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]
(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]
(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]
(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]
(e)2 marks
Explain why steam at 100°C causes more severe burns than water at 100°C.
Model Answer - Q1(e)
Steam contains additional (latent) heat energy that was absorbed during boiling / vaporisation [1]
When steam condenses on skin, it releases this extra energy, transferring more heat to the skin than liquid water at the same temperature [1]
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]
(b)3 marks
Using the kinetic particle theory, explain the following properties:
(b) (i)1 mark
Model Answer - Q2(b)(i)
Gas particles are far apart with large spaces between them, so they can be pushed closer together [1]
(b) (ii)1 mark
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]
(b) (iii)1 mark
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]
(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]
(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]
Question 3: Heating Curves and Energy
Total: 12 marks
Context: A chemist studies the heating of a pure substance X from -20°C to 120°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]
(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]
(c)1 mark
State the name of the energy absorbed during melting.
Model Answer - Q3(c)
Latent heat of fusion [1]
(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]
(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]
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]
(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]
(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]
(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]
(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]
Question 5: Gas Behaviour
Total: 10 marks
Context: Experiments with syringes and balloons.
(a)3 marks
A balloon has a volume of 2.5 litres at sea level where the pressure is 100 kPa. The balloon rises to an altitude where the pressure is 50 kPa. Assuming the temperature remains constant, calculate the new volume of the balloon.
Model Answer - Q5(a)
Using Boyle's Law: P1V1 = P2V2 (at constant temperature) [1]
100 × 2.5 = 50 × V2 [1]
V2 = (100 × 2.5) / 50 = 5.0 litres [1]
(b)2 marks
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]
(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]
(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]
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]
(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]
(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]
(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]
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]
(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]
(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]
(d)3 marks
Calculate the ratio of the rate of diffusion of NH3 to HCl.
Given: Mr(NH3) = 17, Mr(HCl) = 36.5 Hint: Graham's Law states that the rate of diffusion is inversely proportional to the square root of the relative molecular mass.