IGCSE Chemistry 0620/61 May/June 2025 Past Paper with Answers

Question 1

A burette is used to add dilute sulfuric acid to aqueous potassium carbonate until the end-point is reached. The final burette reading is 18.8 cm³ and the initial burette reading is 4.2 cm³. What volume of dilute sulfuric acid was added?

1. 14.6 cm³, found from 18.8 cm³ − 4.2 cm³
2. 23.0 cm³, found from 18.8 cm³ + 4.2 cm³
3. 4.2 cm³, which is just the initial burette reading
4. 18.8 cm³, which is just the final burette reading

Question 2

Potassium carbonate and sulfuric acid react in a 1:1 mole ratio: $\text{K}_2\text{CO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{K}_2\text{SO}_4 + \text{H}_2\text{O} + \text{CO}_2$. A volume of 25.0 cm³ of aqueous potassium carbonate reacts exactly with 14.6 cm³ of dilute sulfuric acid. Which solution is the least concentrated, and why?

1. the potassium carbonate, because the same number of moles is spread through the larger volume (25.0 cm³)
2. the sulfuric acid, because it was the solution measured from the burette
3. the two solutions must have exactly the same concentration as each other
4. it is impossible to compare the concentrations from the volumes given

Question 3

A student needs to measure exactly 25.0 cm³ of aqueous potassium carbonate into a conical flask. Which piece of apparatus measures this fixed volume most accurately?

1. a volumetric pipette
2. a 250 cm³ beaker
3. a conical flask
4. a dropping pipette (teat pipette)

Question 4 · Answer A

Step 1: Methyl orange is a suitable indicator for an acid being added to an alkaline carbonate. Step 2: In the alkaline solution it is yellow, and at the end-point the solution becomes acidic, turning the methyl orange orange. Step 3: So a suitable indicator is methyl orange, changing from yellow to orange at the end-point.

Question 5 · Answer A

Step 1: As the acid is added it must be mixed evenly with the carbonate so the reaction reaches the end-point fairly. Step 2: Swirling the conical flask mixes the two solutions thoroughly. Step 3: So the student should swirl (mix) the contents of the conical flask.

Question 6 · Answer A

Step 1: Repeat the reaction with the known volumes (25.0 cm³ carbonate and 14.6 cm³ acid) but without indicator, so the salt is not contaminated by the indicator. Step 2: Warm the solution in an evaporating basin until it reaches the crystallisation point, when about half the water has gone and crystals begin to form. Step 3: Leave the solution to cool so that pure crystals of potassium sulfate form, then filter them off and dry them.

Question 7 · Answer A

Step 1: The stop-watch shows 0 minutes and 20 seconds, so no whole minute has passed. Step 2: The total time is therefore just 20 seconds, which is already a whole number of seconds. Step 3: So the time recorded for Experiment 1 is 20 s.

Question 8 · Answer A

Step 1: A good graph uses a linear scale (equal steps) chosen so the plotted points fill more than half of the available grid. Step 2: Every result is plotted accurately, then a single smooth line of best fit is drawn through the trend. Step 3: So the correct approach is a linear y-axis scale filling over half the grid, all points plotted accurately and one line of best fit.

Question 9 · Answer A

Step 1: To read an intermediate value, find 1.3 mol/dm³ on the concentration axis. Step 2: Draw a vertical construction line up to the line of best fit, then a horizontal line across to the time axis. Step 3: The value where the horizontal line meets the time axis is the answer, so you draw a vertical line up from 1.3 then read horizontally across to the time axis.

Question 10 · Answer A

Step 1: Mean rate = length ÷ time = 5 cm ÷ 20 s. Step 2: 5 ÷ 20 = 0.25, and the unit is length over time, so cm/s. Step 3: So the mean rate of reaction is 0.25 cm/s.

Question 11 · Answer A

Step 1: A lower concentration of acid gives fewer collisions per second and so a slower reaction. Step 2: Experiment 5 uses the lowest concentration, 0.5 mol/dm³. Step 3: So the mean rate of reaction is slowest in Experiment 5.

Question 12 · Answer A

Step 1: Repeating an experiment lets you compare the results from each run. Step 2: Close (concordant) results show the data is repeatable, and any value that differs a lot can be spotted as an anomaly and left out. Step 3: So repeating each experiment lets you check the results are repeatable and identify anomalous values.

Question 13 · Answer A

Step 1: The temperature change is the final temperature minus the initial temperature. Step 2: That is 46.5 °C − 24.0 °C = 22.5 °C, written to the same one decimal place as the readings. Step 3: So the temperature change is 22.5 °C.

Question 14 · Answer A

Step 1: Temperature is a factor that affects the rate of reaction: a higher temperature speeds the reaction up. Step 2: If the temperature varied between experiments, the rate would change for that reason as well as the concentration, so the comparison would be unfair. Step 3: So keeping the temperature constant is an improvement because a change in temperature would change the rate of reaction.

Question 15 · Answer A

Step 1: The reaction of magnesium with acid is exothermic, so it releases heat. Step 2: Polystyrene is a good insulator, so it prevents that heat escaping to the surroundings. Step 3: With the heat trapped, the temperature still rises, so a polystyrene cup does not keep the temperature constant because it prevents heat escaping.

Question 16 · Answer A

Step 1: To keep a temperature constant you surround the reaction vessel with something held at that temperature. Step 2: A water bath kept at a fixed temperature absorbs or supplies heat to hold the acid at that temperature. Step 3: So you stand the conical flask in a water bath kept at a constant temperature.

Question 17 · Answer A

Step 1: A carbonate reacts with dilute acid to give a salt, water and carbon dioxide, so you see effervescence as the gas bubbles off. Step 2: Carbon dioxide turns limewater milky. Step 3: So the observations are effervescence and the gas turning limewater milky.

Question 18 · Answer A

Step 1: Acidified barium nitrate gives a white precipitate of barium sulfate if sulfate ions are present. Step 2: Here no precipitate forms, so there are no sulfate ions in solution H. Step 3: So the observation is no change, showing that sulfate ions are absent.

Question 19 · Answer A

Step 1: Adding sodium hydroxide to a calcium salt gives a white precipitate of calcium hydroxide. Step 2: Calcium hydroxide is insoluble in excess sodium hydroxide, so the precipitate stays when more is added. Step 3: So a white precipitate forms dropwise and it remains insoluble in excess.

Question 20 · Answer A

Step 1: Acidified silver nitrate gives a precipitate whose colour identifies the halide: white for chloride, cream for bromide, yellow for iodide. Step 2: Solution H was made using hydrochloric acid, so it contains chloride ions, which give a white precipitate. Step 3: So the observation is a white precipitate, showing chloride ions are present.

Question 21 · Answer A

Step 1: A lilac flame shows a potassium ion. Step 2: A cream precipitate with silver nitrate, and an orange colour when chlorine displaces it, both show a bromide ion. Step 3: Combining a potassium ion with a bromide ion gives potassium bromide, $\text{KBr}$.

Question 22 · Answer A

Step 1: To compare fairly, use the same known mass of sodium hydrogencarbonate and the same measured volume of each acid, measured with suitable apparatus in a suitable container. Step 2: Measure the temperature of the acid before mixing, add and stir, then measure the temperature after the reaction. Step 3: Endothermic reactions take in heat, so the temperature falls; the reaction with the larger temperature decrease is the more endothermic.