IGCSE Chemistry Paper 4 (Theory / Extended)

Topic 1: States of Matter -- Mock Exam 1

Duration 1 hour 15 minutes

Total Marks 80

Questions 7

Time Remaining 75:00

Syllabus 0620

Instructions

Answer all questions in the spaces provided.
Show all working for calculations. Answers without working may not receive full marks.
Use appropriate scientific terminology in your explanations.
Your answers will be automatically graded when you submit the exam. The model answers will be shown for review.

Question Navigation

Question 1 -- Properties of Solids, Liquids and Gases

Total: 12 marks

A student in a Mumbai laboratory investigates three unknown substances, A, B and C. She records their properties to determine their state at room temperature.

(a) (i) [3]

Complete the table below to describe the properties of solids, liquids and gases.

Property	Solid	Liquid	Gas
Shape	Fixed	Takes the shape of the container	.......
Volume	.......	Fixed	.......
Compressibility	Cannot be compressed	.......	Easily compressed

Model Answer -- 1(a)(i)

Gas shape: fills the container / no fixed shape [1]

Solid volume: fixed [1]

Gas volume: no fixed volume / fills the container [1]

Liquid compressibility: cannot be compressed / very slightly compressible [1]

Note: Only 3 marks available -- any 3 correct entries scores full marks

1 mark -- gas shape fills container 1 mark -- solid volume fixed 1 mark -- gas volume no fixed fills container

(a) (ii) [2]

Using the kinetic particle theory, explain why a solid has a fixed shape.

Model Answer -- 1(a)(ii)

Particles in a solid are held in fixed positions by strong forces of attraction between them [1]

Particles can only vibrate about their fixed positions and cannot move from place to place, so the solid keeps its shape [1]

1 mark -- particles held fixed positions strong forces attraction 1 mark -- particles vibrate fixed positions cannot move keeps shape

(b) [2]

Explain why gases can be compressed but liquids cannot.

Model Answer -- 1(b)

In a gas, particles are far apart with large spaces between them, so particles can be pushed closer together [1]

In a liquid, particles are already close together with very little space between them, so they cannot be pushed much closer [1]

1 mark -- gas particles far apart large spaces pushed closer 1 mark -- liquid particles close together little space cannot compressed

A substance has a melting point of -39°C and a boiling point of 357°C.
(i) State the physical state of the substance at 25°C.
(ii) State the physical state of the substance at 400°C.

Model Answer -- 1(c)

(i) At 25°C the substance is a liquid because 25°C is above the melting point (-39°C) but below the boiling point (357°C) [1]

(ii) At 400°C the substance is a gas because 400°C is above the boiling point (357°C) [1]

1 mark -- liquid at 25 degrees 1 mark -- gas at 400 degrees

(d) [3]

Explain why the density of a gas is much lower than the density of a solid.

Model Answer -- 1(d)

In a gas, particles are very far apart / widely spaced [1]

In a solid, particles are closely packed together [1]

Therefore, for the same volume, a gas contains far fewer particles than a solid, so the mass per unit volume (density) is much lower [1]

1 mark -- gas particles far apart widely spaced 1 mark -- solid particles closely packed together 1 mark -- fewer particles same volume mass lower density

Question 2 -- Particle Theory and States of Matter

Total: 12 marks

A student in a London school laboratory heats ice in a beaker, recording observations as the ice changes from solid to liquid to gas.

(a) [3]

Draw labelled particle diagrams showing the arrangement of particles in:
(i) ice (solid water)
(ii) liquid water
(iii) steam (gaseous water)

Model Answer -- 2(a)

Ice: particles shown in a regular, closely packed arrangement (touching, in rows/layers) [1]

Water: particles close together but irregularly arranged (no fixed pattern, still touching) [1]

Steam: particles widely spaced and randomly distributed (far apart, with large gaps) [1]

1 mark -- ice regular closely packed arrangement 1 mark -- water close together irregular arrangement 1 mark -- steam widely spaced random distributed

(b) [6]

Describe the arrangement, movement and spacing of particles in each of the three states of matter.

Model Answer -- 2(b)

Solid: Particles are arranged in a regular pattern / lattice [1]

Particles vibrate about fixed positions but cannot move from place to place; particles are very close together with no gaps [1]

Liquid: Particles are arranged irregularly / randomly but still close together [1]

Particles can slide over each other and move around; slightly more spaced than solid [1]

Gas: Particles are randomly arranged and very far apart [1]

Particles move rapidly in all directions at high speed; very large spaces between particles [1]

1 mark -- solid regular pattern lattice arrangement 1 mark -- solid vibrate fixed positions close together 1 mark -- liquid irregular random close together 1 mark -- liquid slide over each other move around 1 mark -- gas random far apart 1 mark -- gas move rapidly high speed all directions

Explain why increasing the temperature of a substance increases the motion of its particles.

