Elements, Compounds and Mixtures Worked ExamplesIGCSE Chemistry 0620

An element contains only one type of atom (single atoms or molecules). A compound has two or more different atoms chemically joined, and every particle is identical. A mixture has two or more different substances not chemically joined to each other.

(a) A (single atoms of X) and B (molecules of $\text{Y}_2$) — each contains only one type of atom.

(b) C — every particle is the same XY molecule, with X and Y chemically joined.

(c) D — molecules of $\text{X}_2$ and molecules of $\text{Y}_2$ mixed together. (d) F — two different compounds, $\text{XY}_2$ and XY. (E is a mixture of an element, $\text{X}_2$, and a compound, XY.)

(a)(i) An element is a substance made of only one type of atom (all atoms have the same proton number). It cannot be broken down into anything simpler by chemical reactions.

(ii) A compound is a substance made of two or more different elements chemically combined (bonded) in a fixed ratio.

(iii) A mixture contains two or more substances that are not chemically combined, in any proportions.

(b) Any two: a mixture can have any proportions, a compound has a fixed ratio; a mixture can be separated by physical methods (e.g. a magnet, filtration, distillation), a compound can only be split up by a chemical reaction; the substances in a mixture keep their own properties, while a compound has different properties from its elements; no energy change is needed to make a mixture, while forming a compound is a chemical reaction.

Check each row one entry at a time. A: air is a mixture (mainly nitrogen and oxygen), not a compound — wrong.

B: graphite is carbon, an element; carbon dioxide, $\text{CO}_2$, is a compound; brass is an alloy — a mixture of copper and zinc. All correct. Answer: B.

C: stainless steel is an alloy (a mixture of iron with other elements), not an element; ethanol, $\text{C}_2\text{H}_5\text{OH}$, is a compound, not a mixture — wrong. D: sea water is a mixture and glucose is a compound — both are in the wrong columns.

In a compound the elements are chemically combined (bonded) in a fixed ratio — in carbon dioxide, $\text{CO}_2$, always one carbon atom to two oxygen atoms. Answer: B.

A is a true statement, but a mixture of carbon and oxygen also contains two types of atom, so it does not explain the difference. C is wrong: filtration is a physical method that cannot separate a compound. D is wrong: a compound has different properties from its elements — carbon dioxide is a colourless gas that does not burn, unlike carbon or oxygen.

(a) Hydrogen, $\text{H}_2$ — an element (one type of atom) and a gas.

(b) Carbon dioxide, $\text{CO}_2$ — carbon and oxygen chemically combined; a gas.

(c) Sodium chloride, NaCl — sodium (a metal) combined with chlorine (a non-metal).

(d) Silicon(IV) oxide, $\text{SiO}_2$ — a solid compound of two non-metals. Graphite is a solid non-metal but it is an element, not a compound.

(e) Brass — an alloy, which is a mixture of copper and zinc. (Stainless steel is a mixture of iron with elements such as chromium, nickel and carbon.)

(a)(i) Box 1 is a mixture — it contains two different substances, $\text{H}_2$ and $\text{Cl}_2$ molecules, that are not chemically joined to each other. (ii) Box 2 is a compound — every molecule is the same, with hydrogen and chlorine atoms chemically bonded together.

(b) Box 1: the chlorine in the mixture keeps its own properties, so the damp blue litmus turns red and then is bleached (turns white). Box 2: hydrogen chloride dissolves to give an acidic solution, so the damp blue litmus turns red (it is not bleached).

(c) In a mixture the substances are not bonded, so any amount of each can be added. In the compound each hydrogen atom is chemically bonded to one chlorine atom, so the ratio is fixed at 1 : 1.

(d) Hydrogen and chlorine are both diatomic. Check: 2 H and 2 Cl on each side.

(a) The other four points lie on a straight line through the origin with mass of copper $= 0.80 \times$ mass of copper(II) oxide. For $4.00$ g the expected mass is $3.20$ g, but the result is $3.35$ g — too high. Reason: the oxide was not heated for long enough, so some copper(II) oxide was not reduced and the solid still contained oxygen.

(b) Mass of oxygen = mass of oxide − mass of copper.

