IGCSE Chemistry Practice Test 20

Correct: (C)

Sublimation is when a solid changes directly into a gas without passing through the liquid state.

Correct: (A)

Gas particles have lots of energy and move quickly and randomly, so they spread out and mix rapidly.

Correct: (D)

Pure water boils at 100°C at atmospheric pressure.

Correct: (B)

Neutrons = mass number – proton number = 35 – 17 = 18.

Correct: (D)

Chlorine has 17 electrons: 2 in the first shell, 8 in the second and 7 in the third shell.

Correct: (C)

Carbon dioxide consists of small molecules with weak intermolecular forces, so little energy is needed to separate them.

Correct: (A)

In a metal, the outer electrons are delocalised and hold the positive ions together in a lattice; this also explains electrical conductivity.

Correct: (B)

Calcium loses its two outer electrons to gain a full outer shell, forming a Ca^2+ ion.

Correct: (D)

Methane is formed between two non-metals sharing electrons; the others are ionic compounds of a metal with a non-metal.

Correct: (C)

X gained two electrons, so the atom had configuration 2,8,6 with 6 outer electrons, placing it in Group 16 (Group VI). It is sulfur.

Correct: (A)

In graphite each carbon is bonded to three others in layers, and the fourth outer electron is delocalised and free to carry charge.

Correct: (D)

Mr = 64 + 32 + (4 × 16) = 160.

Correct: (B)

Moles = volume / 24 = 36 / 24 = 1.5 mol.

Correct: (A)

Mass of oxygen in one mole = 2 × 16 = 32 g. Percentage = (32 / 44) × 100 = 72.7%.

Correct: (C)

The mass of CH2 is 14. 84 / 14 = 6, so the molecular formula is 6 × CH2 = C6H12.

Correct: (D)

25.0 cm^3 = 0.0250 dm^3. Moles = 0.0250 × 0.10 = 0.0025 mol.

Correct: (A)

Moles of CaCO3 = 10 / 100 = 0.10 mol, so 0.10 mol of CO2 forms. Mass = 0.10 × 44 = 4.4 g.

Correct: (B)

Concentration = mass / volume = 8 / 2 = 4 g/dm^3.

Correct: (C)

Hydrogen ions move to the cathode and are discharged to form hydrogen gas.

Correct: (D)

Dissolving aluminium oxide in molten cryolite lowers the working temperature, saving energy and cost.

Correct: (B)

Copper is below hydrogen in the reactivity series, so copper(II) ions are discharged at the cathode, coating it with copper.

Correct: (A)

The anode is the positive electrode; negative ions (anions) move towards it.

Correct: (C)

In the fuel cell hydrogen and oxygen combine to form water only, converting chemical energy directly into electrical energy.

Correct: (D)

Thermal decomposition absorbs heat from the surroundings, so it is endothermic. The others release energy.

Correct: (B)

Energy absorbed breaking bonds = 436 + 242 = 678 kJ. Energy released forming bonds = 2 × 431 = 862 kJ. ΔH = 678 – 862 = -184 kJ/mol.

Correct: (A)

Energy is released to the surroundings, so the products have less energy than the reactants and ΔH is negative.

Correct: (C)

Energy must be put in to break bonds (endothermic), and energy is given out when bonds form (exothermic).

Correct: (D)

Energy = 50 × 4.2 × 10 = 2100 J = 2.1 kJ.

Correct: (A)

Powdered marble has a much larger surface area, so there are more collisions per second with the acid and the rate increases.

Correct: (C)

At a higher temperature particles move faster and collide more often, and a larger proportion of collisions have enough energy to react.

Correct: (B)

The forward reaction is exothermic, so a lower temperature shifts the equilibrium towards SO3 (in industry a compromise temperature is used for a reasonable rate).

Correct: (D)

In the displacement reaction zinc loses electrons (oxidation) and copper(II) ions gain electrons (reduction), so it is redox. The others involve no electron transfer.

