IGCSE Chemistry Paper 4 (Theory/Extended) - Cambridge Challenge

Topic 1: States of Matter | Core + Supplement (1.1 Solids, Liquids and Gases + 1.2 Diffusion)

Duration 75 minutes

Total Marks 80

Questions 7

Time Remaining 75:00

⚡ Cambridge Challenge Level

These questions match real Cambridge IGCSE difficulty. Scoring 50%+ is a solid B, 60%+ is an A, 70%+ is A*. Don't worry if this feels harder -- that's the point!

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.
Where explanations are required, use correct scientific terminology.
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: Advanced States of Matter Analysis

Total: 12 marks

An industrial chemist working in a Mumbai pharmaceutical laboratory analyses several unknown substances. The table below shows the melting points and boiling points of five substances.

Substance	Melting Point / °C	Boiling Point / °C
P	-219	-183
Q	-39	357
R	44	280
S	801	1413
T	-7	59

(a) 3 marks

Using the data in the table, identify the physical state (solid, liquid, or gas) of each substance at:
(i) 25 °C
(ii) 80 °C
Present your answers clearly for all five substances at each temperature.

Model Answer - Q1(a)

At 25 °C: P = gas (25 > -183), Q = liquid (-39 < 25 < 357), R = solid (25 < 44), S = solid (25 < 801), T = liquid (-7 < 25 < 59) [1]

At 80 °C: P = gas, Q = liquid, R = liquid (44 < 80 < 280), S = solid, T = gas (80 > 59) [1]

All answers correct with reasoning that temperature compared to mp and bp determines state [1]

⚠ If you missed marks here: The rule is: below the melting point → solid, between mp and bp → liquid, above the boiling point → gas. The classic slips are calling Q a gas because its mp is -39 (negative doesn't mean gas – 25 °C is still between -39 and 357, so Q is liquid), and missing that T becomes a gas at 80 °C because 80 is above its bp of 59. Check R too: at 80 °C it has melted (80 > 44) but not boiled (80 < 280), so it's a liquid.

1 mark -- correct states at 25 degrees: P gas, Q liquid, R solid, S solid, T liquid 1 mark -- correct states at 80 degrees: P gas, Q liquid, R liquid, S solid, T gas 1 mark -- all ten states correct or reasoning shown using melting and boiling points

(b) 2 marks

Substance Z has a melting point of 0 °C and a boiling point of 100 °C. A student claims that substance Z must be water. Evaluate this claim.

Model Answer - Q1(b)

The claim is not necessarily correct / the student cannot be certain [1]

Other substances could have the same melting and boiling points as water. To identify Z conclusively, further tests would be needed (e.g. density, chemical tests, refractive index). The data is consistent with water but does not prove it [1]

⚠ If you missed marks here: "Evaluate" questions trap students into simply agreeing – if you wrote "yes, 0 °C and 100 °C means it's water" you scored zero. Examiners wanted the sceptical answer: two data points are consistent with water but don't prove it, because another substance could share both values, and you must say what would settle it (density, chemical test, etc.).

1 mark -- claim is not necessarily correct or cannot be certain from melting and boiling points alone 1 mark -- other substances could share same values or further tests needed to confirm identity

Explain why the density of most substances decreases when they melt, but water is an exception. Use particle theory in your answer.

Model Answer - Q1(c)

In most substances, particles are closely packed in a regular arrangement in the solid. When they melt, particles gain energy and move slightly further apart, so volume increases while mass stays the same, giving lower density [1]

In ice, water molecules form an open hexagonal structure due to hydrogen bonding, which holds molecules further apart than in liquid water [1]

When ice melts, this open structure collapses and molecules move closer together, so liquid water is denser than ice [1]

⚠ If you missed marks here: Most lost marks come from only answering half the question – explaining normal melting but skipping WHY water is different. "Ice floats because it's lighter" earns nothing: you need the open hexagonal structure held apart by hydrogen bonding, which collapses on melting so the molecules get CLOSER together. Also watch the wording – density falls because volume increases while mass stays the same, not because particles "get lighter".

