IGCSE Chemistry Practice Test 6

Correct: (C)

Sublimation is the direct change from solid to gas without passing through the liquid state.

Correct: (A)

Gas particles are widely spaced, have weak forces between them, and move randomly and quickly in all directions.

Correct: (B)

With the same particles in a smaller space, collisions with the container walls happen more often, so the pressure rises.

Correct: (D)

The proton number is 6, which is carbon. Atoms are neutral, so the electron number equals the proton number.

Correct: (A)

One electron in the outer shell places the element in Group I; with 19 electrons in total it is potassium.

Correct: (C)

Each oxygen atom shares two pairs of electrons with the other, forming a double covalent bond, so both atoms achieve a complete outer shell.

Correct: (B)

In graphite each carbon atom uses only three of its outer electrons for bonding; the fourth electron is delocalised and can carry charge along the layers.

Correct: (D)

Ionic compounds have giant lattices with strong attractions in all directions, giving high melting points; the ions can only move and carry charge when the lattice is broken down by melting or dissolving.

Correct: (A)

In a metal the atoms lose their outer electrons into a shared 'sea'; the attraction between the positive ions and the delocalised electrons holds the structure together and explains conductivity and malleability.

Correct: (C)

Chemical behaviour depends on the electron arrangement. Both isotopes have 6 protons and 6 electrons arranged identically; only the neutron number differs.

Correct: (D)

Magnesium is in Group II, so its atom loses its two outer electrons to gain a full outer shell, forming an Mg^2+ ion.

Correct: (B)

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

Correct: (D)

Volume = 0.50 × 24 = 12 dm^3.

Correct: (B)

Moles of C = 1.2 / 12 = 0.10; moles of H = 0.2 / 1 = 0.20. Ratio C : H = 1 : 2, so the empirical formula is CH2.

Correct: (C)

Moles = 5.85 / 58.5 = 0.10 mol. Concentration = 0.10 / 0.250 = 0.40 mol/dm^3.

Correct: (D)

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

Correct: (A)

The equation ratio N2 : H2 is 1 : 3, so 10 cm^3 of nitrogen reacts with 3 × 10 = 30 cm^3 of hydrogen.

Correct: (B)

Moles of CaCO3 = 5.0 / 100 = 0.050 mol, which gives 0.050 mol of CO2. Mass = 0.050 × 44 = 2.2 g.

Correct: (C)

Water is decomposed: hydrogen is discharged at the cathode and oxygen at the anode, with twice the volume of hydrogen because the equation is 2H2O → 2H2 + O2.

Correct: (D)

The only positive ions present are Na^+, so sodium metal is discharged at the cathode. Chlorine is formed at the anode.

Correct: (A)

Opposite charges attract, so positive cations move to the negative electrode, the cathode, where they gain electrons.

Correct: (B)

The impure copper is made the anode so that copper atoms lose electrons and dissolve as Cu^2+ ions, which then plate out as pure copper on the cathode. The impurities fall away as sludge.

Correct: (C)

The more reactive metal, zinc, loses electrons more readily and becomes the negative terminal; electrons flow through the wires from zinc to copper.

Correct: (D)

Dissolving ammonium nitrate takes in energy from the surroundings, so the temperature drops. The other changes give out energy.

Correct: (A)

Energy taken in breaking bonds = 436 + 193 = 629 kJ. Energy released forming bonds = 2 × 366 = 732 kJ. Energy change = 629 – 732 = -103 kJ/mol.

Correct: (B)

In an exothermic reaction energy is given out, so the products end at a lower energy level than the reactants; the difference is the energy released.

Correct: (D)

A catalyst provides an alternative pathway of lower activation energy, so a greater proportion of collisions lead to reaction.

Correct: (A)

Hydrogen burns in oxygen to form only water, so it does not release carbon dioxide or other pollutants at the point of use.

Correct: (B)

Powder exposes far more surface, so acid particles can collide with the solid much more frequently, increasing the rate.

Correct: (C)

As carbon dioxide escapes, the total mass falls; recording mass against time gives a direct measure of the rate of reaction.

Correct: (C)

There are 3 moles of gas on the left and 2 moles on the right. Increasing the pressure favours the side with fewer gas molecules, so equilibrium shifts to the right.

Correct: (B)

Lower temperatures favour the forward exothermic reaction, but the rate becomes very slow, so a compromise temperature of about 450°C is used with a vanadium(V) oxide catalyst.

