N₂ molecule stays whole.
• “The nitrogen joins with oxygen to make a new gas.” — Nothing reacts: the gas is nitrogen, the same N₂ molecules as the liquid.
• “The molecules get bigger when they turn into a gas.” — The molecules stay the same size. The spaces between them get much bigger.

2. PQ8 · Practice

Options as numbered on the pupil sheet: 1 — Chemical change; 2 — Physical change.

Model answer. The puddle (1) evaporates, the coffee (3) dissolves and the paper (5) only changes shape: no new substance, so they are physical. The bleach (2) reacts with the coloured substance in the stain and makes new, colourless substances; the copper roof (4) reacts with the air to make a green compound; the fish going off (6) makes new, bad-smelling substances: these are chemical.

A full answer includes

• I matched 1 (the puddle) to physical change.
• I matched 2 (the bleach) to chemical change.
• I matched 3 (the coffee) to physical change.
• I matched 4 (the copper roof) to chemical change.
• I matched 5 (the paper) to physical change.
• I matched 6 (the fish) to chemical change.
• I did not match the drying puddle to chemical change: the water is still water, as a vapour.

Also accept

• 1, 3 and 5 physical; 2, 4 and 6 chemical

Watch for

• “The puddle has gone, so the water has become something new.” — The water evaporates: it is still water, now as a vapour in the air.
• “The green on the roof is only dirt on the copper.” — The copper reacts with gases in the air and makes a new green compound on its surface.
• “The bleach only washes the stain out of the cloth.” — Water alone does not shift the stain. The bleach reacts with the coloured substance and makes new, colourless substances.

3. PQ9 · Practice

Model answer. In Q, hydrogen molecules and oxygen molecules become water molecules: a new kind of particle, so a new substance. In P the same water molecules spread out (boiling), and in R the same nitrogen and oxygen molecules mix: no new particles, so both are physical changes.

A full answer includes

• I chose Q.
• I did not choose P or R: spreading out and mixing keep the same particles.

Also accept

• Q

Watch for

• “P is a chemical change, because the molecules have spread out.” — In P the same water molecules are only farther apart, as in boiling. There is no new kind of particle.
• “R is a chemical change, because the two gases have mixed.” — In R the same N₂ and O₂ molecules are only mixed together. There is no new kind of particle.
• “P and R are both chemical changes, because the particles moved.” — Moving, spreading out and mixing change the arrangement, not the particles. Look for a box with a new kind of particle after the change.

4. PQ10 · Practice

Model answer. (a) R starts with nitrogen molecules and oxygen molecules and ends with the same molecules, mixed: no new kind of particle, so a physical change. (b) Water, H₂O. (c) Before: four H₂ (8 hydrogen atoms) and two O₂ (4 oxygen atoms). After: four H₂O, also 8 hydrogen and 4 oxygen atoms. No atom is made or destroyed; they are joined in new ways.

A full answer includes

• (a) I wrote that the same nitrogen and oxygen molecules are there before and after.
• (a) I wrote that they are only mixed, so no new substance is made.
• (b) I named water.
• (c) I found 8 hydrogen atoms before and 8 after.
• (c) I found 4 oxygen atoms before and 4 after.
• (c) I wrote that the atoms are the same, only joined in new ways.
• (c) I did not count 2 oxygen atoms before: each O₂ molecule has two atoms.

Also accept

• (a) The same molecules, only mixed. (b) Water. (c) 8 H and 4 O before and after: the same atoms, joined in new ways.

Watch for

• “In a chemical change some atoms are destroyed.” — Atoms are never made or destroyed. They join up in new ways, so new substances form.
• “Mixed particles are a new substance.” — In R each N₂ and O₂ molecule is still the same molecule. Mixing changes only where they are.

5. PQ11 · Practice

Model answer. The crystal dissolves: its particles spread out among the water particles, too small to see, and colour the water purple. They are still potassium manganate(VII): evaporate the water and purple crystals are left. No new substance, so it is a physical change.

A full answer includes

• I chose: physical, because the same particles spread out in the water.
• I did not choose "the purple colour spreads": a colour that spreads is not a new substance.