Model Answer -- 2(c)

When a substance is heated, energy is transferred to the particles [1]

This energy increases the kinetic energy of the particles [1]

Particles with more kinetic energy move faster / vibrate more vigorously [1]

1 mark -- heated energy transferred particles 1 mark -- increases kinetic energy particles 1 mark -- move faster vibrate more vigorously

Question 3 -- Changes of State

Total: 12 marks

A chemist in Delhi heats a pure solid substance from -20°C to well above its boiling point, recording the temperature at regular time intervals.

(a) [2]

(i) Define the term melting point.
(ii) Define the term boiling point.

Model Answer -- 3(a)

(i) Melting point: the temperature at which a solid changes into a liquid (at normal atmospheric pressure) [1]

(ii) Boiling point: the temperature at which a liquid changes into a gas throughout the liquid (at normal atmospheric pressure) [1]

1 mark -- melting point temperature solid changes liquid 1 mark -- boiling point temperature liquid changes gas

(b) [4]

The diagram below shows a heating curve for a pure substance heated at a constant rate.

(i) Identify the state of matter present in each section A-E.
(ii) Explain why the temperature remains constant in sections B and D.

Model Answer -- 3(b)

(i) Section A: solid; Section B: solid + liquid (melting); Section C: liquid; Section D: liquid + gas (boiling); Section E: gas [2]

(ii) In section B (melting), energy is being used to break the forces of attraction between particles / to overcome intermolecular forces, not to increase kinetic energy [1]

In section D (boiling), energy is again used to break the remaining intermolecular forces to separate particles completely, not to increase temperature [1]

1 mark -- correct states for sections A B C 1 mark -- correct states for sections D E 1 mark -- energy used break forces attraction between particles during melting 1 mark -- energy used overcome intermolecular forces during boiling not increase temperature

Explain the difference between evaporation and boiling.

Model Answer -- 3(c)

Evaporation occurs at any temperature below the boiling point, whereas boiling occurs only at the boiling point [1]

Evaporation occurs only at the surface of the liquid, whereas boiling occurs throughout the liquid with bubbles forming [1]

Evaporation is a slow, gradual process, whereas boiling is rapid and vigorous [1]

1 mark -- evaporation any temperature boiling only at boiling point 1 mark -- evaporation surface only boiling throughout liquid bubbles 1 mark -- evaporation slow gradual boiling rapid vigorous

(d) [2]

(i) Name the process by which a gas changes directly into a liquid.
(ii) Name the process by which a solid changes directly into a gas without becoming a liquid first.

Model Answer -- 3(d)

(i) Condensation [1]

(ii) Sublimation [1]

1 mark -- condensation 1 mark -- sublimation

(e) [1]

Explain why the temperature of a pure liquid stays constant while it is boiling.

Model Answer -- 3(e)

All the energy supplied is used to break the intermolecular forces / bonds between particles rather than increasing the kinetic energy / temperature of the particles [1]

1 mark -- energy used break intermolecular forces bonds not increasing kinetic energy temperature

Question 4 -- Heating and Cooling Curves

Total: 12 marks

A student records the temperature of naphthalene as it cools from 100°C. Naphthalene is a pure substance with a melting point of 80°C. Readings are taken every minute for 14 minutes.

(a) [3]

Sketch and label a cooling curve for pure naphthalene cooling from 100°C. Mark the melting/freezing point clearly on the temperature axis.

Model Answer -- 4(a)

Temperature decreases from 100°C as liquid naphthalene cools [1]

A horizontal plateau at 80°C where freezing occurs (temperature remains constant) [1]

After all liquid has solidified, temperature continues to decrease as the solid cools; axes labelled correctly [1]

1 mark -- temperature decreases from 100 liquid cooling 1 mark -- horizontal plateau at 80 degrees freezing point 1 mark -- temperature continues decrease solid cooling axes labelled

(b) [3]

Using the kinetic particle theory, explain what is happening at the plateau on the cooling curve.

Model Answer -- 4(b)

At the plateau, the substance is changing state from liquid to solid (freezing) [1]

Energy is being released / given out as bonds / forces of attraction form between particles [1]

The energy released compensates for heat lost to the surroundings, so the temperature remains constant even though the substance is still losing energy [1]

1 mark -- substance changing state liquid to solid freezing 1 mark -- energy released bonds forces attraction form between particles 1 mark -- energy released compensates heat lost temperature constant

Explain how the cooling curve would differ if the naphthalene sample were impure.

