IGCSE Chemistry 0620/42 October/November 2025 Past Paper with Answers

Question 1

For solids, liquids and gases, which row correctly describes the particle separation, arrangement and motion?

1. solid: particles touching, regular arrangement, vibrating about fixed positions; liquid: particles touching, random arrangement, moving around each other; gas: particles far apart, random arrangement, moving quickly in all directions
2. solid: particles far apart, random arrangement, moving quickly; liquid: touching, regular, vibrating; gas: touching, regular, moving slowly
3. solid: touching, random, moving freely; liquid: far apart, regular, vibrating; gas: touching, random, vibrating
4. solid: touching, regular, moving freely in all directions; liquid: far apart, random, vibrating about fixed positions; gas: touching, regular, moving slowly

Question 2

Three physical changes are: solid to liquid, gas to liquid, and solid to aqueous (a solid forming a solution). Which row names all three correctly?

1. solid to liquid = melting; gas to liquid = condensing; solid to aqueous = dissolving
2. solid to liquid = freezing; gas to liquid = evaporating; solid to aqueous = subliming
3. solid to liquid = condensing; gas to liquid = melting; solid to aqueous = freezing
4. solid to liquid = subliming; gas to liquid = freezing; solid to aqueous = melting

Question 3

Why do gases diffuse, in terms of their particles?

1. the particles are in constant random motion, so they move and spread out to fill the available space
2. the particles are held in fixed positions and only vibrate
3. the particles are packed closely together and cannot move past one another
4. the particles attract each other strongly and stay close together

Question 4 · Answer A

Step 1: At a given temperature, lighter gas particles move faster than heavier ones. Step 2: The mass of each molecule is its relative molecular mass, $M_r$. Step 3: So the relative rate of diffusion is determined by the relative molecular mass: gases with a lower $M_r$ diffuse faster.

Question 5 · Answer A

Step 1: Oxides of nitrogen react in sunlight to form photochemical smog. Step 2: They are also toxic and irritate the lungs, causing respiratory (breathing) problems. Step 3: So two further adverse effects are photochemical smog and respiratory problems.

Question 6 · Answer A

Step 1: Incomplete combustion happens when there is not enough oxygen, producing carbon monoxide instead of carbon dioxide. Step 2: Carbon monoxide is a toxic gas. Step 3: So the toxic gaseous product removed in the catalytic converter is carbon monoxide.

Question 7 · Answer A

Step 1: In the catalytic converter the toxic carbon monoxide reduces the oxides of nitrogen. Step 2: The nitrogen oxides are converted to harmless nitrogen gas, and the carbon monoxide is oxidised to carbon dioxide. Step 3: So the two non-toxic gases formed are nitrogen and carbon dioxide.

Question 8 · Answer A

The main iron ore used in the blast furnace, which contains iron(III) oxide, is called hematite.

Question 9 · Answer A

Step 1: Hot air is blasted into the furnace where it reacts with the coke (carbon). Step 2: The carbon burns in the oxygen of the air, and this combustion is strongly exothermic. Step 3: So the main heat comes from the burning of carbon (coke).

Question 10 · Answer A

Step 1: Coke reacts with limited oxygen and carbon dioxide to form carbon monoxide. Step 2: Carbon monoxide is the gas that reduces iron(III) oxide to iron. Step 3: So the gaseous reducing agent is carbon monoxide.

Question 11 · Answer A

Step 1: Each carbon monoxide gains an oxygen to become carbon dioxide, while iron(III) oxide loses its oxygen to form iron. Step 2: $\text{Fe}_2\text{O}_3$ has 3 oxygen atoms, so 3 CO molecules are needed and 3 $\text{CO}_2$ molecules form. Step 3: Balancing the iron gives 2 Fe, so the equation is $3\text{CO} + \text{Fe}_2\text{O}_3 \rightarrow 2\text{Fe} + 3\text{CO}_2$.

Question 12 · Answer A

Step 1: The limestone first decomposes in the heat: $\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$. Step 2: The basic calcium oxide then reacts with the acidic silicon(IV) oxide impurity: $\text{CaO} + \text{SiO}_2 \rightarrow \text{CaSiO}_3$ (slag). Step 3: So the two equations are the thermal decomposition of limestone and the reaction of calcium oxide with silicon(IV) oxide.

