IGCSE Chemistry 0620/12 October/November 2025 Past Paper with Answers

Question 1

Water can exist as solid ice, liquid water and steam. In which of these states can a sample of water easily be compressed to half of its original volume?

1. steam only
2. ice, liquid water and steam
3. liquid water and steam only
4. liquid water only

Question 2

The melting point of sulfur is 113 °C. A sample of sulfur is kept at 25 °C, so it is a solid. Which row describes both the arrangement and the movement of the particles in this sample of sulfur?

1. the particles are in a regular arrangement and vibrate about fixed positions
2. the particles are in a regular arrangement and move randomly
3. the particles are in a random arrangement and vibrate about fixed positions
4. the particles are in a random arrangement and move randomly

Question 3

A solid, Y, is added to an excess of hot water. A blue solution forms and a brown solid remains. The brown solid is filtered off, dried, and is found to conduct electricity. What is Y?

1. a mixture that contains a metal
2. a compound that contains a metal
3. a pure substance that is a metal
4. a pure substance that is a non-metal

Question 4 · Answer A

Step 1: A neutron has a relative mass of 1 and carries no charge. Step 2: A proton has a relative mass of 1 but a positive charge, not a negative one. Step 3: An electron has almost no mass (about 1/2000) and a negative charge, so only the statement about neutrons is correct.

Question 5 · Answer A

Step 1: An argon atom (proton number 18) has 18 electrons arranged 2,8,8 and argon-40 has nucleon number 40, so student 1 is correct. Step 2: A chlorine atom has only 17 electrons (2,8,7) and a nucleon number of about 35, so student 2 is wrong. Step 3: Calcium has 20 electrons, but a Ca²⁺ ion has lost 2 to leave 18 electrons (2,8,8), and calcium-40 has nucleon number 40, so student 3 is correct.

Question 6 · Answer A

The term for different atoms of the same element that have the same number of protons but different numbers of neutrons is isotopes.

Question 7 · Answer A

Step 1: Q has 2 electrons in its outer shell, so it loses them to form a Q²⁺ ion. Step 2: R has 6 electrons in its outer shell, so it gains 2 electrons to form an R²⁻ ion. Step 3: The 2+ and 2− charges balance in a 1:1 ratio, so the formula is RQ.

Question 8 · Answer A

Step 1: Simple molecular substances have weak forces between their molecules, so they have low melting and boiling points (property 3 is correct). Step 2: They have no free electrons or ions, so they do not conduct electricity. Step 3: Malleability is a property of metals, not simple molecules, so only property 3 is typical.

Question 9 · Answer A

Step 1: In diamond each carbon atom forms four strong covalent bonds, giving a rigid giant structure that is very hard. Step 2: All four outer electrons of every carbon are used in bonding, so there are no delocalised (free) electrons. Step 3: With no free electrons to carry charge, diamond does not conduct electricity.

Question 10 · Answer A

Step 1: Complete combustion of a hydrocarbon produces only carbon dioxide and water. Step 2: Balancing $2\text{C}_4\text{H}_{10}$ gives 8 carbon dioxide and 10 water molecules. Step 3: This needs 13 oxygen molecules, giving $2\text{C}_4\text{H}_{10} + 13\text{O}_2 \rightarrow 8\text{CO}_2 + 10\text{H}_2\text{O}$.

Question 11 · Answer A

Step 1: Add the relative atomic mass of calcium (40) and of sulfur (32) to give 72. Step 2: There are four oxygen atoms, so add $4 \times 16 = 64$. Step 3: The total is $40 + 32 + 64 = 136$.

Question 12 · Answer A

Step 1: When a molten binary salt is electrolysed, the metal is produced at the cathode, so silver forms at the cathode. Step 2: The non-metal is produced at the anode, so iodine forms at the anode. Step 3: There is no water present, so no hydrogen or oxygen is formed.

Question 13 · Answer A

Step 1: Electrolysing dilute sulfuric acid produces oxygen at the anode and hydrogen at the cathode; both gases are colourless. Step 2: Twice as much hydrogen is made as oxygen, so the volume of gas at the cathode is double that at the anode. Step 3: This gives 10 cm³ of colourless gas at the anode and 20 cm³ of colourless gas at the cathode.

