Mix it, then make it — answer key

1. EQ1 · Practice

Options as numbered on the pupil sheet: 1 — The stirred powders; 2 — Silver sulfide.

Model answer. 1, 4 and 6 describe the stirred powders; 2, 3 and 5 describe silver sulfide. In the stirred powders, silver and sulfur keep their own properties: shiny grey grains and yellow grains can be seen and picked apart, and the two powders can be in any proportions. Silver sulfide is a new substance made by a chemical reaction. It is black all through, every sample has the same make-up (87% silver by mass), and its silver atoms are joined to sulfur atoms, so only another chemical reaction can get the silver back.

A full answer includes

• I matched 1, 4 and 6 to the stirred powders.
• I matched 2, 3 and 5 to silver sulfide.
• I did not match 6 to silver sulfide: its silver atoms are joined to sulfur atoms, so it has no loose silver grains.
• I did not match 2 to the stirred powders: their make-up changes from one sample to another.

Also accept

• Stirred powders: 1, 4, 6. Silver sulfide: 2, 3, 5.

Watch for

• “Silver sulfide contains silver, so tweezers could pick silver grains out of it.” — Its silver atoms are joined to sulfur atoms all through the black solid. There are no loose silver grains to pick out: only a chemical reaction gets the silver back.
• “Statement 2 describes the stirred powders, because they contain silver too.” — Stirred powders can be in any proportions, so their make-up changes from sample to sample. Only a compound has the same make-up in every sample.
• “Statement 3 could describe the stirred powders if they are stirred very finely.” — However fine the grains, stirred silver and sulfur keep their own colours. Black all through is a new property: a new substance has formed.

2. EQ2 · Practice

Model answer. Row A. J is a compound: strong heating splits it into two new, simpler substances, a liquid and a gas, always in the same ratio by mass. K is an element: it can react with other substances to make compounds, but it can't be split into anything simpler. L is a mixture: water, a physical method, dissolves one kind of crystal and leaves the other, so its two substances are not joined to each other.

A full answer includes

• I chose row A.
• I deduced that J is a compound: strong heating splits it into two simpler substances.
• I deduced that K is an element: nothing simpler has ever been got from it.
• I deduced that L is a mixture: water, a physical method, separates its two kinds of crystal.
• I did not call K a compound just because it reacts with other substances.

Also accept

• Row A

Watch for

• “Row B.” — J's liquid and gas are new substances, made by strong heating and always in the same ratio by mass. That is a compound splitting up, not a mixture coming apart.
• “Row C.” — Reacting with other substances makes new compounds, but K itself has never been split into anything simpler. That makes K an element.
• “Row D.” — Water, a physical method, separates L's two kinds of crystal, so they are not joined to each other: L is a mixture.

3. EQ3 · Practice

Model answer. 5.0 g of sulfur combines with twice its mass of zinc: 2 × 5.0 g = 10 g. There is 13 g of zinc, more than 10 g, so all the sulfur reacts and some zinc is left over. (a) Mass of zinc sulfide = 10 g + 5.0 g = 15 g. (b) Mass of zinc left over = 13 g − 10 g = 3.0 g. The element left over is zinc: all 13 g of zinc would need 6.5 g of sulfur, but there is only 5.0 g, so the sulfur runs out first.

Checked automatically

• (a) Mass of zinc sulfide formed: 15 g (within ±0.1 g)
  • Common wrong answer 18 : 18 g is all the zinc and all the sulfur. 5.0 g of sulfur can't take all 13 g of zinc, so some zinc doesn't react.
  • Common wrong answer 19.5 : 19.5 g is 13 g of zinc with 6.5 g of sulfur. There is only 5.0 g of sulfur, so not all the zinc can react.
  • Common wrong answer 7.5 : 7.5 g uses the ratio the wrong way round. Each 1 g of sulfur takes 2 g of zinc, not 0.5 g.
• (b) Mass of the element left over: 3.0 g (within ±0.1 g)
  • Common wrong answer 8 : 8 g is 13 g − 5.0 g. Subtract the mass of zinc that reacted, not the mass of sulfur.
  • Common wrong answer 0 : Not all the zinc can react: 5.0 g of sulfur can't take all 13 g of it.
  • Common wrong answer 10.5 : 10.5 g uses the ratio the wrong way round. Each 1 g of sulfur takes 2 g of zinc, not 0.5 g.
  • Common wrong answer 1.5 : 1.5 g is how much more sulfur all the zinc would need. The question asks for the mass of zinc that has no sulfur to combine with.