Check with relative atomic masses: in CuO the ratio is $A_r(\text{Cu}) : A_r(\text{O}) = 64 : 16 = 4 : 1$. ✔

(c) Every (correct) result gives the same ratio of copper to oxygen, $4 : 1$ — the points lie on one straight line through the origin. A compound has a fixed composition; a mixture could have any proportions. The oxygen can only be removed by a chemical reaction, not a physical method.

(d) Hydrogen removes the oxygen as water (steam at this temperature). Check: Cu 1, O 1, H 2 on each side.

(a) Use a magnet to remove the iron filings (iron is magnetic; salt and sand are not).

Add water to the salt and sand and stir: the salt dissolves, the sand does not. Filter: the sand stays on the filter paper; wash it with a little water and dry it.

Heat the salt solution to evaporate some of the water until crystals start to form, then leave it to cool and crystallise; filter off the crystals and dry them (e.g. between filter papers).

(b) P is the residue (sand). Q is the filtrate (salt solution) — the liquid that has passed through the filter.

(c) In the mixture the three substances are not chemically combined, so they keep their own properties (magnetism, solubility) and can be separated by physical methods. In sodium chloride, sodium and chlorine are chemically combined in a compound, so a chemical reaction (for example electrolysis of the molten salt) is needed to separate them.

(a) About 78% nitrogen, $\text{N}_2$, and 21% oxygen, $\text{O}_2$. The remainder is a mixture of noble gases (mainly argon) and carbon dioxide, $\text{CO}_2$.

(b) Any two: its composition varies (e.g. the amount of water vapour or carbon dioxide can change) — a compound has a fixed composition; it has no chemical formula; the gases in it keep their own properties (e.g. oxygen in air still relights a glowing splint, though less brightly than pure oxygen); it can be separated by a physical method (fractional distillation of liquid air).

(c) The substances have different boiling points, so use fractional distillation. As the liquid air warms up, the substance with the lowest boiling point boils first.

Order the boiling points: $-196 < -186 < -183$. Nitrogen ($-196^{\circ}\text{C}$) boils off first, then argon, then oxygen.

(a)(i) K — an alloy is a mixture of a metal with other elements, so it shows mostly one kind of metal atom with some atoms of a different size scattered among them.

(ii) J — all the atoms are identical, so it is a single element (a pure metal). (iii) L — a regular lattice of alternating positive and negative ions.

(b) A compound always has a fixed ratio of its elements (a fixed formula). Brass can be made with different proportions of copper and zinc, and the copper and zinc atoms are not chemically combined — so brass is a mixture.

(a) $\text{O}_3$ and $\text{S}_8$ are elements: each contains only one type of atom (oxygen only; sulfur only).

(b) For a bracket, multiply everything inside by the number outside. $(\text{NH}_4)_2\text{SO}_4$: elements N, H, S, O — 4 elements; atoms $2 \times (1 + 4) + 1 + 4$.

$\text{Al}_2(\text{SO}_4)_3$: elements Al, S, O — 3 elements.

$\text{C}_6\text{H}_{12}\text{O}_6$: C, H, O — 3 elements. NaCl: Na, Cl — 2 elements.

(c) Whether a substance is an element or a compound depends on the number of types of atom, not the number of atoms. All eight atoms in $\text{S}_8$ are sulfur atoms, so it is an element. A compound must contain atoms of two or more different elements chemically combined.

(a) Tube A is over the negative electrode (cathode), where hydrogen forms. Tube B is over the positive electrode (anode), where oxygen forms.

The volumes agree: tube A has twice the volume of tube B ($24 : 12 = 2 : 1$), matching two hydrogen atoms for every oxygen atom in water.

(b) Hydrogen: a lighted splint gives a (squeaky) pop. Oxygen: a glowing splint relights.

(c) Check: 4 H and 2 O on each side.

(d) Boiling is a physical change: the water molecules are unchanged and simply move apart, so steam is still water. If water were a mixture, heating would release hydrogen and oxygen gas, and the proportions could vary.

Only a chemical change (electrolysis) splits water into hydrogen and oxygen, and it always gives them in the fixed 2 : 1 ratio. The products have very different properties from water. So water is a compound: hydrogen and oxygen chemically combined in a fixed ratio.