Model Answer -- 4(c)

The freezing point would be lower than 80°C / impurities lower the freezing point [1]

The plateau would slope downwards / not be horizontal / freezing would occur over a range of temperatures rather than at a fixed temperature [1]

1 mark -- freezing point lower impurities depress 1 mark -- plateau slopes downward range temperatures not horizontal

(d) [2]

A different pure substance produces the following heating curve data when heated steadily:

The temperature rises from -10°C to 44°C, then remains constant at 44°C for 3 minutes, then rises again to 118°C, where it remains constant for 4 minutes before rising further.

(i) What is the melting point of this substance?
(ii) What is the boiling point of this substance?

Model Answer -- 4(d)

(i) Melting point = 44°C (the first plateau) [1]

(ii) Boiling point = 118°C (the second plateau) [1]

1 mark -- melting point 44 degrees 1 mark -- boiling point 118 degrees

(e) [2]

Using the cooling curve of naphthalene shown in part (a), the plateau starts at approximately 4 minutes and ends at approximately 10 minutes.
(i) Calculate the time taken for the naphthalene to completely freeze.
(ii) Suggest why the freezing process takes several minutes even though the freezing point is a fixed temperature.

Model Answer -- 4(e)

(i) Time = 10 - 4 = 6 minutes [1]

(ii) It takes time for all particles to form bonds and arrange into the solid structure / energy must be removed gradually from all the particles [1]

1 mark -- time 6 minutes calculation 1 mark -- takes time particles form bonds arrange solid structure energy removed gradually

Question 5 -- Gas Laws and Kinetic Theory

Total: 10 marks

A student in a British school uses a gas syringe connected to a sealed flask to investigate the relationship between pressure, volume and temperature of a gas.

(a) [2]

State what happens to the volume of a gas when:
(i) the temperature is increased at constant pressure
(ii) the pressure is increased at constant temperature

Model Answer -- 5(a)

(i) The volume increases when temperature increases at constant pressure [1]

(ii) The volume decreases when pressure increases at constant temperature [1]

1 mark -- volume increases temperature increases 1 mark -- volume decreases pressure increases

(b) [3]

Using the kinetic particle theory, explain why the pressure of a gas increases when the temperature is raised at constant volume.

Model Answer -- 5(b)

When temperature increases, the particles gain more kinetic energy and move faster [1]

The particles collide with the walls of the container more frequently [1]

The particles also collide with greater force, so the total force on the container walls increases, resulting in higher pressure [1]

1 mark -- particles gain kinetic energy move faster 1 mark -- collide walls container more frequently 1 mark -- collide greater force total force increases higher pressure

A gas has a volume of 200 cm³ at a pressure of 100 kPa. The temperature is kept constant.
Calculate the new volume of the gas when the pressure is increased to 400 kPa.

Model Answer -- 5(c)

p₁V₁ = p₂V₂

Correct formula stated (Boyle's law) [1]

100 x 200 = 400 x V₂

Correct substitution [1]

V₂ = (100 x 200) / 400 = 20000 / 400 = 50 cm³

Correct answer: 50 cm³ [1]

1 mark -- correct formula Boyle law p1V1 equals p2V2 1 mark -- correct substitution values 1 mark -- correct answer 50 cm3

(d) [2]

Explain why a sealed container of gas might burst if it is heated too much.

Model Answer -- 5(d)

Heating increases the kinetic energy of the gas particles so they move faster and hit the container walls harder and more often [1]

Since the container is sealed (fixed volume), the pressure inside increases until it exceeds the strength of the container, causing it to burst [1]

1 mark -- heating kinetic energy particles faster hit walls harder 1 mark -- sealed fixed volume pressure increases exceeds strength burst

Question 6 -- Diffusion

Total: 12 marks

Students at an international school perform a series of experiments to investigate diffusion.

(a) [2]

Define the term diffusion.

Model Answer -- 6(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]

1 mark -- net movement particles 1 mark -- higher concentration to lower concentration random motion

(b) [3]

A glass jar of bromine vapour (reddish-brown) is placed at the bottom, and a second jar of air is placed upside down on top. After some time, the reddish-brown colour spreads upward into the top jar.

Describe the observations over time and explain them using the kinetic particle theory.

Model Answer -- 6(b)

The reddish-brown colour gradually spreads from the bottom jar into the top jar until both jars are the same colour [1]

Bromine particles are in constant random motion and move in all directions, including upward [1]

The bromine particles spread from a region of high concentration (bottom jar) to a region of low concentration (top jar) until evenly distributed [1]

1 mark -- brown colour gradually spreads both jars same colour 1 mark -- bromine particles constant random motion all directions 1 mark -- spread high concentration low concentration evenly distributed

Explain why diffusion is faster in gases than in liquids.

