IGCSE Chemistry 0620/12 May/June 2025 Past Paper with Answers

Question 1

Substance L melts at -7 °C and is a brown liquid at room temperature (about 20 °C). A pure substance boils at one definite temperature, not over a range. What is the boiling point of pure L?

1. 59 °C
2. -77 °C
3. -7 °C to +7 °C
4. 107 °C to 117 °C

Question 2

A fixed mass of gas is studied in two separate changes. In the first change the temperature of the gas is increased while the pressure is kept constant. In the second change the pressure on the gas is increased while the temperature is kept constant. Which row correctly describes how the volume of the gas changes in each case?

1. when the temperature is increased at constant pressure the volume increases, and when the pressure is increased at constant temperature the volume decreases
2. when the temperature is increased at constant pressure the volume decreases, and when the pressure is increased at constant temperature the volume decreases
3. when the temperature is increased at constant pressure the volume decreases, and when the pressure is increased at constant temperature the volume increases
4. when the temperature is increased at constant pressure the volume increases, and when the pressure is increased at constant temperature the volume increases

Question 3

A lower gas jar is filled with red-brown nitrogen dioxide gas, which is denser than air, and an upper gas jar is filled with colourless air. The two jars are placed mouth to mouth, separated by a glass plate. The glass plate is then removed and the jars are left for 24 hours. What is the appearance of the two gas jars after 24 hours?

1. both jars are filled with an even, pale red-brown colour because the gases have mixed completely
2. the lower jar stays red-brown and the upper jar stays colourless, with no mixing
3. the red-brown colour moves entirely into the upper jar, leaving the lower jar colourless
4. a sharp boundary remains, with red-brown gas only in the lower half

Question 4 · Answer A

Step 1: Oxygen ($\text{O}_2$) is an element and distilled water ($\text{H}_2\text{O}$) is a compound, mixed together. Step 2: Glucose dissolved in water is two compounds (glucose and water), and clean dry air is a mixture of several gases. Step 3: Stainless steel is a mixture of metal elements, so only oxygen in water is one element plus one compound.

Question 5 · Answer A

Step 1: The atomic number, or proton number, counts the protons in the nucleus of an atom. Step 2: The total number of particles in the nucleus is the nucleon number, not the atomic number. Step 3: So the atomic number is the number of protons in the nucleus.

Question 6 · Answer A

Step 1: Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. Step 2: A different number of neutrons changes the nucleon number (protons + neutrons). Step 3: So isotopes differ in nucleon number.

Question 7 · Answer A

Step 1: A chlorine atom has 7 outer electrons and needs one more to fill its outer shell. Step 2: It gains one electron from sodium to become a chloride ion with a 1- charge. Step 3: So each chlorine atom gains one electron.

Question 8 · Answer A

Step 1: Carbon in methane forms four bonds, using all four of its outer electrons. Step 2: Hydrogen has only one outer electron and uses it in its single bond. Step 3: Chlorine in HCl and oxygen in water keep non-bonding pairs, so only carbon and hydrogen use all their outer electrons.

Question 9 · Answer A

Step 1: A giant covalent structure is a large network of atoms joined by covalent bonds. Step 2: Graphite is a giant covalent structure made of carbon atoms. Step 3: Sodium chloride and magnesium chloride are giant ionic, and ammonia is a small molecule, so graphite is correct.

Question 10 · Answer A

Step 1: Counting atoms, molecule 1 is $\text{C}_4\text{H}_8$, while molecules 2, 3 and 4 are each $\text{C}_3\text{H}_6\text{O}$. Step 2: So molecules 2, 3 and 4 share the same molecular formula, $\text{C}_3\text{H}_6\text{O}$, and molecule 1 is different. Step 3: That means exactly three of the molecules have the same molecular formula.

Question 11 · Answer A

Step 1: Moles of magnesium $= 6 \div 24 = 0.25$ mol. Step 2: The equation shows 2 mol of magnesium reacts with 1 mol of $\text{O}_2$, so 0.25 mol of Mg reacts with 0.125 mol of $\text{O}_2$. Step 3: Mass of oxygen $= 0.125 \times 32 = 4$ g.

Question 12 · Answer A

Step 1: The electrode connected to the positive terminal is the anode, so X is the anode and Y (negative) is the cathode. Step 2: A salt only conducts electricity when it is molten or dissolved (aqueous), because the ions are then free to move. Step 3: So Z is an aqueous salt, giving X anode, Y cathode, Z aqueous salt.

