IGCSE Chemistry Practice Test 12

Correct: (A)

Sublimation is the change of a solid directly into a gas (or a gas directly into a solid) without passing through the liquid state, as happens with iodine and solid carbon dioxide.

Correct: (B)

Evaporation happens at any temperature, when the faster-moving particles at the surface of the liquid escape. Boiling, in contrast, occurs at one temperature and throughout the liquid.

Correct: (C)

In a solid the particles are held in fixed positions in a regular arrangement by strong forces, so they can only vibrate about those positions. This gives a solid a fixed shape and volume.

Correct: (D)

Proton number = nucleon number – number of neutrons = 24 – 12 = 12. The element is magnesium.

Correct: (A)

The configuration contains 2 + 8 + 1 = 11 electrons, so the atom has 11 protons, which is sodium. (Magnesium is 2,8,2; aluminium is 2,8,3; chlorine is 2,8,7.)

Correct: (B)

Water is a simple molecule in which hydrogen and oxygen atoms are joined by shared pairs of electrons. Sodium chloride and magnesium oxide are ionic, and calcium carbonate contains both ionic and covalent bonds.

Correct: (C)

In graphite each carbon atom forms only three bonds, leaving one electron per atom delocalised. These electrons can move along the layers and carry charge.

Correct: (D)

A chlorine atom has 17 electrons. A chloride ion is formed when the atom gains one electron, so the ion has 17 + 1 = 18 electrons.

Correct: (A)

Number of neutrons = nucleon number – proton number = 19 – 9 = 10.

Correct: (B)

A magnesium atom (2,8,2) loses its two outer electrons to form Mg^2+ with the stable configuration 2,8.

Correct: (C)

Sulfur is a simple molecular substance with no ions and no delocalised electrons, so it cannot conduct in any state. Molten sodium chloride conducts because its ions are free to move, and the metals conduct using delocalised electrons.

Correct: (C)

Mr = 64 + 32 + (4 × 16) = 64 + 32 + 64 = 160.

Correct: (D)

Each mole of CO2 contains 1 mol of carbon atoms, so 0.50 mol of CO2 contains 0.50 mol of carbon. Mass = 0.50 × 12 = 6 g.

Correct: (A)

Moles of NaCl = 5.85 / 58.5 = 0.100 mol. Volume = 500 cm^3 = 0.500 dm^3. Concentration = 0.100 / 0.500 = 0.20 mol/dm^3.

Correct: (B)

Volume = moles × molar gas volume = 2.0 × 24 = 48 dm^3.

Correct: (C)

Moles: C = 2.4/12 = 0.20; H = 0.4/1 = 0.40. The ratio is 0.20 : 0.40 = 1 : 2, so the empirical formula is CH2. (C2H4 has the same ratio but is not the simplest whole-number formula.)

Correct: (D)

From the equation CaCO3 + 2HCl → CaCl2 + H2O + CO2, 1 mol of CaCO3 produces 1 mol of CO2, so 0.050 mol of CaCO3 is needed. Mass = 0.050 × 100 = 5.0 g.

Correct: (A)

Percentage purity = (mass of pure substance / mass of sample) × 100 = (9.0 / 10.0) × 100 = 90%.

Correct: (B)

Bromide ions, Br-, move to the anode where they lose electrons and form bromine. Lead is formed at the cathode, where Pb^2+ ions gain electrons.

Correct: (C)

Oxidation is the loss of electrons (OILRIG: Oxidation Is Loss). At the anode, negative ions such as Br- lose electrons, for example 2Br- → Br2 + 2e-, so they are oxidised.

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 they give the largest voltage.

Correct: (A)

Copper ions are discharged at the cathode and coat it with a pink-brown layer of copper metal. Oxygen is formed at the anode, and the blue colour slowly fades as copper ions are removed.

Correct: (B)

Sodium is above hydrogen in the reactivity series, so in aqueous sodium hydroxide the hydrogen ions (from water) are discharged at the cathode instead, forming hydrogen. Copper is discharged from both copper electrolytes, and molten sodium chloride gives sodium.

Correct: (C)

Thermal decomposition of calcium carbonate (CaCO3 → CaO + CO2) needs continuous heating and takes in energy, so it is endothermic. Combustion and neutralisation are exothermic.

Correct: (D)

Energy needed to break bonds = 945 + (3 × 436) = 2253 kJ. Energy released forming bonds = 6 × 391 = 2346 kJ (each NH3 has 3 N-H bonds). ΔH = 2253 – 2346 = -93 kJ/mol.

Correct: (A)

Breaking bonds takes in energy and forming bonds releases energy. If less energy is absorbed in breaking bonds than is released in forming them, the extra energy is transferred to the surroundings and the reaction is exothermic.

Correct: (B)

In a hydrogen fuel cell, hydrogen reacts with oxygen to form only water, so no carbon dioxide or other pollutants are produced. Hydrogen is actually difficult to store.

Correct: (C)

Energy = mass × c × temperature rise = 50 × 4.2 × 15 = 3150 J = 3.15 kJ.

Correct: (D)

Rate depends on temperature, concentration (or pressure for gases), surface area and catalysts. The colour of a reactant has no effect on the frequency or energy of collisions.

Correct: (A)

Manganese(IV) oxide speeds up the decomposition of hydrogen peroxide (2H2O2 → 2H2O + O2) and is unchanged at the end, so it is a catalyst.

