Answers · Elements, compounds and mixtures

What the particles show — answer key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. EQ6 · Practice

Options as numbered on the pupil sheet: 1 — An element; 2 — A compound; 3 — A mixture.

Model answer. A element, B compound, C mixture, D element. A has only one kind of atom, as single atoms. D also has only one kind of atom; its atoms are joined in pairs, but they're all the same kind, so it's still an element. In B every particle is the same molecule, made of two kinds of atom joined, so it's a compound. C has two kinds of particle, pairs of one kind of atom and single atoms of another, not joined to each other: a mixture of two elements. B and C both contain two kinds of atom, but in B the two kinds are joined in every particle, while in C each particle has only one kind of atom and the particles are not joined to each other.

A full answer includes

I matched A to an element: one kind of atom, as single atoms.
I matched D to an element: its atoms are joined in pairs, but all are one kind.
I matched B to a compound.
I matched C to a mixture.
I explained that in B the two kinds of atom are joined in every particle.
I explained that in C the two kinds of atom are in separate particles, not joined to each other.
I did not call C a compound just because it contains two kinds of atom.

Also accept

A element; B compound; C mixture; D element.

Watch for

“D is a compound, because its atoms are joined.” — Joined atoms make a compound only if they're different kinds. All the atoms in D are the same colour, so D is an element whose particles are molecules of two atoms.
“C is a compound, because it contains two kinds of atom.” — In C the two kinds of atom aren't joined to each other: there are two kinds of particle. That's a mixture.

2. EQ7 · Practice

Model answer. 1: 2, carbon and hydrogen. 2: compound: every particle is the same, with two kinds of atom joined. 3: 2, methane molecules and oxygen molecules. 4: mixture: two kinds of particle, not joined to each other, so the jar does not hold a pure substance.

A full answer includes

1: I wrote 2 (carbon and hydrogen).
2: I wrote compound.
3: I wrote 2 (methane molecules and oxygen molecules).
4: I wrote mixture.
I did not write 5 in gap 1: four hydrogen atoms are still only one kind of atom.

Also accept

1: 2 (two). 2: compound. 3: 2 (two). 4: mixture.

Watch for

“Methane has five kinds of atom, one for each atom in the molecule.” — Count kinds, not atoms. The four hydrogen atoms are all the same kind, so methane has two kinds of atom.
“The gas jar holds one kind of particle, because it is all gas.” — Look inside the jar: methane molecules and oxygen molecules are different particles. Two kinds of particle make a mixture.
“Methane is a mixture, because it contains carbon and hydrogen.” — In methane the carbon and hydrogen atoms are joined in every molecule, and all the molecules are the same: one substance, a compound.

3. EQ8 · Practice

Model answer. R. It contains two kinds of particle that aren't joined to each other, so it's a mixture. One kind is pairs of the same atom, an element; the other is molecules with two kinds of atom joined, a compound. P is also a mixture, but both its kinds of particle have only one kind of atom: a mixture of two elements. Q has one kind of particle with two kinds of atom, so it's a compound. S has two kinds of particle, but both contain two kinds of atom joined: a mixture of two compounds.

A full answer includes

I chose R.
I explained that R has two kinds of particle, not joined to each other, so it is a mixture.
I explained that one kind of particle in R has only one kind of atom (an element) and the other has two kinds joined (a compound).
I identified one other box with its reason: P a mixture of two elements, Q a compound, or S a mixture of two compounds.
I did not choose P or S just because it has two kinds of particle.

Also accept

R. P mixture of two elements, Q compound, S mixture of two compounds.

Watch for

“S is a mixture of an element and a compound, because it has two kinds of particle.” — S is a mixture, but look inside each particle. Both kinds contain two kinds of atom joined, so both are compounds.
“P is a mixture of an element and a compound, because its green atoms are joined in pairs.” — Joined atoms make a compound only if they are different kinds. The green pairs and the purple atoms each have only one kind of atom, so P is a mixture of two elements.
“Q is a mixture of an element and a compound, because each of its particles has a black atom and a red atom.” — Q has only one kind of particle: every molecule is the same, so Q is a pure compound, not a mixture.

4. EQ9 · Practice

Model answer. Each ammonia molecule has 1 + 3 = 4 atoms, so 48 atoms make 48 ÷ 4 = 12 molecules. Each molecule has 3 hydrogen atoms, so the diagram shows 12 × 3 = 36 hydrogen atoms (and 12 nitrogen atoms). In pure ammonia every particle is the same, so there are always three hydrogen atoms for every nitrogen atom. In the mixture, the nitrogen molecules and hydrogen molecules are not joined to each other and can be in any proportions, so 48 atoms could be mostly nitrogen or mostly hydrogen: the number of hydrogen atoms can't be calculated.

Checked automatically

Number of hydrogen atoms in the diagram: 36 (exact value)
- Common wrong answer 12 : 12 is the number of ammonia molecules, which is also the number of nitrogen atoms. Each molecule has three hydrogen atoms.
- Common wrong answer 16 : 16 is 48 ÷ 3. Each molecule has four atoms, not three: one nitrogen atom and three hydrogen atoms.
- Common wrong answer 144 : 144 is 48 × 3, more atoms than the diagram shows. Find the number of molecules first.
- Common wrong answer 24 : 24 is half the atoms. In ammonia, three of every four atoms are hydrogen.

