Answers · Elements, compounds and mixtures

Elements, compounds and mixtures — final check key

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

1. EK1 · Final check

Model answer. (a) Either observation: the magnesium burns with a bright white light, or it keeps burning when it is taken out of the flame. Both show that the change itself gives out energy, not just the flame. (b) The white powder has new properties: it is not a shiny grey metal or a colourless gas. A mixture of magnesium and oxygen would keep the properties of both elements, so a new substance, a compound, has formed. (c) Sample 1: mass of oxygen = 5.0 g − 3.0 g = 2.0 g. Sample 2: mass of oxygen = 8.0 g − 4.8 g = 3.2 g. Magnesium ÷ oxygen = 3.0 ÷ 2.0 = 1.5 in sample 1, and 4.8 ÷ 3.2 = 1.5 in sample 2: 3 g of magnesium for every 2 g of oxygen in both, or 60% magnesium by mass. The two samples have the same ratio, as every sample of a compound must.

A full answer includes

(a) I predicted a bright white light (this line or the next one is enough).
(a) Or: I predicted that the strip keeps burning when it is taken out of the flame.
(a) I did not count 'it gets hot in the flame': the flame heats a mixture too.
(b) I explained that the white powder has new properties, unlike a shiny grey metal or a colourless gas.
(b) I explained that a mixture would keep the properties of its elements.
(c) I calculated the oxygen in sample 1 as 5.0 g − 3.0 g.
(c) I wrote 2.0 g for sample 1.
(c) I calculated the oxygen in sample 2 as 8.0 g − 4.8 g.
(c) I wrote 3.2 g for sample 2.
(c) I compared magnesium with oxygen in each sample: 3.0 ÷ 2.0 and 4.8 ÷ 3.2.
(c) I found 1.5 for both (3 : 2), or 60% magnesium in both.
(c) I deduced that the two samples have the same ratio, as every sample of a compound must.
(c) I did not compare only the masses of magnesium: 3.0 g and 4.8 g differ, but the ratio does not.

Also accept

(a) A bright white light, or it keeps burning out of the flame. (b) New properties: a white powder, not a shiny metal or a colourless gas; a mixture would keep the old properties. (c) 2.0 g and 3.2 g of oxygen; both ratios are 1.5 (3 : 2), so the same.

Watch for

“Getting hot in the flame shows that a reaction happened.” — The flame heats any solid. The evidence is energy given out by the change itself: a bright light, or burning that goes on after the strip leaves the flame.
“Magnesium oxide is a mixture because it is made from two elements.” — Being made from two elements doesn't make it a mixture. They're chemically combined in a fixed ratio, so it's a compound with new properties.
“Sample 2 has more magnesium, so its ratio is different.” — Sample 2 is a bigger sample, so it has more of both elements. Compare the magnesium with the oxygen inside each sample, not the two samples' magnesium.

2. EK2 · Final check

Model answer. (a) X is a mixture: it has two kinds of particle, molecules of one black atom joined to two red atoms and molecules of one white atom joined to one green atom, not joined to each other. Both kinds are compounds, so X is a mixture of two compounds. Y is a compound: every particle is the same molecule, a white, a black and a blue atom chemically combined. Z is an element: all its atoms are the same kind, as single atoms. (b) The number of kinds of atom doesn't decide it. In Y the three kinds of atom are joined in every particle, and all the particles are the same, so there is only one substance: a compound. A mixture needs two or more kinds of particle that are not joined to each other.

A full answer includes

(a) X is a mixture: it has two kinds of particle, not joined to each other.
(a) Y is a compound: every particle is the same, with different kinds of atom joined.
(a) Z is an element: it has only one kind of atom.
(b) I explained that in Y the three kinds of atom are joined in every particle.
(b) I explained that all the particles in Y are the same, so Y is one substance.
(b) I explained that a mixture needs more than one kind of particle, not more than one kind of atom.
I did not call X a compound because each of its particles has joined atoms.

Also accept

(a) X mixture (two kinds of particle, not joined); Y compound (identical molecules of three kinds of atom joined); Z element (one kind of atom). (b) Y's three kinds of atom are joined in every particle and all its particles are the same: one substance, a compound. A mixture needs more than one kind of particle.

Watch for

“X is a compound, because the atoms in each of its particles are joined.” — Look at the particles, not only inside them. X has two different kinds of particle, not joined to each other: two substances, so a mixture.
“Y is a mixture, because it contains three kinds of atom.” — A compound can have three or more kinds of atom. In Y they are joined in every particle, and every particle is the same, so Y is one substance, a compound.
“Z is not an element, because its atoms are not joined to each other.” — An element just needs one kind of atom. Its atoms can be single, as in Z, or joined to each other.