Question 13 · Answer A

Step 1: They are all iron, so they have the same number of protons (26), which defines the element. Step 2: Their nucleon numbers differ (54, 56, 57, 58), which means they have different numbers of neutrons. Step 3: Atoms of the same element with the same protons but different neutrons are isotopes.

Question 14 · Answer A

Step 1: Both ions are iron, so each has 26 protons. Step 2: Neutrons = nucleon number - protons, so $^{54}\text{Fe}$ has 54 - 26 = 28 neutrons and $^{58}\text{Fe}$ has 58 - 26 = 32 neutrons. Step 3: A 2+ ion has lost 2 electrons (26 - 2 = 24) and a 3+ ion has lost 3 electrons (26 - 3 = 23).

Question 15 · Answer A

Step 1: Aqueous metal ions are tested by adding aqueous sodium hydroxide (or aqueous ammonia). Step 2: Iron(II) ions give a green precipitate of iron(II) hydroxide. Step 3: So the test is to add sodium hydroxide and look for a green precipitate.

Question 16 · Answer A

Step 1: An oxidising agent causes another substance to be oxidised (to lose electrons). Step 2: In doing so it gains those electrons, so it is itself reduced. Step 3: Hence an oxidising agent oxidises another substance and is itself reduced.

Question 17 · Answer A

Step 1: The charge increases from 2+ to 3+, which means the ion has become more positive. Step 2: An ion becomes more positive by losing a (negative) electron. Step 3: Loss of electrons is the definition of oxidation, so $\text{Fe}^{2+} \rightarrow \text{Fe}^{3+}$ is oxidation.

Question 18 · Answer A

Step 1: Potassium manganate(VII) acts as an oxidising agent in acidic conditions. Step 2: The reaction needs hydrogen ions, so the solution must be acidified. Step 3: So the condition required is that the solution is acidified.

Question 19 · Answer A

Step 1: The iodide ions act as the reducing agent, so they are themselves oxidised. Step 2: Iodide ions lose electrons and are oxidised to iodine. Step 3: So while $\text{Fe}^{3+}$ becomes $\text{Fe}^{2+}$, the other product is iodine.

Question 20 · Answer A

Step 1: Barium sulfate is insoluble, so it forms as a solid precipitate, $\text{BaSO}_4(\text{s})$. Step 2: Sodium chloride is soluble and stays in solution as $\text{NaCl}(\text{aq})$. Step 3: Balancing the chlorine and sodium needs 2 NaCl, giving $\text{BaSO}_4(\text{s}) + 2\text{NaCl}(\text{aq})$.

Question 21 · Answer A

Step 1: The relative formula mass of $\text{BaCl}_2$ = 137 + (2 x 35.5) = 208. Step 2: Mass = moles x $M_r$ = 0.100 x 208. Step 3: This gives a mass of 20.8 g.

Question 22 · Answer A

Step 1: Volume in $\text{dm}^3$ = moles / concentration = 0.100 / 1.25 = 0.0800 $\text{dm}^3$. Step 2: Convert to $\text{cm}^3$ by multiplying by 1000. Step 3: 0.0800 x 1000 = 80 $\text{cm}^3$.

Question 23 · Answer A

Step 1: Barium sulfate is an insoluble salt that precipitates out of the mixture. Step 2: Barium sulfate is a white solid. Step 3: So the colour of the solid formed is white.

Question 24 · Answer A

The solid left behind in the filter paper after filtration is called the residue (the liquid that passes through is the filtrate).

Question 25 · Answer A

Step 1: When the mixture is filtered, the residue is wet with the solution, which contains dissolved sodium chloride. Step 2: If the residue is not washed, this sodium chloride dries onto it and adds extra mass. Step 3: Rinsing the residue with distilled water washes the soluble sodium chloride away, so the additional step is to rinse with water and the mass was high because of the leftover sodium chloride.

Question 26 · Answer A

Making an insoluble salt by mixing two soluble solutions so the insoluble salt forms as a solid is called precipitation.

