IGCSE Chemistry Paper 4 (Theory/Extended) - Mock 2
Topic 1: States of Matter | Core + Supplement (1.1 Solids, Liquids and Gases / 1.2 Diffusion)
75 minutes
80
7
75:00
Instructions
Answer ALL questions in the spaces provided.
Show all working for calculation questions. Marks are awarded for method even if the final answer is incorrect.
Your answers will be automatically graded when you submit the exam. The model answers will be shown for review.
Question Navigation Score: 0 / 80
Question 1: States of Matter in Everyday Life
Total: 12 marks
Context: Cooking and food preparation in an Indian kitchen.
(a)3 marks
State the three states of matter and give one example of each that can be found in a kitchen.
Model Answer - Q1(a)
Solid - e.g. ice / salt / sugar / metal pan [1]
Liquid - e.g. water / cooking oil / milk / vinegar [1]
Gas - e.g. steam / air / carbon dioxide from baking / LPG cooking gas [1]
⚠ If you missed marks here: Each mark needs the state AND a matching kitchen example, so listing "solid, liquid, gas" alone scores nothing. Choose examples that really are in that state: smoke (tiny solid particles in air) and flames are poor gas examples, whereas steam or air are reliable choices.
(b)3 marks
A pressure cooker uses steam at high pressure to cook food faster. Explain, using particle theory, why increasing the pressure raises the boiling point of water.
Model Answer - Q1(b)
Increasing the pressure forces the gas particles closer together / makes it harder for liquid particles to escape the surface [1]
Particles need more (kinetic) energy to overcome the increased pressure and escape into the gas phase [1]
Therefore a higher temperature is needed for boiling to occur / boiling point increases [1]
⚠ If you missed marks here: "The pressure cooker traps the heat" does not explain WHY the boiling point rises. The chain is: higher pressure above the water makes it harder for particles to escape from the liquid, so they need more kinetic energy, which they only have at a higher temperature – so the water boils above 100°C.
(c)2 marks
When spices are ground into a powder, their surface area increases. Explain, using particle theory, why the aroma from ground spices spreads faster than from whole spices.
Model Answer - Q1(c)
Greater surface area means more aroma particles are exposed and can escape into the air at any given time [1]
These particles mix with air particles by diffusion / random motion and spread faster because more particles are released per second [1]
⚠ If you missed marks here: Grinding does not make the aroma MOLECULES smaller or faster – it breaks the spice into smaller pieces, exposing more surface so that more aroma particles escape each second. Once in the air they spread by diffusion (random motion), so both ideas are needed: more particles released, then random mixing with the air particles.
(d)2 marks
Ice cream is stored at -18°C. Describe the particle arrangement and motion in the ice cream at this temperature.
Model Answer - Q1(d)
Particles are closely packed in a regular / fixed arrangement, held together by strong forces of attraction [1]
Particles vibrate about their fixed positions but do not move from place to place [1]
⚠ If you missed marks here: At −18°C the ice cream is frozen solid, so describe a solid: particles closely packed in a regular arrangement, held by strong forces. "The particles stop moving because it is frozen" loses the second mark – they still vibrate about fixed positions, just less than at higher temperatures.
(e)2 marks
Explain why steam at 100°C causes more severe burns than water at 100°C.
Model Answer - Q1(e)
Steam at 100°C holds extra energy that was taken in when the water boiled, used to overcome the forces of attraction between its particles [1]
When steam condenses on the skin, the forces of attraction form again and this extra energy is released, so the skin receives much more energy than from water at the same temperature [1]
⚠ If you missed marks here: "Steam is hotter" is wrong – the question says both are at 100°C. The difference is the extra energy steam took in to become a gas: when it CONDENSES on the skin it gives that energy out as well, on top of the heat the hot water would release.
Question 2: Kinetic Particle Theory
Total: 12 marks
Context: A series of experiments in a school laboratory in Nairobi.
(a)3 marks
State three main assumptions of the kinetic particle theory.
