IGCSE Chemistry Practice Test 15

Correct: (B)

This is diffusion. Particles move randomly from a region of high concentration to a region of low concentration until they are evenly spread.

Correct: (D)

Condensation is the change from the gas state to the liquid state, when particles lose energy and come closer together.

Correct: (A)

On melting, particles gain enough energy to overcome the forces holding them in a regular arrangement, so they can slide past each other in the liquid.

Correct: (C)

Protons = proton number = 11. Neutrons = 23 – 11 = 12. A neutral atom has the same number of electrons as protons = 11.

Correct: (B)

Isotopes have the same proton number (so they are the same element) but different nucleon numbers because they contain different numbers of neutrons.

Correct: (D)

The group number equals the number of electrons in the outer shell, which is 6, so the element (sulfur) is in Group VI.

Correct: (A)

Magnesium forms Mg^2+ ions by losing two electrons. Oxygen forms O^2- ions by gaining two electrons. The compound is held together by ionic bonding.

Correct: (C)

Nitrogen forms three single covalent bonds, one with each hydrogen atom, and it also has one lone pair of electrons.

Correct: (B)

In a metal, the outer electrons are delocalised. These free electrons carry the charge through the solid lattice.

Correct: (D)

In diamond each carbon atom forms four covalent bonds in a rigid giant (macromolecular) structure, which makes diamond very hard with a very high melting point.

Correct: (A)

Ar = (75 × 35 + 25 × 37) / 100 = (2625 + 925) / 100 = 35.5.

Correct: (C)

Moles = mass / Mr = 8.0 / 16 = 0.50 mol.

Correct: (B)

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

Correct: (D)

250 cm^3 = 0.250 dm^3. Concentration = 0.50 / 0.250 = 2.0 mol/dm^3.

Correct: (A)

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

Correct: (C)

Percentage yield = (actual yield / theoretical yield) × 100 = (32 / 40) × 100 = 80%.

Correct: (B)

Moles of CaCO3 = 5.0 / 100 = 0.050 mol. The equation CaCO3 + 2HCl → CaCl2 + H2O + CO2 shows 1 mol of CaCO3 gives 1 mol of CO2, so mass of CO2 = 0.050 × 44 = 2.2 g.

Correct: (D)

Percentage of calcium = (40 / 100) × 100 = 40%.

Correct: (A)

Pb^2+ ions move to the negative cathode and are discharged as lead metal. Br^- ions move to the positive anode and are discharged as bromine.

Correct: (C)

At the anode, hydroxide ions from the water are discharged to form oxygen gas. Copper is deposited at the cathode.

Correct: (B)

The cathode is the negative electrode. Positive ions (cations) move to it and gain electrons, so reduction occurs at the cathode.

Correct: (D)

Electroplating coats an object with a thin layer of metal to improve appearance and to protect the object underneath from corrosion.

Correct: (C)

Aluminium oxide has a very high melting point. Dissolving it in cryolite lowers the temperature needed to keep the mixture molten, which saves energy and cost.

Correct: (A)

In an exothermic reaction energy is given out to the surroundings, so the temperature rises and the enthalpy change, ΔH, is negative.

Correct: (B)

Energy taken in to break bonds = 436 + 242 = 678 kJ. Energy released when bonds form = 2 × 431 = 862 kJ. ΔH = 678 – 862 = -184 kJ/mol.

Correct: (D)

The products have less energy than the reactants, so energy has been released to the surroundings. The reaction is exothermic and ΔH is negative.

Correct: (C)

In the fuel cell hydrogen and oxygen react together: 2H2 + O2 → 2H2O. Water is the only product, and chemical energy is converted directly to electrical energy.

Correct: (A)

Thermal decomposition needs a continuous supply of heat energy to break down the compound, so it is endothermic. Combustion, respiration and neutralisation are all exothermic.

Correct: (D)

At a higher temperature particles move faster, so collisions are more frequent, and more of the collisions have enough energy to react.

Correct: (B)

A catalyst lowers the activation energy by providing a different reaction pathway. It is chemically unchanged at the end of the reaction and does not change the yield.

Correct: (C)

Both experiments start with the same mass (amount) of calcium carbonate reacting with excess acid, so the same total amount of carbon dioxide is produced. The powder only reacts faster because of its larger surface area.

Correct: (A)

Blue hydrated copper(II) sulfate loses water on heating to give white anhydrous copper(II) sulfate, and adding water reverses the change. CuSO4.5H2O ⇌ CuSO4 + 5H2O.