Question 13 · Answer A

Step 1: A hydrogen-oxygen fuel cell reacts hydrogen with oxygen to make water and release energy. Step 2: The balanced equation is $2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$. Step 3: The other equations make hydrogen peroxide or split water, so they are wrong.

Question 14 · Answer A

Step 1: A rise in the temperature of the surroundings means the reaction is exothermic and gives out energy. Step 2: In an energy level diagram an exothermic reaction has products below reactants, and the larger the drop the more energy is released. Step 3: So the diagram with the largest downward drop from reactants to products gives the largest temperature increase.

Question 15 · Answer A

Step 1: A chemical change makes one or more new substances. Step 2: Neutralising copper(II) oxide with an acid forms a new salt and water, so it is a chemical change. Step 3: Dissolving, distilling and freezing only change state or separate substances, so they are physical changes.

Question 16 · Answer A

Step 1: On a volume against time graph, the rate of reaction is shown by the steepness (gradient) of the curve. Step 2: The curve is steepest at the very start, when the reactants are most concentrated. Step 3: So the point at the start where the curve is steepest shows the fastest rate.

Question 17 · Answer A

Step 1: Two half-arrows pointing in opposite directions are the symbol for a reversible reaction. Step 2: They show the reaction can go both forwards and backwards. Step 3: So the symbol means the reaction is reversible.

Question 18 · Answer A

Step 1: A redox reaction involves both oxidation and reduction (a transfer of oxygen or electrons). Step 2: Incomplete combustion of propane and rusting of iron both involve oxygen being gained, so they are redox. Step 3: The silver nitrate test is a precipitation reaction and the decomposition of calcium carbonate is not redox, so the answer is 1 and 2.

Question 19 · Answer A

Step 1: A pH of 14 is strongly alkaline, and alkaline solutions contain hydroxide ions. Step 2: A pH of 1 is strongly acidic (not alkaline), pH 5 is acidic (not neutral), and pH 9 is alkaline so it turns universal indicator blue or purple, not yellow. Step 3: So only the pH 14 solution containing hydroxide ions is correct.

Question 20 · Answer A

Step 1: Basic oxides are formed by metals, so we need a part of the table that contains only metals. Step 2: Group I contains only reactive metals, which all form basic oxides. Step 3: Period 3 also contains non-metals (acidic oxides) and Group VIII are unreactive noble gases that do not form these oxides, so Group I only is correct.

Question 21 · Answer A

Step 1: Group VII atoms have 7 outer electrons and gain one electron to form ions, so their charge is $7 - 8 = -1$, matching the rule. Step 2: Elements in a group share the same number of outer electrons but not the same full configuration. Step 3: Metallic character decreases across a period and the table is ordered by proton number, so only the Group VII statement is correct.

Question 22 · Answer A

Step 1: Going down Group I the melting point decreases. Step 2: The density generally increases down the group. Step 3: The reactivity increases down the group, so the trends are melting point decreasing, density increasing, reactivity increasing.

Question 23 · Answer A

Step 1: The halogens are all diatomic molecules, so statement 1 is correct. Step 2: They are not all gases (bromine is a liquid and iodine is a solid at room temperature), so statement 2 is wrong. Step 3: Density increases down the group (statement 3 correct) but reactivity decreases down the group (statement 4 wrong), so the answer is 1 and 3.

Question 24 · Answer A

Step 1: Transition elements are metals with high densities and high melting points. Step 2: They conduct heat and electricity and often form coloured compounds. Step 3: So a correct property of zirconium is its high density.

Question 25 · Answer A

Step 1: Argon and krypton are noble gases that exist as single atoms, are unreactive and have eight outer electrons. Step 2: Helium is a noble gas but has only two outer electrons, not eight, so it does not fit. Step 3: Oxygen exists as $\text{O}_2$ molecules and is reactive, so the answer is 2 and 4 only.

Question 26 · Answer A

Step 1: Calcium is above magnesium in the reactivity series, so it reacts faster with cold water (statement 1 correct). Step 2: All metals conduct electricity, so potassium and silver are both good conductors (statement 4 correct). Step 3: Aluminium is harder to extract than iron and copper does not react with dilute acid, so statements 2 and 3 are wrong, giving 1 and 4.