Model Answer - Q2(a)
All matter is made up of very small particles (atoms, molecules or ions) [1]
Particles are in constant motion / move continuously [1]
The higher the temperature, the faster the particles move / particles have more kinetic energy at higher temperatures [1]
⚠ If you missed marks here: These are statements about ALL matter, not about one state: "solids have a fixed shape" or "gas particles are far apart" describe states of matter and do not count. Say that everything is made of tiny particles, that the particles are always moving, and that they move faster (more kinetic energy) at a higher temperature.
(b)3 marks
Using the kinetic particle theory, explain the following properties:
(b) (i)1 mark
A gas can be compressed (squashed into a smaller volume).
Model Answer - Q2(b)(i)
Gas particles are far apart with large spaces between them, so they can be pushed closer together [1]
⚠ If you missed marks here: It is the SPACES between gas particles that get smaller, not the particles: "the gas particles get squashed" scores nothing. Say the particles are far apart with large gaps between them, so they can be pushed closer together into a smaller volume.
(b) (ii)1 mark
A liquid can flow and takes the shape of its container.
Model Answer - Q2(b)(ii)
Liquid particles are close together but can slide over each other / are not held in fixed positions, so they can flow and take the shape of the container [1]
⚠ If you missed marks here: Liquid particles are still CLOSE together – giving them big gaps describes a gas and loses the mark. A liquid flows because its particles are not held in fixed positions, so they slide past one another and the liquid settles into the shape of its container.
(b) (iii)1 mark
A solid has a fixed shape.
Model Answer - Q2(b)(iii)
Solid particles are held in fixed positions by strong forces of attraction and can only vibrate, so the solid keeps a definite shape [1]
⚠ If you missed marks here: "The particles are tightly packed" is not enough on its own – liquid particles are tightly packed too, yet a liquid has no fixed shape. The mark needs strong forces holding the particles in FIXED positions so that they can only vibrate; "the particles cannot move at all" is wrong, because they vibrate.
(c)3 marks
A student heats a beaker of water from 20°C to 100°C. Describe how the behaviour of the water particles changes as the temperature increases from 20°C to 100°C.
Model Answer - Q2(c)
As temperature increases, the particles gain kinetic energy and move faster [1]
The forces of attraction between particles are increasingly overcome / particles move further apart on average [1]
At 100°C, particles at the surface have enough energy to escape as gas (steam) / boiling occurs as particles throughout the liquid form bubbles of gas [1]
⚠ If you missed marks here: Describe the change across the whole range: the particles gain kinetic energy and speed up, move further apart as the forces between them are overcome, and at 100°C have enough energy to form bubbles of gas throughout the liquid. Stopping at "the particles move faster" earns one of the three marks, and "the particles expand" earns none – particles never change size.
(d)3 marks
The diagrams below show the particle arrangement in each state of matter. Study them and then draw labelled diagrams on paper showing the particle arrangement in (i) a crystalline solid, (ii) a liquid, and (iii) a gas.
Model Answer - Q2(d)
Crystalline solid: particles drawn touching, in a regular ordered pattern (rows and columns) [1]
Liquid: particles drawn close together but in an irregular / disordered arrangement [1]
Gas: particles drawn far apart, randomly spaced, with arrows showing random movement in all directions [1]
⚠ If you missed marks here: In the liquid drawing the particles must still TOUCH each other, just with no pattern; only the gas has big gaps, so spacing the liquid out like a gas is the commonest lost mark. Keep every particle the same size in all three, and add arrows pointing in different directions on the gas particles to show their random movement.
Question 3: Heating Curves and Energy
Total: 12 marks
Context: A chemist studies the heating of a pure substance X from -20°C to 80°C.
(a)2 marks
The heating curve for substance X is shown below. Identify the melting point and boiling point of substance X from the curve.