Question 14 · Answer A

Step 1: An exothermic reaction releases heat, so the temperature rises; the most exothermic gives the biggest rise. Step 2: Experiments A and C have temperatures that fall, so they are endothermic and can be ignored. Step 3: Experiment B rises from 20 °C to 32 °C (a rise of 12 °C) while experiment D rises by only 9 °C, so B has the largest rise and is the most exothermic.

Question 15 · Answer A

Step 1: Dissolving sodium chloride in water makes no new substance and the salt can be recovered by evaporation, so it is a physical change. Step 2: An iron nail in water rusts, forming a new substance, which is a chemical change. Step 3: Zinc with acid and calcium carbonate with acid both produce gases (new substances), so those are chemical changes; only test-tube A is physical.

Question 16 · Answer A

Step 1: Measuring a rate always needs timing, so a stop-watch is essential. Step 2: To study the effect of temperature you must measure the temperature, so a thermometer is essential. Step 3: A balance or gas syringe is only useful when the reaction changes mass or makes a gas, so they are not needed for every reaction.

Question 17 · Answer A

Step 1: Heating turns pink into blue, and adding water turns blue back into pink, so the change goes both ways and is reversible. Step 2: The pink form contains water (it gives off water when heated), so it is hydrated. Step 3: Therefore the reaction is reversible and the pink cobalt(II) chloride is hydrated.

Question 18 · Answer A

Step 1: Each oxygen has an oxidation number of −2, and there are three oxygens, giving a total of −6. Step 2: The compound is neutral, so the two iron atoms must total +6. Step 3: That means each iron atom has an oxidation number of +6 ÷ 2 = +3.

Question 19 · Answer A

Step 1: Acids react with metal oxides, carbonates and hydroxides to form a salt, so copper(II) oxide, copper(II) carbonate and copper(II) hydroxide all give copper(II) chloride. Step 2: Copper metal is below hydrogen in the reactivity series, so it does not react with dilute hydrochloric acid. Step 3: Therefore only substances 2, 3 and 4 react.

Question 20 · Answer A

Step 1: With X alone, methyl orange is red and thymolphthalein is colourless, which shows X is acidic. Step 2: After adding excess Y, thymolphthalein turns blue and methyl orange turns yellow, which are the colours in an alkaline solution. Step 3: Since Y changes the solution from acidic to alkaline, Y must be an alkali, sodium hydroxide.

Question 21 · Answer A

Step 1: Filtration separates a soluble solid (which dissolves and passes through) from an insoluble solid (which stays on the filter). Step 2: Ammonium chloride is soluble and silver chloride is insoluble, so they can be separated this way. Step 3: The other pairs are either both insoluble (sulfates, hydroxides) or both soluble (nitrates), so filtration cannot separate them.

Question 22 · Answer A

Step 1: Metals are found on the left of the Periodic Table; they lose electrons to form positive ions. Step 2: Non-metals are found on the right; they gain electrons to form negative ions. Step 3: So the left is metallic forming positive ions and the right is non-metallic forming negative ions.

Question 23 · Answer A

Step 1: Going down Group I, the density of the metals generally increases. Step 2: The melting point decreases down the group, not increases. Step 3: The reactivity increases down the group and each atom has more electrons, so only the density increasing is correct.

Question 24 · Answer A

Step 1: At room temperature and pressure iodine is a grey-black solid. Step 2: Chlorine is a yellow-green gas, not colourless and not a liquid. Step 3: Iodine is a solid at room temperature (it only gives a purple vapour when warmed), so the grey-black solid description is correct.

Question 25 · Answer A

Step 1: Transition elements have high melting points and high densities. Step 2: They typically form coloured compounds. Step 3: So osmium should have a high melting point, a high density and form coloured compounds.

Question 26 · Answer A

Step 1: Neon is a noble gas in Group VIII (Group 0), so its atoms have a full outer shell of electrons. Step 2: A full outer shell makes neon very unreactive, so it does not react with oxygen. Step 3: Because it is so stable, neon exists as single atoms, not diatomic molecules.

Question 27 · Answer A

Step 1: Producing hydrogen with dilute acid and forming basic oxides are reactions of a reactive metal. Step 2: Magnesium is a reactive metal, so it reacts with acid, steam and oxygen in these ways. Step 3: Phosphorus and silicon are non-metals (they form acidic oxides) and silver is too unreactive to react with acid or steam, so G is magnesium.