Question 27 · Answer A

Step 1: The more bubbles of hydrogen a metal produces with acid, the more reactive it is. Step 2: P gives the most bubbles, then Q (few), then S (very few), and R gives none. Step 3: So from most to least reactive the order is P, Q, S, R.

Question 28 · Answer A

Step 1: Iron needs both water and oxygen to rust. Step 2: Only test-tube 1 has both water and air present. Step 3: Test-tube 2 has no water and test-tube 3 has no oxygen (boiled water under oil), so only the nail in test-tube 1 rusts.

Question 29 · Answer A

Step 1: An alloy contains atoms of different sizes, which disrupt the regular layers of atoms. Step 2: This makes it harder for the layers to slide, so the alloy is harder and keeps its shape better than the pure metal. Step 3: So alloys are used because they are harder and stronger.

Question 30 · Answer A

Step 1: Air is about 21% oxygen, so 100 $\text{cm}^3$ of air contains about 21 $\text{cm}^3$ of oxygen. Step 2: The heated copper removes the oxygen, leaving mainly nitrogen. Step 3: So the volume left is about $100 - 21 = 79$ $\text{cm}^3$.

Question 31 · Answer A

Step 1: Acid rain is caused by acidic gases dissolving in rainwater. Step 2: Both sulfur dioxide and oxides of nitrogen form acids in rainwater, so both contribute to acid rain. Step 3: Carbon monoxide is a toxic gas but does not cause acid rain, so the answer is oxides of nitrogen and sulfur dioxide.

Question 32 · Answer A

Step 1: Compounds 1 ($\text{C}_2\text{H}_4$) and 2 ($\text{C}_4\text{H}_8$) both fit the general formula $\text{C}_n\text{H}_{2n}$, the alkenes. Step 2: Compound 3 ($\text{C}_4\text{H}_{10}$) fits $\text{C}_n\text{H}_{2n+2}$, the alkanes. Step 3: So only 1 and 2 are in the same homologous series.

Question 33 · Answer A

Step 1: Gasoline (petrol) is separated from petroleum, so the answer to the first question is yes. Step 2: Gasoline is used as a fuel for cars, so the answer to the second question is also yes. Step 3: Following yes then yes through the flowchart leads to outcome A.

Question 34 · Answer A

Step 1: Aqueous bromine is only decolourised by an unsaturated hydrocarbon (one with a carbon-carbon double bond). Step 2: Methane and ethane have only single bonds (saturated), so the bromine stays orange. Step 3: Ethene has a double bond (unsaturated), so it changes the bromine from orange to colourless.

Question 35 · Answer A

Step 1: Ethene reacts with steam in an addition reaction using a catalyst. Step 2: The water adds across the double bond to form ethanol. Step 3: So the product is ethanol.

Question 36 · Answer A

Step 1: Ethanoic acid is an acid, so its aqueous solution has a pH below 7, not 10, making statement 1 wrong. Step 2: Like other acids, it reacts with metal carbonates to give carbon dioxide (statement 2 correct). Step 3: It also reacts with reactive metals such as magnesium to give hydrogen (statement 3 correct), so the answer is 2 and 3 only.

Question 37 · Answer A

Step 1: Poly(ethene) is a very unreactive polymer that is not broken down by natural processes. Step 2: Because it does not decompose in water, it stays in the environment for a very long time. Step 3: So the bags build up in the oceans because poly(ethene) does not decompose in water.

Question 38 · Answer A

Step 1: The sample spot must stay out of the solvent at the start. Step 2: If the solvent is at or above the spot, the sample dissolves straight into the solvent and washes away instead of separating up the paper. Step 3: So the solvent level must start below the sample spot.

Question 39 · Answer A

Step 1: Liquids with different boiling points can be separated by boiling them off one at a time. Step 2: Fractional distillation uses a fractionating column to separate liquids by their boiling points. Step 3: So fractional distillation is the correct process.

Question 40 · Answer A

Step 1: Sodium ions give no precipitate with aqueous sodium hydroxide, so there is no visible reaction. Step 2: Chloride ions give a white precipitate of silver chloride with acidified aqueous silver nitrate. Step 3: So sodium chloride gives no visible reaction with sodium hydroxide and a white precipitate with silver nitrate.