Model Answer - Q3(a)
Melting point = 0°C (the temperature at which section B is horizontal / flat) [1]
Boiling point = 60°C (the temperature at which section D is horizontal / flat) [1]
⚠ If you missed marks here: Read each FLAT section across to the temperature axis: B is at 0°C (melting) and D is at 60°C (boiling). Giving the letters B and D instead of temperatures, or the temperature where the heating started (−20°C), scores nothing.
(b)3 marks
Explain why the temperature remains constant at sections B and D of the heating curve, even though heat energy is being supplied continuously.
Model Answer - Q3(b)
At B (melting) and D (boiling), a change of state is occurring [1]
The heat energy supplied is used to break / overcome the forces of attraction between particles (intermolecular forces) [1]
The energy increases the potential energy of the particles, not the kinetic energy, so the temperature does not rise [1]
⚠ If you missed marks here: Heat is still flowing in during B and D, so "no energy is being absorbed" is wrong. The energy is being used to overcome the forces of attraction between the particles (a change of state), so their kinetic energy – and therefore the temperature – does not rise until the change of state is complete.
(c)1 mark
State the physical state or states of substance X during section B of the heating curve.
Model Answer - Q3(c)
Solid and liquid together: X is melting, so both states are present at 0°C [1]
⚠ If you missed marks here: During B the substance is melting, so solid AND liquid are present together – "liquid" on its own loses the mark, and "melting" names the change rather than the states. The temperature stays at 0°C until the last of the solid has melted; only then does section C begin.
(d)3 marks
During which section(s) of the heating curve is the kinetic energy of particles increasing? Explain your answer.
Model Answer - Q3(d)
Sections A, C and E [1]
In these sections the temperature is rising / increasing [1]
Temperature is a measure of the average kinetic energy of the particles, so when temperature rises, the particles move faster / have more kinetic energy [1]
⚠ If you missed marks here: Kinetic energy rises only where the TEMPERATURE rises – sections A, C and E. Including B or D because "heat is supplied all the time" loses the first mark: on the flat sections the energy goes into overcoming the forces between particles, so their kinetic energy stays the same.
(e)3 marks
Sketch or describe how the heating curve would differ if substance X were impure (contained an impurity).
Model Answer - Q3(e)
The melting point would be lower than for the pure substance [1]
The boiling point would be higher than for the pure substance [1]
The flat sections (B and D) would no longer be completely horizontal / temperature would rise slightly during the change of state / melting and boiling would occur over a range of temperatures [1]
⚠ If you missed marks here: An impurity moves the two changes of state in OPPOSITE directions – the melting point goes DOWN but the boiling point goes UP – so "both would be lower" loses a mark. The third mark is for the shape: B and D are no longer perfectly flat, because the impure substance melts and boils over a RANGE of temperatures.
Question 4: Cooling and Condensation
Total: 12 marks
Context: Weather patterns and dew formation in the British countryside.
(a)3 marks
Morning dew forms on grass overnight. Explain this observation using kinetic particle theory.
Model Answer - Q4(a)
At night, the temperature of the grass surface drops / the grass becomes cold [1]
Water vapour particles in the air lose kinetic energy when they come into contact with the cold grass surface [1]
The particles slow down enough for the forces of attraction between them to hold them together, so the gas condenses into liquid water droplets (dew) [1]
⚠ If you missed marks here: Dew does not fall like rain or soak up from the soil – it is water VAPOUR from the air condensing on the cold grass, so "evaporation" is the wrong process. The particle marks need the vapour particles losing kinetic energy on the cold surface and slowing down enough for the forces of attraction to hold them together as liquid droplets.
(b)3 marks
A student records cooling data for melted stearic acid as it cools from 85°C. The cooling curve is shown below. Describe what is happening to the particles at each stage of the cooling curve.
