IGCSE Chemistry 0620/41 October/November 2025 Past Paper with Answers

Question 1

A proton has a relative mass of 1 and a relative charge of +1. What are the relative masses and relative charges of a neutron and an electron?

1. neutron: relative mass 1, relative charge 0; electron: relative mass $\tfrac{1}{1840}$, relative charge -1
2. neutron: relative mass 1, relative charge -1; electron: relative mass $\tfrac{1}{1840}$, relative charge 0
3. neutron: relative mass $\tfrac{1}{1840}$, relative charge 0; electron: relative mass 1, relative charge -1
4. neutron: relative mass 0, relative charge +1; electron: relative mass 1, relative charge -1

Question 2

Which statement correctly defines isotopes?

1. atoms of the same element that have the same number of protons but different numbers of neutrons
2. atoms of different elements that have the same number of neutrons but different numbers of protons
3. atoms of the same element that have the same number of neutrons but different numbers of protons
4. atoms of the same element that have different numbers of electrons but the same number of neutrons

Question 3

Why do all isotopes of the same element have the same chemical properties?

1. they have the same number of electrons and so the same electronic configuration
2. they have the same number of neutrons in the nucleus
3. they have the same nucleon (mass) number
4. they have different numbers of protons in the nucleus

Question 4 · Answer A

Step 1: The number of protons is the proton number, and 22 protons means the element is titanium (Ti). Step 2: The nucleon number is protons + neutrons = 22 + 28 = 50. Step 3: There are 2 fewer electrons (20) than protons (22), so the particle has lost 2 electrons and carries a 2+ charge, giving $\text{Ti}^{2+}$.

Question 5 · Answer A

Nucleon number means the total number of protons and neutrons (nucleons) in the nucleus, so it is another name for mass number.

Question 6 · Answer A

Step 1: Number of moles of argon = mass / $M_r$ = 2.00 / 40 = 0.0500 mol. Step 2: Number of atoms = moles x Avogadro constant = 0.0500 x $6.02\times10^{23}$. Step 3: This gives $3.01\times10^{22}$ atoms.

Question 7 · Answer A

Step 1: The number of occupied electron shells equals the period (row) number in the Periodic Table. Step 2: Five occupied shells means the element is in Period 5. Step 3: The Group II element in Period 5 is strontium, so strontium is the answer.

Question 8 · Answer A

Step 1: Group II elements are metals, so they have metallic bonding. Step 2: In a metal the atoms form a regular lattice of positive ions and the outer electrons become delocalised, forming a sea of mobile electrons. Step 3: The bond is the electrostatic attraction between the positive metal ions and the sea of delocalised electrons.

Question 9 · Answer A

Step 1: Calcium reacts with water to release hydrogen gas, so bubbles (effervescence) are seen. Step 2: The calcium is used up in the reaction, so the solid dissolves and gets smaller. Step 3: The other product is calcium hydroxide, an alkali, so the universal indicator turns blue/purple.

Question 10 · Answer A

Step 1: A reactive metal reacting with water gives a metal hydroxide and hydrogen gas. Step 2: For calcium this gives calcium hydroxide and hydrogen. Step 3: So the two products are calcium hydroxide and hydrogen.

Question 11 · Answer A

Step 1: Burning calcium produces a characteristic brick-red, orange-red flame. Step 2: Lilac, green and yellow flames belong to potassium, copper and sodium respectively. Step 3: So calcium burns with an orange-red flame.

Question 12 · Answer A

Step 1: Calcium forms $\text{Ca}^{2+}$ and oxygen forms $\text{O}^{2-}$, so calcium oxide is CaO. Step 2: Oxygen gas is $\text{O}_2$, so two calcium atoms are needed to use both oxygen atoms. Step 3: Balancing gives $2\text{Ca} + \text{O}_2 \rightarrow 2\text{CaO}$.

Question 13 · Answer A

Water held within a crystal structure in a fixed ratio is called water of crystallisation.