Question 27 · Answer A

Step 1: A different soluble barium salt to replace barium chloride is barium nitrate (nitrates are soluble). Step 2: A different soluble sulfate to replace sodium sulfate is potassium sulfate (or ammonium sulfate). Step 3: A different insoluble barium salt that precipitation could make is barium carbonate, which is also insoluble.

Question 28 · Answer A

Step 1: An acid is a substance that releases hydrogen ions when dissolved in water. Step 2: Nitric, sulfuric and ethanoic acid all release the hydrogen ion. Step 3: So the common cation produced is $\text{H}^+$.

Question 29 · Answer A

Step 1: A strong acid is one that dissociates (ionises) completely in water, releasing all its hydrogen ions. Step 2: Nitric acid does this, so it is a strong acid. Step 3: A weak acid only partially dissociates, releasing few hydrogen ions; ethanoic acid does this, so it is a weak acid.

Question 30 · Answer A

Step 1: The higher the concentration of hydrogen ions, the lower the pH. Step 2: Ethanoic acid is weak and only partially dissociates, so it has the lowest hydrogen ion concentration. Step 3: The lowest hydrogen ion concentration gives the highest pH, so ethanoic acid has the highest pH.

Question 31 · Answer A

Step 1: Thymolphthalein is colourless in acidic and neutral solutions and blue in alkaline solutions. Step 2: All three solutions here are acidic. Step 3: So the thymolphthalein is colourless in each of the dilute acids.

Question 32 · Answer A

Step 1: Sulfuric acid, $\text{H}_2\text{SO}_4$, loses two $\text{H}^+$ to leave the sulfate ion, $\text{SO}_4^{2-}$. Step 2: Ethanoic acid, $\text{CH}_3\text{COOH}$, loses one $\text{H}^+$ from the -COOH group to leave the ethanoate ion, $\text{CH}_3\text{COO}^-$. Step 3: So the anions are $\text{SO}_4^{2-}$ and $\text{CH}_3\text{COO}^-$.

Question 33 · Answer A

Step 1: Dilute sulfuric acid is essentially acidified water, so electrolysis splits the water. Step 2: At the cathode (negative electrode) hydrogen ions are reduced to hydrogen gas. Step 3: At the anode (positive electrode) the water/hydroxide is oxidised to oxygen, so the two gases are hydrogen and oxygen.

Question 34 · Answer A

Step 1: A metal reacting with a carboxylic acid forms a salt and hydrogen. Step 2: Ethanoic acid forms ethanoate salts, and the metal is calcium. Step 3: So the salt formed is calcium ethanoate.

Question 35 · Answer A

A base is a substance that is a proton (hydrogen ion) acceptor.

Question 36 · Answer A

Step 1: An alkali is defined as a soluble base, one that dissolves in water. Step 2: Aluminium hydroxide is a base but is not an alkali. Step 3: Therefore it must be insoluble in water.

Question 37 · Answer A

Step 1: Nitric acid reacts with bases such as metal oxides and hydroxides to form the metal nitrate. Step 2: Aluminium oxide is a base containing aluminium, so it reacts with nitric acid to give aluminium nitrate and water. Step 3: So aluminium oxide also forms aluminium nitrate with dilute nitric acid.

Question 38 · Answer A

Step 1: $\text{Al(NO}_3)_3$ has 9 oxygen atoms, contributing 9 x (-2) = -18, and aluminium contributes +3. Step 2: The compound is neutral, so the three nitrogen atoms must total +18 - 3 = +15. Step 3: Sharing +15 across the 3 nitrogen atoms gives +15 / 3 = +5 for each nitrogen.

Question 39 · Answer A

Step 1: A homologous series shares one general formula; for the alkanes this is $\text{C}_n\text{H}_{2n+2}$. Step 2: Each member also differs from the next by a $\text{CH}_2$ unit. Step 3: So two further characteristics are the same general formula and a $\text{CH}_2$ difference between consecutive members.

Question 40 · Answer A

Step 1: As the carbon chain gets longer, the alkane molecules get larger. Step 2: Larger molecules have stronger intermolecular forces, which need more energy to overcome. Step 3: So the boiling points increase as the chain length increases.