A full answer includes

I found that each ammonia molecule has 1 + 3 = 4 atoms.
I calculated 48 ÷ 4 × 3.
I wrote 36.
I explained that the two gases in the mixture can be in any proportions.
I explained that only in a compound is the ratio of the atoms fixed, because every particle is the same.
I did not give 12: that is the number of molecules, and of nitrogen atoms.

Also accept

36 hydrogen atoms. The mixture's gases can be in any proportions, so it has no fixed ratio of hydrogen atoms to nitrogen atoms.

Watch for

“Ammonia is half nitrogen and half hydrogen, because it has two elements.” — Count atoms, not elements: each molecule has one nitrogen atom and three hydrogen atoms, so three of every four atoms are hydrogen.
“The mixture has the same ratio as ammonia, because it has the same two elements.” — In the mixture the nitrogen and hydrogen molecules are not joined, so any amount of each can be mixed. Only a compound fixes the ratio of its atoms.

5. EQ12 · Practice

Model answer. Row A. Chlorine contains only chlorine atoms, so it is an element, although its atoms are joined in pairs. In hydrogen chloride, hydrogen and chlorine atoms are joined in every molecule, so it is a compound. Hydrochloric acid is two substances, hydrogen chloride and water, together; it can be made more or less concentrated, so the proportions can change. That makes it a mixture.

A full answer includes

I chose row A.
I identified chlorine as an element: its atoms are joined, but all are chlorine atoms.
I identified hydrogen chloride as a compound: two elements joined in every molecule.
I identified hydrochloric acid as a mixture of two substances, hydrogen chloride and water.
I did not call hydrochloric acid a compound just because it has one name.

Also accept

Row A

Watch for

“Row B.” — Cl₂ has its atoms joined, but they are all chlorine atoms, so chlorine is an element. And the acid is two substances, hydrogen chloride and water.
“Row C.” — The acid has one name, but it is hydrogen chloride and water together, and it can be dilute or concentrated. A compound has a fixed ratio, so the acid is a mixture.
“Row D.” — In hydrogen chloride a hydrogen atom and a chlorine atom are joined in every molecule, and all the molecules are the same. That is a compound.

6. EQ10 · Practice

Model answer. (a) Pure ethanol is a compound: carbon, hydrogen and oxygen atoms chemically combined, in identical molecules. Ethanol mixed with water is a mixture of two compounds. (b) The pure ethanol diagram would show only one kind of particle: identical ethanol molecules, each with two carbon, six hydrogen and one oxygen atom joined. The other diagram would show ethanol molecules and water molecules spread among each other, not joined: two kinds of particle.

A full answer includes

(a) I stated that pure ethanol is a compound.
(a) I stated that ethanol mixed with water is a mixture.
(b) I described the pure ethanol diagram: only one kind of particle, identical ethanol molecules.
(b) I described the other diagram: ethanol molecules and water molecules, two kinds of particle.
(b) I said that the ethanol and water molecules are not joined to each other.
I did not call pure ethanol a mixture because it contains three kinds of atom.

Also accept

(a) Pure ethanol: compound; ethanol mixed with water: mixture. (b) Pure ethanol: identical ethanol molecules only; the mixture: ethanol molecules and water molecules, two kinds of particle not joined to each other.

Watch for

“Pure ethanol is a mixture, because it contains three elements.” — In pure ethanol the three kinds of atom are joined in every molecule, and all the molecules are the same: one substance, a compound.
“Ethanol and water form a compound, because they mix completely and look like one liquid.” — Looking like one liquid doesn't make a compound. The ethanol and water molecules aren't joined, and you can mix them in any proportions: a mixture.

7. ET2 · Apply

Model answer. (a) Brass is a mixture. The amount of copper can be anything from about 60% to 90%, and the properties change with it, but a compound always has its elements in a fixed ratio. (b) A diagram of brass with more copper would show more copper atoms for each zinc atom than a diagram of brass with less copper: the ratio of the two kinds of atom changes from one kind of brass to another. Two diagrams of iron sulfide would both show one iron atom for every sulfur atom, because iron sulfide is a compound and its ratio is fixed.

A full answer includes

(a) I stated that brass is a mixture.
(a) I gave the evidence that the amount of copper can vary, from 60% to 90%.
(a) I linked it to the rule that a compound always has a fixed ratio.
(b) I described that the two brass diagrams would show different ratios of copper atoms to zinc atoms.
(b) I explained that every iron sulfide diagram shows the same ratio, one iron atom for every sulfur atom.
(b) I explained that iron sulfide is a compound, so its ratio is fixed.
I did not say that brass is a compound because it is one shiny solid.

Also accept

(a) Mixture: the amount of copper varies (60–90%), and a compound has a fixed ratio. (b) Two kinds of brass show different ratios of copper atoms to zinc atoms; every iron sulfide diagram shows one iron atom for every sulfur atom, because a compound's ratio is fixed.

Watch for

“Brass is a compound, because it's one shiny solid made from two elements.” — Looking like one solid doesn't make it a compound. A compound has a fixed ratio, but brass can have different amounts of copper.
“Melting the metals together makes them react.” — Melting changes the state, not the substances. The copper and zinc mix as liquids and stay mixed when the brass sets.
“Two diagrams of brass would look the same, because both are brass.” — Different kinds of brass have different amounts of copper, so their diagrams show different numbers of copper atoms for each zinc atom.