Correct: (B)

The Haber process uses a temperature of about 450°C, a pressure of about 200 atm and an iron catalyst. These are compromise conditions giving an acceptable yield at an acceptable rate.

Correct: (C)

Hydrogen removes oxygen from copper(II) oxide, so hydrogen is the reducing agent. The hydrogen itself is oxidised to water, and the copper(II) oxide is the oxidising agent.

Correct: (D)

At equilibrium both reactions continue at the same rate, so there is no overall change. The concentrations stay constant but are not necessarily equal.

Correct: (A)

Average rate = volume of gas / time = 100 / 40 = 2.5 cm^3/s.

Correct: (B)

A higher concentration means more particles packed into the same volume, so they collide more frequently and the reaction is faster. Concentration does not change particle speed or activation energy.

Correct: (C)

Zinc oxide is amphoteric: it reacts with both acids and alkalis to form salts. Sodium oxide is basic, and carbon dioxide and sulfur dioxide are acidic oxides.

Correct: (D)

pH 9 is just above the neutral value of 7, so the solution is a weak alkali (a weak alkaline solution). Strong alkalis have a pH of about 13-14.

Correct: (A)

Insoluble salts are made by precipitation: two soluble salts are mixed, the insoluble salt forms as a precipitate, and it is filtered off, washed with distilled water and dried. (Method C fails because the insoluble lead(II) sulfate coats the lead and stops the reaction.)

Correct: (B)

A pale blue precipitate of copper(II) hydroxide forms first, but it dissolves in excess aqueous ammonia to give a deep blue solution. This is a test for copper(II) ions.

Correct: (C)

Acids produce hydrogen ions, H+(aq), when dissolved in water. It is these ions that make the solution acidic, whatever the particular acid is.

Correct: (D)

Silver ions react with chloride ions to form a white precipitate of silver chloride; the nitric acid removes ions such as carbonate that could interfere. (Test A identifies sulfate ions instead.)

Correct: (A)

Sodium sulfate is a soluble salt of a reactive metal, so it is made by titrating aqueous sodium hydroxide with dilute sulfuric acid using an indicator. Barium sulfate and lead(II) chloride are insoluble (made by precipitation), and calcium carbonate is not made in solution.

Correct: (B)

Halogen reactivity decreases down the group: fluorine is the most reactive and iodine the least, because the outer electrons are further from the nucleus and an extra electron is gained less easily. They form 1- ions, and bromine is a liquid at room temperature.

Correct: (C)

A more reactive halogen displaces a less reactive halogen from a solution of its halide: Br2 + 2KI → 2KBr + I2. The solution turns brown as iodine is formed.

Correct: (D)

The atom has 12 electrons, so 12 protons: magnesium. (Sodium is 2,8,1; aluminium is 2,8,3; silicon is 2,8,4.)

Correct: (A)

Transition elements and their compounds are often used as catalysts (for example iron in the Haber process), which Group I metals are not. Both groups conduct electricity, but Group I metals are the soft, low-density ones.

Correct: (B)

Bauxite is purified aluminium oxide, the ore from which aluminium is extracted by electrolysis. Iron is extracted from haematite.

Correct: (C)

Aluminium (usually as an alloy) is light because of its low density, and its surface forms a protective oxide layer so it resists corrosion. Both properties are essential for aircraft.

Correct: (D)

Iron reacts with steam but not with cold water, and iron(III) oxide is reduced by carbon in the blast furnace. Sodium reacts violently with cold water, copper does not react with steam, and magnesium oxide cannot be reduced by carbon.

Correct: (A)

Stainless steel is an alloy of iron with chromium (and often nickel). It is hard and, unlike iron, does not rust, so it stays clean and shiny for cutlery.

Correct: (B)

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

Correct: (C)

Sulfur dioxide, released when fossil fuels containing sulfur are burned, dissolves in rainwater to form acid rain. (Oxides of nitrogen also contribute; carbon dioxide makes rainwater only very weakly acidic naturally.)

Correct: (D)

The K in NPK stands for potassium. Nitrogen, phosphorus and potassium are the three elements that fertilisers must replace in the soil for healthy plant growth.

Correct: (A)

Butane is the alkane with 4 carbon atoms. Using CnH2n+2 with n = 4 gives C4H10. C3H8 is propane and C5H12 is pentane.

Correct: (B)

Alkenes are unsaturated hydrocarbons containing a C=C double bond, with general formula CnH2n. The double bond makes them reactive; they decolourise bromine water by addition.

Correct: (C)

Fermentation uses enzymes in yeast at about 30-35°C under anaerobic conditions (no air): C6H12O6 → 2C2H5OH + 2CO2. Conditions B describe the hydration of ethene instead.

Correct: (D)

Addition polymerisation of chloroethene monomers gives poly(chloroethene), commonly called PVC, a hard, waterproof plastic used for pipes and window frames.

Correct: (A)

CH3COOH is ethanoic acid, the carboxylic acid found in vinegar. (Methanoic acid is HCOOH; ethyl ethanoate is an ester, CH3COOCH2CH3.)

Correct: (B)

A pipette delivers one fixed, accurate volume of a solution (for example 25.0 cm^3). A measuring cylinder is much less accurate, and the conical flask is the vessel the solution is delivered into.

Correct: (D)

Carbon dioxide turns limewater (calcium hydroxide solution) milky or cloudy, which is the standard test for the gas. Relighting a glowing splint tests for oxygen, the 'pop' tests for hydrogen, and bleaching is a test for chlorine.