Question 41 · Answer A

Step 1: Viscosity (how thick or sticky a liquid is) is another physical property that shows a trend in the alkanes. Step 2: As the carbon chain gets longer, the molecules are larger and the forces between them stronger. Step 3: This makes the alkanes more viscous, so viscosity increases as the chain length increases.

Question 42 · Answer A

Step 1: A saturated compound contains only single bonds between its carbon atoms. Step 2: In alkanes every carbon-carbon bond is a single bond, with no double bonds. Step 3: So alkanes are described as saturated because all their carbon-carbon bonds are single.

Question 43 · Answer A

Step 1: The substitution of an alkane by chlorine needs energy to start the reaction. Step 2: This energy is provided by ultraviolet light (sunlight). Step 3: So the condition needed is ultraviolet light.

Question 44 · Answer A

Step 1: Monosubstitution replaces just one hydrogen atom of propane with a chlorine atom. Step 2: Replacing a hydrogen on an end carbon gives $\text{CH}_3\text{CH}_2\text{CH}_2\text{Cl}$, which is 1-chloropropane. Step 3: Replacing a hydrogen on the middle carbon gives $\text{CH}_3\text{CHClCH}_3$, which is 2-chloropropane.

Question 45 · Answer A

Structural isomers are compounds with the same molecular formula but different structural formulae (different arrangements of the atoms).

Question 46 · Answer A

Step 1: An ester linkage is formed between a carboxylic acid and an alcohol. Step 2: It contains the carbonyl carbon, the oxygen double-bonded to it (C=O) and the oxygen single-bonded between the two carbon chains. Step 3: So the ester linkage is the -C(=O)-O- group, taking in the carbon and both of its oxygen atoms.

Question 47 · Answer A

Step 1: An ester is named as alkyl + ...oate: the alcohol gives the alkyl part and the acid gives the -oate part. Step 2: The acid part $\text{CH}_3\text{CH}_2\text{CH}_2\text{COO}$ comes from butanoic acid (4 carbons), giving butanoate. Step 3: The $\text{OCH}_2\text{CH}_3$ comes from ethanol, giving the ethyl part, so the ester is ethyl butanoate.

Question 48 · Answer A

Step 1: The molecular formula is $\text{C}_6\text{H}_{12}\text{O}_2$ (6 C, 12 H, 2 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 $\text{C}_3\text{H}_6\text{O}$.

Question 49 · Answer A

Step 1: An ester is made by reacting a carboxylic acid with an alcohol. Step 2: The butanoate part comes from butanoic acid and the ethyl part comes from ethanol. Step 3: So ethyl butanoate is made from butanoic acid and ethanol.

Question 50 · Answer A

Step 1: Esterification of a carboxylic acid with an alcohol is catalysed by an acid. Step 2: Concentrated sulfuric acid is the usual catalyst. Step 3: So a suitable catalyst is an acid catalyst such as concentrated sulfuric acid.

Question 51 · Answer A

Step 1: An ester is an acid part joined to an alcohol part, and together they must total 5 carbons. Step 2: The unbranched possibilities are: methyl butanoate (1+4), ethyl propanoate (2+3), propyl ethanoate (3+2) and butyl methanoate (4+1). Step 3: That gives 4 different unbranched esters with the formula $\text{C}_5\text{H}_{10}\text{O}_2$.

Question 52 · Answer A

Step 1: Each $\text{C}_5\text{H}_{10}\text{O}_2$ gives 5 $\text{CO}_2$ and 5 $\text{H}_2\text{O}$. Doubling avoids a fraction of oxygen, giving 10 $\text{CO}_2$ and 10 $\text{H}_2\text{O}$. Step 2: The right-hand side has (10 x 2) + 10 = 30 oxygen atoms, and the two ester molecules supply 4 oxygens, so $\text{O}_2$ must supply 30 - 4 = 26 oxygen atoms = 13 $\text{O}_2$. Step 3: The balanced equation is $2\text{C}_5\text{H}_{10}\text{O}_2 + 13\text{O}_2 \rightarrow 10\text{CO}_2 + 10\text{H}_2\text{O}$.