IGCSE Chemistry Practice Test 16

Correct: (A)

Evaporation is the escape of the most energetic particles from the surface of a liquid. It happens at any temperature, unlike boiling, which occurs at one fixed temperature throughout the liquid.

Correct: (B)

As a liquid cools, its particles lose energy and move more slowly. On freezing they are locked into a regular arrangement by strong forces, vibrating about fixed positions.

Correct: (D)

The particles in a liquid are close together, so the liquid has a fixed volume, but they can slide past each other, so the liquid takes the shape of its container.

Correct: (B)

Nucleon number = number of protons + number of neutrons = 6 + 8 = 14. (This atom is an isotope of carbon, carbon-14.)

Correct: (C)

The nucleus at the centre of the atom contains protons and neutrons. The electrons move around the nucleus in shells.

Correct: (A)

The element (magnesium) has 2 electrons in its outer shell. It loses both electrons to form a Mg^2+ ion with a full outer shell.

Correct: (D)

Ionic compounds have giant lattice structures. Strong attractions between oppositely charged ions act in all directions, so a lot of energy is needed to melt them.

Correct: (B)

A double bond consists of two shared pairs of electrons, which is 4 electrons in total.

Correct: (C)

In graphite each carbon forms only three covalent bonds. The fourth outer electron from each atom joins a pool of delocalised electrons that can carry charge through the structure. In diamond all four electrons are used in bonds.

Correct: (A)

Silicon dioxide has a giant covalent (macromolecular) structure like diamond, with strong covalent bonds throughout, so it has a very high melting point.

Correct: (D)

Ar = (80 × 24 + 10 × 25 + 10 × 26) / 100 = (1920 + 250 + 260) / 100 = 24.3.

Correct: (B)

Moles = mass / Mr = 25 / 100 = 0.25 mol.

Correct: (D)

Volume = moles × molar volume = 0.50 × 24 = 12 dm^3.

Correct: (A)

20 cm^3 = 0.020 dm^3. Moles = concentration × volume = 2.0 × 0.020 = 0.040 mol.

Correct: (C)

Moles: C = 27.3/12 = 2.275, O = 72.7/16 = 4.54. Ratio = 1 : 2, so the empirical formula is CO2.

Correct: (B)

Percentage yield = (15 / 20) × 100 = 75%.

Correct: (A)

Moles of Mg = 2.4 / 24 = 0.10 mol. The equation 2Mg + O2 → 2MgO shows 1 mol of Mg gives 1 mol of MgO, so mass of MgO = 0.10 × 40 = 4.0 g.

Correct: (C)

Mr = (2 × 1) + 32 + (4 × 16) = 2 + 32 + 64 = 98.

Correct: (D)

K^+ ions move to the negative cathode, where they gain electrons and are discharged as potassium metal. Chlorine is formed at the anode.

Correct: (B)

Hydrogen ions are discharged at the cathode to give hydrogen, and oxygen is given off at the anode. Twice as much hydrogen is produced as oxygen (a 2 : 1 ratio by volume).

Correct: (A)

In electrolysis the cathode is connected to the negative terminal of the power supply, so it attracts positively charged cations, which gain electrons there.

Correct: (C)

The object to be plated is made the cathode (negative electrode), so that positive metal ions from the solution are discharged onto its surface as a layer of metal.

Correct: (B)

Oxygen is discharged at the carbon anodes and reacts with them: C + O2 → CO2. The anodes are gradually used up and must be replaced.

Correct: (D)

Combustion reactions give out energy to the surroundings, so they are exothermic. In (C) energy is taken in from the surroundings, which is why the temperature falls.

Correct: (A)

Bonds broken: 4 C-H + 2 O=O = (4 × 410) + (2 × 500) = 1640 + 1000 = 2640 kJ. Bonds formed: 2 C=O + 4 O-H = (2 × 800) + (4 × 460) = 1600 + 1840 = 3440 kJ. ΔH = 2640 – 3440 = -800 kJ/mol.

Correct: (C)

A fall in temperature shows that the reaction takes in energy from the surroundings, so the reaction is endothermic and ΔH is positive.

Correct: (B)

Energy must be supplied to break bonds, and energy is released when new bonds form. A reaction is exothermic when more energy is released in bond making than is taken in by bond breaking.

Correct: (D)

The voltage of a simple cell is larger when the two metals are further apart in the reactivity series. Magnesium and copper are the furthest apart of these pairs, so this cell gives the largest voltage.

Correct: (A)

A higher concentration means more particles in a given volume, so the frequency of collisions increases and the reaction is faster.

Correct: (C)

A catalyst increases the rate of a reaction by providing a pathway with a lower activation energy, and it is not used up; it can be recovered unchanged at the end.

Correct: (B)

Average rate = volume of gas / time = 24 / 30 = 0.8 cm^3/s.

Correct: (D)

Increasing the temperature shifts the position of equilibrium in the direction of the endothermic reaction (the backward reaction here), so less sulfur trioxide is present at equilibrium, although the reaction is reached faster.