Model Answer - Q4(b)
Liquid cooling stage: particles lose kinetic energy and slow down, temperature decreases [1]
Freezing plateau: particles form bonds / forces of attraction pull particles into a regular arrangement. Energy is released (latent heat) but temperature stays constant [1]
Solid cooling stage: particles are now in fixed positions and vibrate less and less as they continue to lose energy to surroundings [1]
⚠ If you missed marks here: The flat section is not "nothing happening": as the liquid freezes, the particles are pulled into a regular arrangement, and the energy this releases balances the heat lost, so the temperature holds steady. In the final stage the solid's particles still VIBRATE, just less and less – "the particles stop moving" loses that mark.
(c)2 marks
At what temperature does stearic acid freeze? Explain how you can determine this from the cooling curve.
Model Answer - Q4(c)
Stearic acid freezes at 69°C [1]
This is the temperature at which the cooling curve is flat / horizontal, because the temperature remains constant during the change of state from liquid to solid [1]
⚠ If you missed marks here: The freezing point is the temperature of the FLAT part of the curve, 69°C – not 85°C, where the cooling started, and not a time. The second mark needs the reason: the temperature stays constant while the liquid is changing into a solid.
(d)2 marks
Compare the cooling curve of pure stearic acid with that of an impure sample of stearic acid.
Model Answer - Q4(d)
The impure sample would have a lower freezing point than the pure sample [1]
The impure sample would freeze over a range of temperatures (no flat section) / the plateau would slope downward instead of being flat [1]
⚠ If you missed marks here: Impurities LOWER the freezing point, so drawing the impure sample's flat part above 69°C is the wrong way round. The second mark is for the shape: instead of a sharp horizontal plateau, an impure sample freezes over a range of temperatures, so that part of its curve slopes gently downward.
(e)2 marks
Explain why wet clothes dry faster on a windy day than on a still day.
Model Answer - Q4(e)
Wind blows away water vapour particles from near the surface of the clothes, preventing the air near the surface from becoming saturated [1]
This increases the rate of evaporation because more water particles can escape from the surface into the drier air [1]
⚠ If you missed marks here: Wind does not heat the clothes or make the water boil – it carries water vapour away from the cloth, so the air next to it never becomes saturated. With drier air kept next to the surface, more of the water particles that escape stay in the air, so the rate of evaporation goes up.
Question 5: Gas Behaviour
Total: 10 marks
Context: Experiments with syringes and balloons.
(a)3 marks
A student fills one gas syringe with 50 cm³ of air and an identical syringe with 50 cm³ of water. She seals the nozzle of each and pushes hard on both plungers. The air is easily squeezed into a much smaller volume; the water will not compress at all. Explain this difference in terms of the arrangement, separation and motion of the particles.
Model Answer - Q5(a)
In a gas the particles are very far apart in a random, disordered arrangement, moving quickly in all directions, so most of the syringe is empty space [1]
In a liquid the particles are already touching one another, randomly arranged but sliding over each other, with almost no empty space between them [1]
Pushing the plunger on the air syringe simply moves the gas particles closer together into that empty space, so the volume falls a great deal; in the water syringe there is no space left to close up and the particles themselves cannot be squashed, so the volume hardly changes [1]
⚠ If you missed marks here: Water's particles are touching but RANDOMLY arranged and sliding past each other – describing them as fixed in rows is a solid and loses the liquid mark. The final mark needs the idea that compressing only closes up empty space: air is mostly empty space, water has none left, and the particles themselves cannot be squashed.
(b)2 marks
A balloon has a volume of 2.5 litres at sea level, where the atmospheric pressure is 100 kPa. It rises to an altitude where the atmospheric pressure is only 50 kPa. The temperature is the same at both heights, and the balloon is seen to expand. Using kinetic particle theory, explain why the balloon expanded as it rose to a higher altitude.
Model Answer - Q5(b)
At higher altitude, the external atmospheric pressure is lower, so there are fewer air particles pushing inward on the balloon [1]
The gas particles inside the balloon push outward with the same force (as temperature is constant), so the balloon expands until the internal and external pressures balance [1]
⚠ If you missed marks here: The question keeps the temperature the same, so "the gas heats up and expands" scores nothing. The balloon grows because FEWER air particles hit its outside at altitude, while the gas particles inside keep pushing outwards, so it expands until the pressures inside and outside balance.