Question 14 · Answer A

Step 1: Moles of $\text{Ca(NO}_3)_2$ = 3.28 / 164 = 0.0200 mol. Step 2: Moles of water = 1.44 / 18 = 0.0800 mol. Step 3: x = moles of water / moles of salt = 0.0800 / 0.0200 = 4.

Question 15 · Answer A

Step 1: Acid + alkali gives a salt plus water, so the products are sodium sulfate and water. Step 2: The sodium sulfate stays dissolved, so it is (aq), and the water is a liquid, so it is (l). Step 3: Two water molecules are needed to balance the hydrogen and oxygen, giving $\text{Na}_2\text{SO}_4(\text{aq}) + 2\text{H}_2\text{O}(\text{l})$.

Question 16 · Answer A

A reaction between an acid and an alkali to form a salt and water is called neutralisation.

Question 17 · Answer A

Step 1: Concentration = moles / volume in $\text{dm}^3$, and 40.0 $\text{cm}^3$ = 0.0400 $\text{dm}^3$. Step 2: Concentration = 0.0100 / 0.0400. Step 3: This gives 0.25 $\text{mol/dm}^3$.

Question 18 · Answer A

A burette is used to add an accurately measured, variable volume of acid during a titration.

Question 19 · Answer A

Step 1: From the equation, 2 mol NaOH react with 1 mol $\text{H}_2\text{SO}_4$, so moles of acid = 0.0100 / 2 = 0.00500 mol. Step 2: Volume in $\text{dm}^3$ = moles / concentration = 0.00500 / 0.200 = 0.0250 $\text{dm}^3$. Step 3: Converting to $\text{cm}^3$ gives 25 $\text{cm}^3$.

Question 20 · Answer A

Step 1: Methyl orange is yellow in an alkali, so it starts yellow in the sodium hydroxide. Step 2: At the end-point the solution becomes neutral/just acidic and methyl orange turns orange. Step 3: So the colour change is from yellow to orange.

Question 21 · Answer A

Step 1: The crystals are coloured because the indicator stays in the solution and is left behind on evaporation. Step 2: Once the correct acid volume is known from the titration, the reaction can be repeated with those exact volumes but no indicator. Step 3: Evaporating this indicator-free solution gives white crystals.

Question 22 · Answer A

Step 1: A saturated solution contains as much dissolved solute as it can hold at a particular temperature. Step 2: Because it already holds the maximum amount, no further solute can dissolve at that temperature. Step 3: So a saturated solution is one with the maximum concentration of solute dissolved at that temperature.

Question 23 · Answer A

Step 1: The amount of solute that can dissolve (its solubility) decreases as the temperature falls. Step 2: The cooling solution can no longer hold all the dissolved solute, so the excess separates out. Step 3: This excess solute appears as crystals.

Question 24 · Answer A

Step 1: The mass of salt dissolved in the solution is fixed, whatever method is used to remove the water. Step 2: Whether the water is removed by gentle heating or by leaving it to dry, the same dissolved salt is left behind. Step 3: So drying without gentle heating has no effect on the final mass of crystals collected.

Question 25 · Answer A

Step 1: Because $\Delta H$ is negative the reaction is exothermic, so the products are drawn at a lower energy level than the reactants. Step 2: The activation energy $E_a$ is the energy rise from the reactant level up to the top of the energy barrier (peak), shown as an upward arrow. Step 3: $\Delta H$ is the overall fall in energy from reactants to products, shown as a downward arrow from the reactant level to the lower product level.

Question 26 · Answer A

Step 1: Energy to break the reactant bonds = (4 x 330) + (4 x 460) = 1320 + 1840 = 3160 kJ. Step 2: $\Delta H$ = energy in - energy out, so energy released forming products = 3160 - (-54) = 3214 kJ, and this equals (2 x S=O) + (4 x 570). Step 3: (2 x S=O) = 3214 - 2280 = 934, so the S=O bond energy = 934 / 2 = 467 kJ/mol.