1 mark -- most solids have closely packed particles that move apart on melting so density decreases 1 mark -- ice has open structure or hexagonal arrangement due to hydrogen bonding 1 mark -- open structure collapses on melting so liquid water is denser than ice

(d) 4 marks

A sealed steel cylinder contains a gas at 20 °C. The cylinder is heated to 200 °C. Using kinetic particle theory, explain what happens to:
(i) the speed of the particles
(ii) the frequency of collisions with the walls of the cylinder
(iii) the pressure inside the cylinder

Model Answer - Q1(d)

(i) The particles gain kinetic energy as temperature increases, so they move faster / average speed increases [1]

(ii) Because particles move faster, they hit the walls more frequently / the frequency of collisions increases [1]

(ii continued) Each collision also involves greater force because particles have more momentum [1]

(iii) Since the volume is fixed (sealed cylinder), more frequent and harder collisions with the walls means the pressure increases [1]

⚠ If you missed marks here: The mark almost everyone drops is the third one – saying collisions are "more frequent" but forgetting that each collision is also HARDER (more force/momentum) because the particles move faster. Also avoid "the particles expand" or "the gas expands" – the cylinder is sealed, so volume is fixed; that fixed volume is exactly why the pressure must rise.

1 mark -- particles gain kinetic energy and move faster as temperature increases 1 mark -- particles collide with walls more frequently 1 mark -- each collision involves greater force or momentum 1 mark -- pressure increases because volume is constant and collisions more frequent and forceful

Question 2: Complex Particle Theory Applications

Total: 12 marks

Materials scientists study the behaviour of particles in different states and structures to develop new technologies.

(a) 4 marks

Diamond and graphite are both forms of carbon. Using particle theory, explain why diamond is extremely hard while graphite is soft and slippery.

Model Answer - Q2(a)

In diamond, each carbon atom is bonded to four other carbon atoms by strong covalent bonds in a rigid three-dimensional tetrahedral structure [1]

This means there are no weak points in the structure and a large amount of energy is needed to break the bonds, making diamond extremely hard [1]

In graphite, each carbon atom is bonded to three others forming flat layers/sheets of hexagonal rings [1]

The layers are held together by weak intermolecular forces, so the layers can slide over each other easily, making graphite soft and slippery [1]

⚠ If you missed marks here: The big misconception is "graphite has weaker covalent bonds than diamond" – wrong, the covalent bonds within a graphite layer are just as strong. The real difference is 4 bonds per carbon (rigid 3D network) versus 3 bonds per carbon (flat layers), with only WEAK forces BETWEEN the layers letting them slide. You need both the bonding numbers and the weak inter-layer forces to get all four marks.

1 mark -- diamond has each carbon bonded to four others with strong covalent bonds 1 mark -- rigid three-dimensional structure makes diamond hard because many bonds must be broken 1 mark -- graphite has carbon bonded to three others forming flat layers or hexagonal sheets 1 mark -- weak forces between layers allow them to slide making graphite soft and slippery

(b) 3 marks

Glass is sometimes described as a 'supercooled liquid'. Explain what evidence supports this description using particle theory.

Model Answer - Q2(b)

Glass does not have a regular crystalline structure like a true solid; its particles are arranged irregularly/randomly like a liquid [1]

Glass does not have a sharp/definite melting point -- it softens gradually over a range of temperatures, similar to how a very viscous liquid behaves [1]

Over very long time periods, glass can flow (very old window panes are thicker at the bottom), suggesting its particles can move past each other very slowly like a liquid [1]

⚠ If you missed marks here: Answers that just say "glass is like a liquid" without EVIDENCE score zero – the question asked what evidence supports the description. Each mark needs a specific observation: irregular/random particle arrangement (not crystalline), no sharp melting point (softens over a range), and very slow flow over long timescales. Linking each observation back to liquid-like particle behaviour is what earns the marks.

1 mark -- glass has irregular or disordered particle arrangement not regular crystalline structure 1 mark -- glass softens gradually with no sharp or definite melting point 1 mark -- glass can flow slowly over long periods suggesting particles can move past each other

A student observes Brownian motion of smoke particles under a microscope. Describe what they see and explain the cause.

Model Answer - Q2(c)

The smoke particles appear as bright specks of light that move in random, jerky, zigzag paths [1]

The smoke particles are being bombarded by invisible air molecules that collide with them from all directions [1]

The air molecules move randomly and the unequal numbers of collisions from different sides cause the smoke particles to change direction constantly. This provides evidence that air molecules are in constant random motion [1]

⚠ If you missed marks here: The classic error is saying the smoke particles move "because they are hot" or "because they are gas particles" – the smoke particles are NOT moving themselves; they are being knocked about by invisible air molecules hitting them unequally from different sides. You also lose a mark if you describe the motion vaguely ("they move around") instead of random/jerky/zigzag.