Model Answer -- 6(c)

In gases, particles move much faster than in liquids because they have more kinetic energy at the same temperature / less intermolecular forces [1]

In gases, there are much larger spaces between particles so they can move more freely without as many collisions with other particles [1]

1 mark -- gas particles move faster more kinetic energy 1 mark -- larger spaces between particles move freely fewer collisions

(d) [3]

Cotton wool soaked in concentrated hydrochloric acid (HCl) is placed at one end of a long glass tube, and cotton wool soaked in concentrated ammonia solution (NH₃) is placed at the other end. After some time, a white ring of ammonium chloride (NH₄Cl) forms inside the tube.

Describe the experiment and explain why a white ring forms.

Model Answer -- 6(d)

HCl gas and NH₃ gas diffuse along the tube from opposite ends towards each other [1]

Where the two gases meet, they react to form ammonium chloride: NH₃ + HCl → NH₄Cl [1]

The white ring of NH₄Cl is a solid that appears as a white deposit / smoke inside the tube [1]

1 mark -- HCl NH3 diffuse along tube opposite ends 1 mark -- gases meet react form ammonium chloride NH4Cl 1 mark -- white ring solid deposit smoke inside tube

(e) [2]

Explain why the white ring forms closer to the HCl end rather than in the middle of the tube.

Model Answer -- 6(e)

NH₃ has a smaller relative molecular mass (M_r = 17) than HCl (M_r = 36.5), so NH₃ particles are lighter and move faster [1]

Because NH₃ diffuses faster, it travels further along the tube in the same time, so the gases meet closer to the HCl end [1]

1 mark -- NH3 smaller molecular mass lighter moves faster 1 mark -- NH3 diffuses faster travels further meets closer HCl end

Question 7 -- Diffusion and Molecular Mass

Total: 10 marks

A student compares the diffusion rates of different gases to investigate the relationship between molecular mass and rate of diffusion.

(a) [2]

State the relationship between the relative molecular mass of a gas and its rate of diffusion.

Model Answer -- 7(a)

The greater the relative molecular mass of a gas, the slower its rate of diffusion [1]

This is because heavier particles move more slowly at the same temperature / have less average speed for the same kinetic energy [1]

1 mark -- greater molecular mass slower rate diffusion 1 mark -- heavier particles move slowly same temperature less speed

(b) [3]

Ammonia (NH₃, M_r = 17) and hydrogen chloride (HCl, M_r = 36.5) are released at opposite ends of a tube.

Using Graham's law, calculate the ratio of the rate of diffusion of NH₃ to the rate of diffusion of HCl.
Graham's law: rate₁ / rate₂ = √(M₂ / M₁)

Model Answer -- 7(b)

rate(NH₃) / rate(HCl) = √(M_r of HCl / M_r of NH₃)

Correct formula stated [1]

= √(36.5 / 17) = √2.147

Correct substitution [1]

= 1.47 (to 3 s.f.)

Correct answer: NH₃ diffuses approximately 1.47 times faster than HCl [1]

1 mark -- correct Grahams law formula stated 1 mark -- correct substitution 36.5 divided 17 1 mark -- correct answer ratio approximately 1.47

The tube used in the experiment is 100 cm long. NH₃ is placed at one end and HCl at the other end.

Using the ratio calculated in part (b), predict approximately where the white ring of NH₄Cl will form. State the distance from the HCl end.

Model Answer -- 7(c)

Since NH₃ travels 1.47 times faster than HCl, if HCl travels distance d, then NH₃ travels 1.47d. Together: d + 1.47d = 100, so 2.47d = 100, d = 40.5 cm [1]

The white ring forms approximately 40-41 cm from the HCl end (or equivalently about 59-60 cm from the NH₃ end) [1]

1 mark -- correct method ratio distances calculation 1 mark -- white ring approximately 40 cm from HCl end

(d) [3]

Two gases, X (M_r = 28) and Y (M_r = 44), are released simultaneously at opposite ends of a tube.

(i) Which gas reaches the other end first? Explain your answer.
(ii) Suggest the identity of gas X and gas Y.

Model Answer -- 7(d)

(i) Gas X reaches the other end first because it has a lower relative molecular mass (28 vs 44) [1]

Lighter particles move faster at the same temperature, so gas X diffuses more quickly than gas Y [1]

(ii) Gas X (M_r = 28) could be nitrogen (N₂) or carbon monoxide (CO). Gas Y (M_r = 44) could be carbon dioxide (CO₂) or propane (C₃H₈) [1]

1 mark -- gas X reaches first lower molecular mass 1 mark -- lighter particles move faster diffuses more quickly 1 mark -- correct identification nitrogen carbon monoxide carbon dioxide propane

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