A full answer includes

• I found that 5.0 g of sulfur combines with 2 × 5.0 g = 10 g of zinc.
• (a) I added 10 g + 5.0 g.
• (a) I wrote 15 g.
• (b) I subtracted 13 g − 10 g.
• (b) I wrote 3.0 g.
• I identified zinc as the element left over: 13 g is more than the 10 g of zinc that the sulfur can take.
• I did not add all 18 g: only 10 g of the zinc can combine.
• I did not use 6.5 g of sulfur: there is only 5.0 g.

Also accept

• (a) 15 g of zinc sulfide; (b) 3.0 g of zinc left over.

Watch for

• “All 18 g turns into zinc sulfide.” — A compound has a fixed ratio. 5.0 g of sulfur can combine with only 10 g of zinc; the rest of the zinc has no sulfur to combine with.
• “All 13 g of zinc reacts, so it takes 6.5 g of sulfur.” — There is only 5.0 g of sulfur, less than 6.5 g. The sulfur runs out first, so some zinc is left over.

4. EQ11 · Practice

Model answer. 4 g of copper combines with 1 g of oxygen to make 5 g of copper oxide, so copper is 4/5 of the mass of copper oxide. (a) Mass of copper = 12 g × 4/5 = 9.6 g. (b) 6.0 g of copper combines with 6.0 g ÷ 4 = 1.5 g of oxygen, so it makes 6.0 g + 1.5 g = 7.5 g of copper oxide. The pupil's solid is only 7.0 g, less than 7.5 g, so less oxygen combined: some copper did not react, and the solid is a mixture of copper oxide and leftover copper.

Checked automatically

• (a) Mass of copper needed: 9.6 g (2 significant figures)
  • Common wrong answer 48 : 48 g is 12 × 4. Copper is 4 g out of every 5 g of copper oxide, so it is less than 12 g.
  • Common wrong answer 3 : 3 g is 12 ÷ 4. The ratio 4 : 1 is copper to oxygen, so copper is 4/5 of the oxide, not 1/4.
  • Common wrong answer 2.4 : 2.4 g is the oxygen in 12 g of copper oxide. The copper is the rest of the mass.
• (b) Mass of copper oxide from 6.0 g of copper: 7.5 g (within ±0.05 g)
  • Common wrong answer 1.5 : 1.5 g is the oxygen that 6.0 g of copper takes. Copper oxide is the copper and the oxygen together.
  • Common wrong answer 24 : 24 g is 6.0 × 4. Copper takes only a quarter of its own mass in oxygen.
  • Common wrong answer 4.8 : 4.8 g is less than the 6.0 g of copper you started with. Adding oxygen makes the mass go up.
  • Common wrong answer 7 : 7.0 g is what the pupil measured, not what 6.0 g of copper makes if all of it reacts.

A full answer includes

• (a) I found that 4 g of copper makes 4 g + 1 g = 5 g of copper oxide.
• (a) I calculated 12 g × 4/5.
• (a) I wrote 9.6 g, to 2 significant figures.
• (b) I calculated 6.0 g + 1.5 g of oxygen (or 6.0 g × 5/4).
• (b) I wrote 7.5 g.
• I deduced that the 7.0 g solid still contains copper that did not react, because 7.0 g is less than 7.5 g.
• I did not give 2.4 g, which is the mass of oxygen in 12 g of copper oxide.

Also accept

• (a) 9.6 g; (b) 7.5 g. The 7.0 g solid still contains some copper that did not react.

Watch for

• “The copper in 12 g of copper oxide is 12 g × 4 = 48 g.” — The copper can't have more mass than the copper oxide it is part of. Of every 5 g of copper oxide, 4 g is copper.
• “The solid has less mass than 7.5 g, so some copper oxide must have escaped.” — Copper oxide is a solid and stays in the tube. A smaller mass means less oxygen combined: some copper did not react, so the solid is a mixture of copper oxide and copper.

5. EQ4 · Practice

Model answer. 2, 3 and 5. In zinc sulfide the zinc and sulfur atoms are chemically combined, so it has new properties (it is white all through, with no grey or yellow grains) and a fixed ratio: every sample is about two-thirds zinc by mass. Statement 1 is true of both: the powder and the compound contain the same atoms, and the difference is whether they are joined. Statements 4 and 6 are true only of the powder before heating: a physical method, such as dissolving the sulfur, can separate it, and it can be mixed in any amounts, such as half and half.

A full answer includes

• I chose 2, 3 and 5.
• I did not choose 4: dissolving is a physical method, and it separates only a mixture.
• I did not choose 6: zinc sulfide has a fixed ratio, so it can't be half sulfur.
• I explained that both contain zinc atoms and sulfur atoms, and only in zinc sulfide are they joined.