Correct: (A)

Carbon monoxide gains oxygen (and loses electrons overall) as it converts iron(III) oxide to iron, so CO is the reducing agent; the iron(III) oxide is reduced.

Correct: (D)

Oxidation is gain of oxygen or loss of electrons. Magnesium gains oxygen and its atoms lose electrons to form Mg^2+, so it is oxidised.

Correct: (B)

A higher concentration means more reacting particles per unit volume, so successful collisions happen more often.

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: (D)

Titration is used when both the acid and the alkali are soluble and no insoluble excess reactant can be added, as for potassium chloride from KOH(aq) and HCl(aq).

Correct: (B)

Sulfur dioxide is an acidic oxide; it dissolves to give an acidic solution, one cause of acid rain which damages buildings and trees.

Correct: (C)

Carbonates react with dilute acids to release carbon dioxide, which is identified by turning limewater milky (cloudy).

Correct: (D)

Basic oxides are oxides of metals that neutralise acids, forming a salt and water. Calcium oxide solution (limewater) is also alkaline.

Correct: (A)

Ammonium salts release ammonia gas on warming with an alkali. Ammonia is alkaline and turns damp red litmus blue; this is the test for ammonium ions.

Correct: (B)

For an insoluble base, excess solid is added to make sure all the acid reacts; the leftover solid is filtered off, and the filtrate is heated and left to crystallise.

Correct: (C)

Halogen reactivity decreases down Group VII because the atoms gain an electron less easily as the outer shell is further from the nucleus. Halogens exist as diatomic molecules.

Correct: (D)

The melting points of the alkali metals fall down the group (Li 180°C, Na 98°C, K 63°C) as the metallic bonding weakens with larger atoms.

Correct: (A)

Chlorine, being higher in Group VII and more reactive, displaces iodine from potassium iodide: Cl2 + 2KI → 2KCl + I2. The iodine formed colours the solution brown.

Correct: (A)

Iron catalyses the Haber process for ammonia, and nickel catalyses the hydrogenation of oils to make margarine. Both are transition elements.

Correct: (B)

Carbon sits above zinc in the reactivity series, so on heating it reduces zinc oxide, taking the oxygen to form carbon oxides and leaving zinc metal.

Correct: (D)

Aluminium is above carbon in the reactivity series, so its oxide is too stable to be reduced by carbon; electrolysis of the molten ore must be used instead.

Correct: (A)

Stainless steel contains chromium, which forms a protective oxide layer that stops the iron underneath from rusting.

Correct: (B)

Zinc is higher in the reactivity series, so it reacts with air and water preferentially, sacrificing itself while the iron stays protected.

Correct: (C)

Carbon dioxide, released in large amounts by burning fossil fuels, traps heat radiation from the Earth and is the main human-made greenhouse gas.

Correct: (C)

Sulfur dioxide from coal-burning power stations and nitrogen oxides from car exhausts dissolve in rain clouds to form acid rain.

Correct: (A)

Carbon monoxide is a toxic, colourless, odourless gas that combines with haemoglobin in red blood cells, stopping them carrying oxygen around the body.

Correct: (B)

The fractions with the highest boiling points and longest molecules, such as bitumen, collect near the bottom of the column where it is hottest; bitumen is used for roads and roofing.

Correct: (C)

Alkenes contain one C=C double bond and have the general formula CnH2n, e.g. ethene C2H4 and propene C3H6.

Correct: (A)

Cracking splits one long molecule into a shorter alkane and an alkene: C10H22 → C8H18 + C2H4. The atoms balance (C: 10 = 8 + 2; H: 22 = 18 + 4).

Correct: (D)

Ethanoic acid is a typical (weak) acid: with a reactive metal it forms a salt, magnesium ethanoate, plus hydrogen gas, though more slowly than a strong acid.

Correct: (C)

Condensation polymerisation links monomers by reactions that eliminate a small molecule, usually water, for each link formed — unlike addition polymerisation, where nothing is lost.

Correct: (C)

Anhydrous copper(II) sulfate is white; adding water converts it to the blue hydrated salt, CuSO4.5H2O. (Blue cobalt(II) chloride paper turning pink is an alternative test.)

Correct: (D)

Potassium compounds give a lilac (purple) flame in a flame test. Sodium gives yellow, calcium brick-red, and copper(II) blue-green.