(c)2 marks
A fixed volume of gas is at 20°C. The temperature is raised to 100°C. Explain what happens to the pressure of the gas.
Model Answer - Q5(c)
The pressure increases [1]
Because the particles gain kinetic energy and move faster, so they hit the walls of the container more frequently and with greater force [1]
⚠ If you missed marks here: The volume is fixed, so the gas cannot expand – it is the PRESSURE that rises. Do not claim it rises five times (100 ÷ 20): pressure is not proportional to temperature in °C; the marks are for "increases" plus faster particles hitting the walls more often and with more force.
(d)3 marks
A student pushes a gas syringe to halve the volume of gas inside. The temperature remains constant. Explain, using particle theory, why the pressure of the gas doubles.
Model Answer - Q5(d)
The same number of particles is now in half the volume / space [1]
The particles are closer together, so they collide with the walls of the syringe more frequently [1]
More frequent collisions per unit area means the force per unit area (pressure) on the walls doubles [1]
⚠ If you missed marks here: The temperature is constant, so the particles do NOT move faster or hit harder – answers built on extra speed lose the marks. The pressure doubles only because the same number of particles is now in half the volume, so they strike each part of the walls twice as often.
Question 6: Diffusion Experiments
Total: 12 marks
Context: Laboratory experiments in a school in Singapore.
(a)2 marks
Define the term diffusion.
Model Answer - Q6(a)
Diffusion is the net movement of particles [1]
from a region of higher concentration to a region of lower concentration, due to the random motion of the particles [1]
⚠ If you missed marks here: "Particles moving from a low to a high concentration" is backwards and loses the second mark – the net movement is from HIGHER to LOWER concentration. The word "net" earns the first: individual particles move randomly in every direction, but overall more of them move away from where they are crowded.
(b)3 marks
A student opens a bottle of perfume in the corner of a room. After 5 minutes, a student at the other side of the room can smell it. Explain this observation using kinetic particle theory.
Model Answer - Q6(b)
Perfume particles evaporate / escape from the liquid and enter the air as a gas [1]
The perfume gas particles move randomly in all directions and collide with air particles, gradually spreading out / diffusing [1]
There is a net movement of perfume particles from high concentration (near the bottle) to low concentration (across the room), which is why the student eventually smells it [1]
⚠ If you missed marks here: Start with the change of state – the liquid perfume EVAPORATES – or the first mark is lost. The particles do not fly straight across the room: they keep colliding with air particles and changing direction, which is why it takes minutes, and the smell arrives through the NET movement from high concentration near the bottle to low concentration across the room.
(c)3 marks
Describe and explain an experiment to demonstrate diffusion in liquids using potassium manganate(VII).
Model Answer - Q6(c)
Place a crystal of potassium manganate(VII) at the bottom of a beaker of water (without stirring) [1]
Over time, the purple colour spreads throughout the water, even without stirring [1]
This shows that the potassium manganate(VII) particles are moving by diffusion from the high concentration near the crystal to the lower concentration in the surrounding water, due to random particle motion [1]
⚠ If you missed marks here: Do NOT stir – stirring moves the water itself, so the colour spreading would prove nothing about diffusion, and the method mark is lost. Say what you would see (the purple colour slowly spreads until all the water is coloured) and why: random particle motion gives a net movement from the concentrated region near the crystal into the rest of the water.
(d)4 marks
State and explain two factors that affect the rate of diffusion.
Model Answer - Q6(d)
Factor 1: Temperature - higher temperature increases the rate of diffusion [1]
Explanation: because particles have more kinetic energy and move faster at higher temperatures, so they spread out more quickly [1]
Factor 2: Molecular mass / relative formula mass of the particles [1]
Explanation: lighter / smaller particles diffuse faster than heavier / larger ones because they move at higher speeds at the same temperature [1]
⚠ If you missed marks here: Each factor needs its own explanation in particle terms: "temperature" alone is one mark, and the second comes from saying the particles have more kinetic energy and move faster. For mass the direction catches people out – LIGHTER particles (lower Mr) diffuse faster at the same temperature, not heavier ones.
