IGCSE Chemistry Practice Test 7

Correct: (C)

When a liquid freezes it loses energy, the particles move more slowly and lock into a regular arrangement (a lattice).

Correct: (A)

Ink particles spread through the water by diffusion, moving randomly down the concentration gradient until evenly spread.

Correct: (B)

The same number of particles occupies a smaller volume, so they hit the walls more often, increasing the pressure. Their speed is unchanged because the temperature is constant.

Correct: (D)

Nucleon number = protons + neutrons = 11 + 12 = 23.

Correct: (B)

Isotopes have the same proton (and electron) number but different numbers of neutrons, so their chemistry, which depends on electrons, is the same.

Correct: (C)

17 electrons fill the shells as 2 in the first, 8 in the second and 7 in the third: 2,8,7.

Correct: (A)

Ar = (0.75 × 35) + (0.25 × 37) = 26.25 + 9.25 = 35.5.

Correct: (D)

In diamond every carbon atom is covalently bonded to four others in one giant lattice. Copper is metallic, iodine is simple molecular and sodium chloride is ionic.

Correct: (A)

A magnesium atom (2,8,2) loses its two outer electrons to form Mg^2+, leaving 2,8.

Correct: (C)

MgO is formed by electron transfer from a metal (magnesium) to a non-metal (oxygen), giving Mg^2+ and O^2- ions. The others are covalent.

Correct: (B)

The delocalised electrons in graphite can move along the layers and carry charge; diamond has no free electrons or ions.

Correct: (D)

Moles = mass / Mr = 8 / 16 = 0.5 mol.

Correct: (A)

Volume = moles × 24 = 0.25 × 24 = 6 dm^3.

Correct: (C)

Moles of NaCl = 5.85 / 58.5 = 0.10 mol. Concentration = 0.10 / 0.500 = 0.20 mol/dm^3.

Correct: (B)

Moles: C = 40.0/12 = 3.33, H = 6.7/1 = 6.7, O = 53.3/16 = 3.33. Ratio 3.33 : 6.7 : 3.33 = 1 : 2 : 1, so the empirical formula is CH2O.

Correct: (D)

Moles of CaCO3 = 10.0 / 100 = 0.10 mol, so 0.10 mol of CO2 forms. Volume = 0.10 × 24 = 2.4 dm^3.

Correct: (A)

Percentage yield = (actual / theoretical) × 100 = (30 / 40) × 100 = 75%.

Correct: (C)

56 / 14 = 4, so the molecular formula is 4 × CH2 = C4H8.

Correct: (B)

At the cathode, hydrogen ions are discharged in preference to sodium ions because sodium is high in the reactivity series, so hydrogen gas forms.

Correct: (D)

Pb^2+ + 2e^- → Pb. The positive lead ions move to the negative electrode and gain electrons.

Correct: (A)

Graphite (and platinum) are inert and do not react with the electrolyte or the products, unlike reactive metals such as copper, iron or zinc.

Correct: (C)

Water decomposes as 2H2O → 2H2 + O2, so hydrogen and oxygen are produced in a 2 : 1 volume ratio. Oxygen = 60 / 2 = 30 cm^3.

Correct: (B)

The object to be plated must be the cathode (negative electrode) so that Ag^+ ions are attracted to it and form a layer of silver; the electrolyte must contain silver ions.

Correct: (D)

Combustion releases energy to the surroundings, so it is exothermic. The other three take in energy and are endothermic.

Correct: (A)

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

Correct: (C)

Energy = m × c × ΔT = 100 × 4.2 × 4.0 = 1680 J = 1.68 kJ.

Correct: (B)

In an endothermic reaction bond breaking takes in more energy than bond making releases, so ΔH is positive and the surroundings cool down.

Correct: (D)

In the fuel cell hydrogen is oxidised and the only product is water: 2H2 + O2 → 2H2O.

Correct: (A)

A more concentrated acid has more particles in the same volume, so collisions between particles are more frequent and the rate increases.

Correct: (C)

At a higher temperature particles move faster, collide more often, and, most importantly, more of the collisions have enough energy (the activation energy) to react.

Correct: (B)

A catalyst speeds up the reaction by lowering the activation energy and is chemically unchanged at the end; it does not change the yield or the product energies.

Correct: (D)

Heating drives off the water of crystallisation, leaving white anhydrous copper(II) sulfate. The change is reversible when water is added again.