Correct: (D)

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

Correct: (B)

Zinc atoms lose electrons to form Zn^2+ ions (Zn → Zn^2+ + 2e^-). Loss of electrons is oxidation, so zinc is oxidised and copper(II) ions are reduced.

Correct: (C)

Crushing the chips increases the surface area of the marble exposed to the acid, so collisions happen more frequently and the rate increases.

Correct: (A)

A pH of 1 means the solution is a strong acid, and Universal Indicator turns red in a strong acid.

Correct: (D)

Carbonates react with acids to form a salt, water and carbon dioxide gas, which turns limewater milky.

Correct: (C)

Aluminium oxide is amphoteric: it reacts with both acids and alkalis. Calcium oxide and sodium oxide are basic oxides, and carbon dioxide is an acidic oxide.

Correct: (B)

Insoluble salts are prepared by precipitation: two soluble salts are mixed so the insoluble salt forms as a precipitate, which is then filtered off, washed with distilled water and dried.

Correct: (A)

The indicator shows the exact end-point of the titration. Repeating with the same volumes but no indicator gives a pure salt solution, which is then crystallised.

Correct: (D)

Alkalis react with ammonium salts on warming to release ammonia gas: NH4Cl + NaOH → NaCl + H2O + NH3.

Correct: (C)

Sulfur dioxide is a non-metal oxide and is acidic; it reacts with alkalis and forms sulfurous acid with water. The others are basic metal oxides.

Correct: (B)

Group I metals become more reactive down the group because the outer electron is further from the nucleus and is lost more easily.

Correct: (A)

Chlorine is more reactive than iodine, so it displaces iodine from potassium iodide: Cl2 + 2KI → 2KCl + I2. The iodine colours the solution brown.

Correct: (D)

Transition elements are hard, dense metals with high melting points. They form coloured compounds, have variable oxidation states and often act as catalysts, for example iron in the Haber process.

Correct: (C)

The element has 7 electrons in its outer shell, so it is in Group VII. It is chlorine.

Correct: (B)

In the reactivity series potassium is near the top, zinc is in the middle and copper is near the bottom, so the order is potassium > zinc > copper.

Correct: (D)

Coke burns in the hot air to form carbon dioxide, which then reacts with more coke to form carbon monoxide. The carbon monoxide reduces the iron(III) oxide: Fe2O3 + 3CO → 2Fe + 3CO2.

Correct: (A)

In a pure metal the layers of similar atoms slide over each other easily. In an alloy, atoms of a different size distort the layers and stop them sliding, making the alloy harder and stronger.

Correct: (C)

Zinc is higher in the reactivity series than iron, so it loses electrons and corrodes preferentially. This is called sacrificial protection; galvanising works the same way.

Correct: (B)

Chlorine is a disinfectant that kills harmful microorganisms, making the water safe to drink. Filtration, not chlorination, removes solid particles.

Correct: (A)

When a fuel burns in a limited supply of oxygen, incomplete combustion occurs and carbon monoxide (and sometimes carbon) forms instead of carbon dioxide.

Correct: (D)

Methane is a greenhouse gas, along with carbon dioxide and water vapour. Greenhouse gases absorb infrared radiation and contribute to global warming.

Correct: (C)

With a plentiful supply of air, methane burns completely: CH4 + 2O2 → CO2 + 2H2O.

Correct: (B)

Ethene is an alkene with a C=C double bond, so it reacts with bromine in an addition reaction and the orange solution goes colourless. Ethane is saturated and does not react.

Correct: (A)

Fractions with the smallest molecules and lowest boiling points, such as petroleum gases and petrol, rise highest and are collected near the top of the column. Bitumen has the largest molecules and is collected at the bottom.

Correct: (D)

Polyesters (and nylon) are condensation polymers: the monomers join together with the loss of a small molecule such as water. Poly(ethene), poly(propene) and poly(chloroethene) are addition polymers.

Correct: (C)

Fermentation uses enzymes in yeast, works best at about 30°C, and must be done without air (anaerobic conditions) so the ethanol is not oxidised.

Correct: (A)

Barium chloride gives a white precipitate of barium sulfate with sulfate ions. The mixture is acidified first to remove carbonate ions, which would also give a white precipitate.

Correct: (B)

Rf = distance moved by the substance / distance moved by the solvent front = 4.0 / 8.0 = 0.50.