Correct: (C)

In reaction (C), magnesium is oxidised (it loses electrons to form Mg^2+) and copper(II) ions are reduced. Oxidation and reduction occur together, so it is a redox reaction. The others involve no change in oxidation state.

Correct: (A)

Copper(II) oxide gives oxygen to the hydrogen (and its copper ions gain electrons), so CuO is reduced and it acts as the oxidising agent. Hydrogen is the reducing agent.

Correct: (D)

One large piece has a smaller surface area exposed to the acid than the same mass in small pieces, so it reacts more slowly. Warming, powdering and increasing concentration all increase the rate.

Correct: (B)

A base is a substance that neutralises an acid (a base is a proton acceptor). A base that dissolves in water is called an alkali; not all bases are soluble.

Correct: (C)

A metal oxide is a base, and acids react with bases to give a salt and water: CuO + 2HNO3 → Cu(NO3)2 + H2O.

Correct: (A)

Carbon monoxide is a neutral oxide: it shows neither acidic nor basic behaviour. Sulfur dioxide and nitrogen dioxide are acidic oxides, and magnesium oxide is a basic oxide.

Correct: (D)

An insoluble salt is made by precipitation. Mixing the two solutions forms barium sulfate as a precipitate, which is filtered off, washed to remove soluble impurities and dried.

Correct: (B)

Zinc oxide is an insoluble base. Reacting it with sulfuric acid, filtering off the excess and crystallising the filtrate gives zinc sulfate crystals: ZnO + H2SO4 → ZnSO4 + H2O.

Correct: (C)

The sulfate salt is formed from sulfuric acid: 2NH3 + H2SO4 → (NH4)2SO4.

Correct: (D)

Sodium hydroxide is a strong alkali, so its solution has a high pH, around 13-14, and turns Universal Indicator purple.

Correct: (A)

Reactivity increases down Group I because the single outer electron is further from the nucleus and more easily lost. They are soft metals with low melting points and are stored under oil.

Correct: (B)

A more reactive halogen displaces a less reactive one, but bromine is below chlorine in Group VII, so it cannot displace chlorine from its compounds and no reaction occurs.

Correct: (D)

Transition metals are hard, dense, strong metals with high melting points. They form coloured compounds and ions with variable oxidation states, e.g. Fe^2+ and Fe^3+.

Correct: (D)

The atom has electrons in three shells (2 + 8 + 3), so it is in Period 3. It has 3 outer electrons, so it is also in Group III (aluminium).

Correct: (A)

Calcium reacts with cold water to form calcium hydroxide, an alkali that turns Universal Indicator purple, and hydrogen: Ca + 2H2O → Ca(OH)2 + H2. Copper and zinc do not react with cold water, and iron reacts only very slowly (rusting).

Correct: (C)

The limestone (calcium carbonate) decomposes and the calcium oxide formed reacts with sandy impurities to form molten slag, which is tapped off separately from the molten iron. Carbon (coke) is the fuel and source of the reducing agent.

Correct: (B)

Stainless steel is an alloy of iron with chromium (and other metals). It does not rust, so it stays clean and shiny, making it ideal for cutlery.

Correct: (D)

Magnesium is more reactive than iron, so it corrodes (loses electrons) instead of the iron. Painting, oiling and plastic coating are barrier methods that simply keep out air and water.

Correct: (C)

The water is passed through filter beds (often sand), which trap insoluble solid particles. Chlorination afterwards kills bacteria.

Correct: (A)

Clean dry air contains about 78% nitrogen and 21% oxygen, with about 1% argon and noble gases and 0.04% carbon dioxide.

Correct: (B)

Ammonium nitrate contains a high percentage of nitrogen in a form plants can absorb. Fertilisers supply elements such as nitrogen, phosphorus and potassium (NPK) needed for healthy plant growth.

Correct: (D)

Alkenes have the general formula CnH2n and contain a C=C double bond. C3H6 fits CnH2n. C2H6, CH4 and C3H8 are alkanes with the general formula CnH2n+2.

Correct: (A)

This is an addition reaction: hydrogen adds across the C=C double bond, converting ethene into the saturated alkane ethane: C2H4 + H2 → C2H6.

Correct: (C)

Cracking breaks large hydrocarbon molecules into smaller, more useful ones, typically a shorter-chain alkane and an alkene. The alkenes are used to make polymers and the short alkanes as fuels.

Correct: (B)

Ethene molecules join together by addition polymerisation, opening their double bonds, to form poly(ethene). No other product is formed, and the polymer is non-biodegradable.

Correct: (D)

Ethanol is oxidised (by bacteria in the air, or by acidified potassium dichromate in the laboratory) to ethanoic acid, which is the acid in vinegar and gives the sour taste.

Correct: (C)

Silver nitrate gives a white precipitate of silver chloride with chloride ions. The solution is acidified with dilute nitric acid first. Test (A) is for sulfate ions and test (D) is for ammonium ions.

Correct: (A)

A pure substance melts sharply at one fixed temperature. Impurities lower the melting point and make the substance melt over a range of temperatures.