Also accept

• Physical: the same particles spread out in the water

Watch for

• “The colour spreading through the water proves a chemical change.” — The colour spreads because the purple particles spread out among the water particles. They are still the same substance.
• “The crystal has stopped existing.” — Its particles are still there, spread out among the water particles: that is why the water is purple.
• “The particles split into smaller atoms as they dissolve.” — Dissolving does not split the particles. The same particles are only spread out.

6. PQ12 · Practice

Model answer. If the ammonium chloride only turned into a gas and back, the gas would have the same properties as the solid. But the gas turns damp red litmus blue, which ammonium chloride does not, so heating makes a new substance: a chemical change. (The heat breaks the solid up into two gases, ammonia and hydrogen chloride.) At the cool top the two gases join up again into ammonium chloride, so a new substance is made that way too. The change goes back and is still chemical: whether a change can be turned back does not decide its kind; whether a new substance is made does.

A full answer includes

• I wrote that the litmus shows a gas with a property that ammonium chloride does not have.
• I wrote that a new substance is made when the solid is heated.
• I wrote that heating the ammonium chloride is a chemical change.
• I wrote that a change can go back and still be chemical.
• I did not write that the solid only turned into a gas and back, like dry ice: that gas would not turn the litmus blue.

Also accept

• The gas turns red litmus blue, so a new substance is made: a chemical change; a change can go back and still be chemical.

Watch for

• “If you can reverse it, it is a physical change.” — Some chemical changes go back too, and some physical changes are hard to undo. The test is whether a new substance was made.
• “The ammonium chloride only turns into a gas and back, like dry ice.” — Then the gas would have the same properties as the solid. This gas turns damp red litmus blue, which ammonium chloride does not do, so it is a new substance.

7. PQ13 · Practice

Model answer. Toasting makes a brown crust that is crisp and tastes burnt, unlike the soft, sweet marshmallow: new properties, so new substances, a chemical change. The jelly turns from a liquid into a soft solid as it cools, but it is still jelly: a physical change. Snapped chalk is still chalk, however many pieces there are. Fog is tiny drops of water condensing from the air: the same water, a change of state.

A full answer includes

• I chose the toasted marshmallow: its crust is crisp and tastes burnt.
• I did not choose the jelly setting: it is still jelly, only firmer.

Also accept

• Toasting a marshmallow: its crust is crisp and tastes burnt

Watch for

• “A change from a liquid to a solid makes a new substance.” — Setting is like freezing: the jelly turns from a liquid into a soft solid as it cools, but it is still jelly.
• “More pieces means a new substance.” — Each piece of snapped chalk is still chalk, with the same properties. Breaking changes only the shape.
• “A colour change proves a chemical change.” — Fog looks white because tiny drops of water scatter light. The water vapour has only condensed: it is still water.

8. PT2 · Apply

Model answer. (a) The water in the damp charcoal turns to steam: the same H₂O molecules, now as a gas, a change of state. (b) The glowing is the carbon burning: it reacts with oxygen from the air to make carbon dioxide, a new substance. (c) Each carbon atom joins with two oxygen atoms from the air into a CO₂ molecule. Carbon dioxide is a gas, so it spreads into the air and the charcoal gets smaller. The carbon atoms are not destroyed; they leave as part of a new substance. The ash is the small part of the charcoal that does not burn.

A full answer includes

• (a) I identified water in the damp charcoal turning to steam: evaporating or boiling.
• (a) I did not write that the steam is a new substance: it is the same water, as a gas.
• (b) I identified burning: carbon reacting with oxygen from the air.
• (b) I suggested carbon dioxide.
• (c) I wrote that carbon atoms join with oxygen atoms from the air.
• (c) I wrote that they make carbon dioxide molecules, a gas that goes into the air.
• (c) I did not write that the carbon atoms are destroyed.

Also accept

• (a) Water evaporating. (b) Burning; carbon dioxide. (c) The carbon atoms join with oxygen atoms into carbon dioxide, a gas that leaves.

Watch for

• “The steam is a new substance made by the burning.” — The steam comes off at the start, before the charcoal glows: it is the water that was in the damp charcoal, turned to a gas.
• “Burning is a change of state, like melting.” — Burning is a reaction with oxygen that makes new substances, such as carbon dioxide and water. Melting makes no new substance.
• “In a chemical change some atoms are destroyed.” — Atoms are never made or destroyed. They join up in new ways, so new substances form.