Correct: (A)

There are 4 moles of gas on the left and only 2 moles on the right, so increasing the pressure shifts the equilibrium towards the side with fewer gas molecules, giving more ammonia.

Correct: (C)

Zinc atoms lose electrons to form Zn^2+ ions. Loss of electrons is oxidation, so zinc is oxidised (and is the reducing agent).

Correct: (B)

Magnesium is oxidised (0 to +2) and hydrogen is reduced (+1 to 0), so electrons are transferred. The others are neutralisation, acid-carbonate and precipitation reactions with no oxidation number changes.

Correct: (D)

A pH of 2 is well below 7, showing a high concentration of H^+ ions that only a strong (fully ionised) acid gives at this concentration.

Correct: (A)

Sulfur dioxide is a non-metal oxide and reacts with water and alkalis as an acid. Calcium, magnesium and sodium oxides are basic metal oxides.

Correct: (C)

Copper(II) oxide is insoluble, so the excess-insoluble method is used: react, filter off unreacted oxide, then crystallise the copper(II) sulfate from the filtrate.

Correct: (B)

Ba^2+ ions form insoluble white barium sulfate. The solution is acidified first to remove carbonate ions, which would also give a white precipitate with barium ions.

Correct: (D)

Aluminium oxide (and zinc oxide) are amphoteric: they react with both strong acids and strong alkalis. CaO and Na2O are basic; CO2 is acidic.

Correct: (A)

In every acid-alkali neutralisation the hydrogen ions from the acid react with the hydroxide ions from the alkali to form water; the other ions are spectators.

Correct: (C)

Both reactants are solutions, so the volumes are measured by titration (first with an indicator to find the volumes, then repeated without indicator) to give a pure salt solution.

Correct: (B)

Going down Group I the outer electron is further from the nucleus and more shielded, so it is lost more easily and the metals become more reactive (Li < Na < K).

Correct: (D)

Chlorine is more reactive than iodine and displaces it: Cl2 + 2KI → 2KCl + I2. The iodine formed colours the solution brown.

Correct: (A)

Transition elements have high melting points and densities, form coloured compounds, show variable oxidation numbers and are often used as catalysts.

Correct: (C)

Going down Group VII the molecules get larger, the forces between them get stronger, and melting and boiling points increase, while reactivity decreases and the colour deepens.

Correct: (B)

Sodium (like potassium and calcium) reacts vigorously with cold water. Iron and zinc react with steam, and copper does not react with water at all.

Correct: (D)

Limestone decomposes to calcium oxide, which reacts with sandy impurities (silicon dioxide) to form molten slag. Coke and carbon monoxide do the reducing; coke is the fuel.

Correct: (A)

Zinc is more reactive than iron, so in galvanising the zinc corrodes in preference to the iron, protecting it even if the coating is scratched. Paint, oil and plastic are only barrier methods.

Correct: (C)

In an alloy, differently sized atoms disrupt the regular arrangement, stopping the layers sliding, which makes the alloy harder and stronger.

Correct: (B)

Chlorine is added to sterilise the water by killing bacteria and other microorganisms. Filtration and sedimentation remove solid impurities.

Correct: (D)

Air is about 78% nitrogen and 21% oxygen, with about 1% argon and other gases and about 0.04% carbon dioxide.

Correct: (A)

Potassium nitrate contains both potassium and nitrogen (in the nitrate ion), two of the essential elements (N, P, K) that fertilisers supply.

Correct: (C)

Alkanes are saturated hydrocarbons with general formula CnH2n+2, e.g. methane CH4 and ethane C2H6.

Correct: (B)

Ethene is unsaturated and undergoes addition with bromine across its C=C bond, decolourising the bromine water. This is the standard test for a C=C double bond.

Correct: (D)

Fermentation uses yeast, a warm temperature of about 37°C and anaerobic conditions: C6H12O6 → 2C2H5OH + 2CO2.

Correct: (A)

In condensation polymerisation the monomers join with the loss of a small molecule such as water (e.g. forming nylon or a polyester). Poly(ethene) is made by addition polymerisation, where only the polymer forms.

Correct: (C)

Carboxylic acids such as ethanoic acid contain the -COOH functional group, which gives them their acidic properties.

Correct: (B)

Rf = distance moved by the spot / distance moved by the solvent = 4.0 / 10.0 = 0.4.

Correct: (D)

Anhydrous copper(II) sulfate is a chemical test for water, turning from white to blue. (Limewater tests for carbon dioxide, a glowing splint for oxygen and the pop test for hydrogen.)