Correct: (A)

A reducing agent gives electrons to another substance (reducing it) and is itself oxidised in the process.

Correct: (C)

A catalyst speeds up a reaction but is not used up and is chemically unchanged at the end; it does not change the equilibrium yield.

Correct: (B)

As reactant particles are used up their concentration falls, collisions are less frequent, and the rate decreases until the reaction finishes.

Correct: (D)

A pH of 2 is well below 7, showing a strongly acidic solution.

Correct: (A)

Bases are metal oxides and hydroxides that neutralise acids; copper(II) oxide is a typical base. Carbon dioxide and sulfur dioxide are acidic oxides.

Correct: (C)

Salts of Group I metals and ammonium salts are made by titration, using an indicator to find the exact volume of acid needed.

Correct: (B)

Nitrates are reduced by aluminium in alkaline solution on warming, releasing ammonia, which turns damp red litmus paper blue.

Correct: (D)

An ammonium salt reacts with any alkali, such as calcium hydroxide, on warming to give off ammonia gas.

Correct: (A)

Lead(II) sulfate is insoluble, so it is made by mixing solutions containing Pb^2+ and SO4^2- ions and filtering the precipitate.

Correct: (C)

Zinc is reactive enough to react steadily with dilute acid but not dangerously; copper, silver and gold do not react with dilute sulfuric acid.

Correct: (B)

Chlorine is a green gas, bromine is a red-brown liquid and iodine is a dark solid; reactivity decreases down the group.

Correct: (D)

Potassium is very reactive: it floats, melts into a ball, fizzes violently and the hydrogen may catch fire, leaving an alkaline solution of potassium hydroxide.

Correct: (A)

With a full outer electron shell, noble gas atoms neither lose, gain nor share electrons, so they are very unreactive.

Correct: (C)

The melting points of Group I metals decrease down the group (e.g. lithium 180°C, sodium 98°C, potassium 63°C).

Correct: (B)

Potassium is near the top of the reactivity series and reacts violently with cold water; iron reacts only very slowly and copper and gold do not react.

Correct: (D)

Carbon burns to form carbon monoxide, which removes oxygen from the iron(III) oxide: Fe2O3 + 3CO → 2Fe + 3CO2.

Correct: (A)

In an alloy, atoms of different sizes disturb the regular layers of the metal, preventing them sliding over each other, so the alloy is harder.

Correct: (C)

Zinc is more reactive than iron, so in galvanising the zinc corrodes (reacts) in preference to the iron, protecting it even if the coating is scratched.

Correct: (B)

Fertilisers supply nitrogen, phosphorus and potassium for healthy plant growth; ammonium nitrate is a rich source of nitrogen.

Correct: (D)

Sulfur dioxide, released when fossil fuels containing sulfur are burned, dissolves in rain to form acid rain (oxides of nitrogen also contribute).

Correct: (A)

Green plants absorb carbon dioxide for photosynthesis; combustion, respiration and decomposition release it.

Correct: (C)

Alkenes are unsaturated hydrocarbons containing one C=C bond, with general formula CnH2n, e.g. ethene C2H4.

Correct: (B)

Cracking breaks down less useful long-chain alkanes into shorter alkanes and alkenes, using high temperature and a catalyst (or steam).

Correct: (D)

PVC is the addition polymer of chloroethene monomers, used for pipes and insulation.

Correct: (A)

The C=C double bond of an alkene reacts with bromine in an addition reaction, removing its colour; a saturated alkane does not react in the dark.

Correct: (C)

Acidified potassium dichromate(VI) oxidises ethanol to ethanoic acid, turning from orange to green.

Correct: (B)

Fractional distillation separates miscible liquids because they boil at different temperatures and can be collected separately.

Correct: (B)

Rf = distance moved by dye / distance moved by solvent, so distance = 0.75 × 12.0 = 9.0 cm.