Question 27 · Answer A

Step 1: The forward reaction is exothermic, so raising the temperature favours the endothermic (reverse) direction and the equilibrium moves to the left. Step 2: There are fewer gas moles on the left (3) than the right (5), so increasing the pressure shifts the equilibrium towards the side with fewer moles, the left. Step 3: A catalyst speeds up the forward and reverse reactions equally, so it has no effect on the position of equilibrium.

Question 28 · Answer A

Step 1: Lowering the temperature gives the particles less kinetic energy, so they move more slowly. Step 2: Slower particles collide less frequently, so there are fewer collisions each second. Step 3: Fewer particles also have energy greater than or equal to the activation energy, so a smaller proportion of collisions are successful, and the rate falls.

Question 29 · Answer A

The general formula of the alcohols is $\text{C}_n\text{H}_{2n+1}\text{OH}$, showing the -OH functional group.

Question 30 · Answer A

Step 1: A homologous series is a family of compounds with the same general formula and the same functional group. Step 2: Because they share the functional group, members have similar chemical properties and show a steady trend in physical properties. Step 3: So another shared characteristic is having the same functional group.

Question 31 · Answer A

Step 1: Fermentation (method 1) uses yeast at a warm temperature of about 25-35 C in the absence of oxygen (anaerobic conditions). Step 2: Catalytic hydration of ethene (method 2) uses a temperature of about 300 C, an acid (phosphoric acid) catalyst, and a pressure of about 60 atm. Step 3: Only the first row gives both sets of conditions correctly.

Question 32 · Answer A

Step 1: In fermentation, glucose breaks down to ethanol and carbon dioxide with no other reactant. Step 2: One glucose molecule, $\text{C}_6\text{H}_{12}\text{O}_6$, forms two ethanol and two carbon dioxide molecules. Step 3: The balanced equation is $\text{C}_6\text{H}_{12}\text{O}_6 \rightarrow 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2$.

Question 33 · Answer A

Step 1: Ethene, $\text{C}_2\text{H}_4$, reacts with steam (water) across its C=C double bond. Step 2: One molecule of water adds on to give one molecule of ethanol. Step 3: The balanced equation is $\text{C}_2\text{H}_4 + \text{H}_2\text{O} \rightarrow \text{C}_2\text{H}_5\text{OH}$.

Question 34 · Answer A

Step 1: Count the carbon atoms: there are four CH$_2$ groups, giving 4 carbons. Step 2: Count the hydrogens: 4 x 2 = 8 from the CH$_2$ groups plus 2 from the two -OH groups = 10 hydrogens. Step 3: There are 2 oxygen atoms (one in each -OH), so the molecular formula is $\text{C}_4\text{H}_{10}\text{O}_2$.

Question 35 · Answer A

Step 1: An -OH group reacts with one molecule of a carboxylic acid to make one ester linkage. Step 2: Butane-1,4-diol has two -OH groups, so it can form two ester linkages. Step 3: Therefore one mole of the diol reacts with two moles of ethanoic acid.

Question 36 · Answer A

Step 1: The molecular formula is $\text{C}_4\text{H}_6\text{O}_4$ (4 C, 6 H, 4 O). Step 2: The empirical formula is the simplest whole-number ratio, so divide each number by the highest common factor, 2. Step 3: This gives $\text{C}_2\text{H}_3\text{O}_2$.

Question 37 · Answer A

Step 1: A carboxylic acid reacting with a reactive metal behaves like an acid + metal reaction. Step 2: Acid + reactive metal produces a salt plus hydrogen gas. Step 3: So the gas formed when butanedioic acid reacts with sodium is hydrogen.

Question 38 · Answer A

Step 1: A diol and a dicarboxylic acid react by condensation, where an -OH group and a -COOH group join and a small molecule of water is removed. Step 2: The new linkage formed between the carbon of the acid group and the oxygen of the alcohol group is an ester linkage, -C(=O)-O-. Step 3: Repeating this joins many monomers into a polyester, so the chain is held together by ester linkages.

Question 39 · Answer A

Joining monomers with the loss of a small molecule such as water is called condensation polymerisation.