Question 7: Diffusion, Molecular Mass and Calculations
Total: 10 marks
Context: The classic HCl / NH3 diffusion tube experiment.
(a)3 marks
Describe the HCl/NH3 diffusion tube experiment. The diagram below shows the setup.
Model Answer - Q7(a)
A long glass tube is set up horizontally. Cotton wool soaked in concentrated ammonia solution (NH3) is placed at one end, and cotton wool soaked in concentrated hydrochloric acid (HCl) is placed at the other end [1]
Both ends are sealed / plugged simultaneously and the gases are left to diffuse along the tube towards each other [1]
After some time, a white ring of ammonium chloride (NH4Cl) smoke forms inside the tube where the two gases meet [1]
⚠ If you missed marks here: The plugs must go in at BOTH ends at the same time and the tube be sealed, or one gas gets a head start and the result means nothing – that is the method mark people drop. End with what you would see: a white ring of solid ammonium chloride where the gases meet, which is NOT in the middle of the tube.
(b)3 marks
Explain why the white ring of NH4Cl forms closer to the HCl end of the tube.
Model Answer - Q7(b)
NH3 has a smaller relative molecular mass (Mr = 17) than HCl (Mr = 36.5) [1]
Lighter molecules move faster / have a higher rate of diffusion at the same temperature [1]
Therefore NH3 diffuses further along the tube in the same time, so the ring forms closer to the HCl end [1]
⚠ If you missed marks here: Check the Mr values: HCl is 1 + 35.5 = 36.5 (not 35.5) and NH3 is 14 + 3 = 17. The ring forms nearer the HCl end because ammonia is lighter, moves faster and so covers MORE of the tube in the same time; saying the heavier HCl "pushes further" reverses the logic.
(c)1 mark
The tube is 60 cm long. NH3 travels approximately 36 cm before the white ring forms. Calculate the distance HCl travels.
Model Answer - Q7(c)
Distance HCl travels = 60 - 36 = 24 cm [1]
⚠ If you missed marks here: The two distances add up to the whole tube, so HCl travels 60 − 36 = 24 cm. Answering 30 cm (half the tube) ignores that the gases move at different speeds – a shorter distance for the heavier HCl is exactly what you should expect.
(d)3 marks
The experiment is repeated in an identical tube at the same temperature, but the cotton wool at the right-hand end is now soaked in concentrated hydrobromic acid, which gives off hydrogen bromide gas, HBr. Ammonia and hydrogen bromide react to form ammonium bromide, NH4Br, which is also a white solid.
(Ar: H = 1, N = 14, Br = 80; Mr(HCl) = 36.5)
Predict how the position of the white ring compares with its position in the experiment in (a). Explain your answer using relative molecular masses.
Model Answer - Q7(d)
Mr of HBr = 1 + 80 = 81, more than twice the Mr of HCl (36.5) [1]
So HBr molecules move more slowly and HBr diffuses more slowly than HCl did, while the ammonia (Mr 17) diffuses at the same rate as before [1]
The gases therefore meet even closer to the HBr end: the white ring forms further from the ammonia end than before (ammonia covers more than 36 cm of the 60 cm tube) [1]
⚠ If you missed marks here: Work out Mr(HBr) = 1 + 80 = 81 – using 80 forgets the hydrogen. The heavier the gas, the MORE SLOWLY it diffuses, so HBr covers less of the tube than HCl did while the ammonia is unchanged; the gases therefore meet even nearer the HBr end. Saying the ring moves towards the ammonia, or stays in the same place, reverses or ignores the effect of the heavier gas.
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80
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A* : 56+
A : 48-55
B : 40-47
C : 32-39
Below C : <32
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