Also accept

• 2, 3, 5. Statement 1 is true of both; 4 and 6 are true only of the powder before heating.

Watch for

• “Only zinc sulfide contains zinc atoms and sulfur atoms.” — The powder before heating contains the same atoms. The reaction joins them to each other; it doesn't make new atoms.
• “A liquid that dissolves sulfur would wash the sulfur out of zinc sulfide too.” — Dissolving is a physical method: it separates substances that aren't joined. In zinc sulfide the sulfur atoms are joined to zinc atoms, so only another chemical reaction can get them out.
• “Zinc sulfide can be half sulfur if you heat the zinc with more sulfur.” — A compound has a fixed ratio. Extra sulfur doesn't go into the zinc sulfide; it is left over beside it.

6. EQ5 · Practice

Model answer. (a) The black substance is copper sulfide, a compound, and the pink-brown grains are copper that did not react. The copper grains are not joined to the copper sulfide, so the solid is a mixture of a compound and an element. (b) No yellow powder is left, so all the sulfur reacted. Copper grains are left, so some copper had no sulfur to combine with. Copper and sulfur combine in a fixed ratio, so the sulfur ran out before all the copper had reacted: Kofi used too little sulfur for his copper.

A full answer includes

• (a) I identified copper sulfide, the black substance.
• (a) I identified copper, the pink-brown grains.
• (a) I deduced that the solid is a mixture.
• (b) I used the evidence that no yellow powder is left: all the sulfur reacted.
• (b) I used the evidence that copper grains are left: some copper had no sulfur to combine with.
• (b) I linked this to the fixed ratio: the copper needed more sulfur than there was.
• I did not call the whole solid a compound: the copper grains are not joined to the copper sulfide.

Also accept

• (a) Copper sulfide and copper: a mixture. (b) No sulfur is left but some copper is, so the sulfur ran out first: the fixed ratio needed more sulfur.
• (b) All the sulfur was used up (no yellow left) while copper was left over, so there wasn't enough sulfur for all the copper.

Watch for

• “The pink-brown grains are copper sulfide, because copper sulfide contains copper.” — Copper sulfide is black: its copper atoms are joined to sulfur atoms, and it has new properties. Pink-brown grains have copper's own colour, so they are copper that did not react.
• “The solid is a compound, because copper sulfide formed.” — Copper sulfide is a compound, but the solid also holds grains of copper that are not joined to it. A compound with an element beside it is a mixture.
• “If some copper is left over, some sulfur must be left over too.” — Copper and sulfur combine in a fixed ratio. When the sulfur runs out, the extra copper stays; no yellow powder means that no sulfur was left.

7. ET1 · Apply

Model answer. (a) Any two of these. The spark started a reaction that gave out a lot of energy, in the explosion, so a chemical reaction made a new substance. Water has new properties: it is a liquid, not two colourless gases. And water always contains 1 g of hydrogen for every 8 g of oxygen, a fixed ratio, although the gases in the balloon could be in any proportions. So the hydrogen and oxygen in water are chemically combined: water is a compound. (b) The vapour condenses back into water; no hydrogen or oxygen comes off. Boiling is a physical method: it changes the state of water but doesn't split it, because its hydrogen and oxygen are chemically combined. Only another chemical reaction can get them back.

A full answer includes

• (a) I used at least two of the three pieces of evidence below.
• (a) Energy was given out in the explosion, so a chemical reaction happened.
• (a) Water has new properties: it is a liquid, not two gases.
• (a) Water always has 1 g of hydrogen to 8 g of oxygen, a fixed ratio, although the gases could be in any proportions.
• (b) I predicted that the pupil collects water again, not hydrogen or oxygen.
• (b) I explained that boiling is a physical method, which can't separate the elements in a compound.
• I did not say that water is a compound just because it contains two elements.

Also accept

• (a) Any two: energy given out in the explosion; new properties (a liquid, not two gases); a fixed ratio of 1 g : 8 g although the gases could be in any proportions. (b) Water again, not hydrogen and oxygen: boiling is a physical method and can't separate a compound.

Watch for

• “Water is a mixture because it contains hydrogen and oxygen.” — Containing two elements isn't enough to be a mixture. In water they're chemically combined in a fixed ratio, and water's properties are not those of either gas.
• “The gases in the balloon are a compound because they're in one container.” — Being in one container doesn't join them. Until the spark, each gas keeps its own properties, and they can be in any proportions: a mixture.
• “Boiling water gives off hydrogen and oxygen, because water is made of them.” — Boiling only turns liquid water into water vapour. The vapour is still water molecules, which condense back to water. Splitting water needs a chemical reaction.