Question 28 · Answer A

Step 1: Aluminium has a low density and conducts electricity well. Step 2: These two properties make it ideal for overhead power cables, which must be light and conduct. Step 3: The other rows give a wrong property, such as a high density for aluminium or low ductility for copper, so they do not match.

Question 29 · Answer A

Step 1: A barrier method covers the iron to keep out water and oxygen. Step 2: Coating with plastic, greasing and painting all form a physical layer over the surface, so they are barrier methods. Step 3: Alloying changes the metal itself (for example making stainless steel) rather than forming a surface barrier, so it is not a barrier method.

Question 30 · Answer A

Step 1: Thermal decomposition is when a single compound breaks down into simpler substances on heating. Step 2: Calcium carbonate breaks down on heating into calcium oxide and carbon dioxide, which fits this definition. Step 3: The other reactions involve two reactants combining or a reduction (redox), so they are not thermal decompositions.

Question 31 · Answer A

Step 1: Clean, dry air is about 78% nitrogen and 21% oxygen. Step 2: It also contains a small amount of carbon dioxide (about 0.04%). Step 3: Carbon monoxide is a pollutant and is not a normal part of clean air, so the correct row is nitrogen 78%, oxygen 21% and a small amount of carbon dioxide.

Question 32 · Answer A

Step 1: Microbes are tiny living organisms such as bacteria. Step 2: Sewage contains harmful microbes that can cause disease. Step 3: Metal compounds, nitrates and phosphates are chemical pollutants, not living microbes, so the answer is sewage.

Question 33 · Answer A

Step 1: Digestion in animals produces methane, so gas X is methane. Step 2: Burning methane with too little oxygen (incomplete combustion) makes carbon monoxide, so gas Y is carbon monoxide. Step 3: Burning methane with plenty of oxygen (complete combustion) makes carbon dioxide, so gas Z is carbon dioxide.

Question 34 · Answer A

Step 1: Photosynthesis uses carbon dioxide and water, with light energy, to make glucose and oxygen. Step 2: It releases oxygen, so it increases (not decreases) the oxygen in the atmosphere. Step 3: It removes carbon dioxide (a greenhouse gas) and does not need such high temperatures, so the correct statement is that it is a reaction between carbon dioxide and water.

Question 35 · Answer A

Step 1: Decolourising aqueous bromine shows series L is unsaturated, so L is the alkenes with general formula $\text{C}_n\text{H}_{2n}$. Step 2: A pH below 7 shows series M is acidic, so M is the carboxylic acids with general formula $\text{C}_n\text{H}_{2n+1}\text{COOH}$. Step 3: Alcohols ($\text{C}_n\text{H}_{2n+1}\text{OH}$) are neutral and alkanes ($\text{C}_n\text{H}_{2n+2}$) do not decolourise bromine, so the correct pair is $\text{C}_n\text{H}_{2n}$ and $\text{C}_n\text{H}_{2n+1}\text{COOH}$.

Question 36 · Answer A

Step 1: A hydrocarbon contains only carbon and hydrogen, which rules out structure B because it contains oxygen. Step 2: Unsaturated means there is a carbon to carbon double bond. Step 3: Only structure D has a carbon to carbon double bond (it is ethene), so structure D is the unsaturated hydrocarbon; A and C are saturated.

Question 37 · Answer A

Step 1: Petroleum is a mixture of hydrocarbons. Step 2: Hydrocarbons are compounds that contain only carbon and hydrogen. Step 3: Oxygen is not present, so the elements in all of these compounds are carbon and hydrogen only.

Question 38 · Answer A

Step 1: A fuel is a substance that is burned to release useful energy. Step 2: Ethanol releases energy when it is burned, which is exactly why it is chosen as a fuel. Step 3: Being colourless, having a low density, or forming carbon dioxide do not explain why it is useful as a fuel.

Question 39 · Answer A

Step 1: A white precipitate with acidified silver nitrate shows the chloride ion is present (bromide would give a cream precipitate). Step 2: A green precipitate with sodium hydroxide that dissolves in excess shows chromium(III) ions (iron(II) gives a green precipitate that does not dissolve in excess). Step 3: Putting the two results together, the solid is chromium(III) chloride.

Question 40 · Answer A

Step 1: In chromatography, a dye in the mixture gives a spot at the same height as its pure reference sample. Step 2: W has two spots, one matching P and one matching S, so W contains P and S. Step 3: The spots of Q and R do not line up with any of W's spots, so they are not present; W contains P and S only.