1 mark -- smoke particles move in random jerky zigzag paths as bright specks 1 mark -- caused by collisions with invisible air molecules from all directions 1 mark -- unequal collisions cause direction changes providing evidence air molecules in constant random motion

(d) 2 marks

Explain why gases have much lower densities than solids, using specific reference to particle spacing and mass.

Model Answer - Q2(d)

In a gas, the particles are very far apart compared to their size, with large spaces between them. In a solid, particles are closely packed with very little space between them [1]

Although the particles themselves have the same mass, in a given volume a gas contains far fewer particles than a solid, so the mass per unit volume (density) is much lower [1]

⚠ If you missed marks here: The trap is writing "gas particles are lighter" – a water molecule in steam has exactly the same mass as one in ice. The correct logic: particles are far apart in a gas, so a given volume holds far FEWER particles, so mass ÷ volume is smaller. The question explicitly asked for both spacing AND mass, so an answer that mentions only spacing gets 1 of the 2 marks.

1 mark -- gas particles are far apart while solid particles are closely packed 1 mark -- fewer particles in a given volume means lower mass per unit volume so lower density

Question 3: Complex Heating/Cooling Curve Analysis

Total: 12 marks

A student in a Bristol school conducts an experiment with an unknown substance Y. The substance is heated steadily from -20 °C and the temperature is recorded every minute. The heating curve below shows the results.

(a) 2 marks

Using the heating curve, identify the melting point and boiling point of substance Y.

Model Answer - Q3(a)

Melting point = 40 °C (the temperature at which the first plateau occurs, section B) [1]

Boiling point = 120 °C (the temperature at which the second plateau occurs, section D) [1]

⚠ If you missed marks here: Melting and boiling points are read off the TEMPERATURE axis at the flat sections (plateaus B and D), not the time axis – if you wrote "3–6 minutes" you gave a time, not a melting point. The first plateau (40 °C) is always melting and the second (120 °C) is boiling; swapping them is the other common slip. Always include the °C unit.

1 mark -- melting point is 40 degrees C from first plateau 1 mark -- boiling point is 120 degrees C from second plateau

(b) 2 marks

Calculate the rate of temperature rise during section A (before melting), if the temperature rises from -20 °C to 40 °C in 3 minutes.

Model Answer - Q3(b)

Rate = change in temperature / time = (40 - (-20)) / 3 = 60 / 3 [1]

Rate = 20 °C per minute [1]

⚠ If you missed marks here: If you got 6.7 °C/min you subtracted 40 – 20 = 20 instead of 40 – (–20) = 60 – subtracting a negative ADDS, so the temperature change is 60 °C, and 60 ÷ 3 = 20. A bare "20" without the unit °C per minute (or °C/min) also drops the second mark.

1 mark -- correct calculation 40 minus negative 20 equals 60 divided by 3 1 mark -- correct answer 20 degrees per minute with unit

Section C shows the temperature rising from 40 °C to 120 °C. Compare and contrast what is happening to the particles during section A and section C.

Model Answer - Q3(c)

Similarity: In both sections A and C, the temperature is rising, meaning the kinetic energy of the particles is increasing and particles move faster [1]

Difference: In section A, the substance is a solid and the particles are vibrating faster about fixed positions as they gain energy [1]

In section C, the substance is a liquid and the particles are moving around more freely and faster, sliding past each other with increasing speed. The particles are further apart than in section A [1]

⚠ If you missed marks here: "Compare and contrast" means you MUST give a similarity as well as differences – most students only write the differences and lose the first mark (in both A and C, kinetic energy is increasing as temperature rises). The other slip is saying particles in the solid (section A) "move around faster" – in a solid they VIBRATE faster about fixed positions; only in the liquid (section C) do they slide past each other.

1 mark -- both sections show temperature rising and kinetic energy increasing 1 mark -- section A is solid with particles vibrating faster about fixed positions 1 mark -- section C is liquid with particles moving freely and sliding past each other

(d) 3 marks

The student repeats the experiment with an impure sample of Y. Sketch how the heating curve would differ and explain TWO differences.

Model Answer - Q3(d)

Difference 1: The melting point would be lower than 40 °C because impurities lower/depress the melting point [1]

Difference 2: The boiling point would be higher than 120 °C because impurities raise/elevate the boiling point [1]

Difference 3: The plateaus would slope upward rather than being flat, because an impure substance melts/boils over a range of temperatures rather than at a sharp fixed temperature [1]

⚠ If you missed marks here: Get the directions right: impurities LOWER the melting point (below 40 °C) but RAISE the boiling point (above 120 °C) – students often say both go up or both go down. The frequently forgotten third point is that the plateaus stop being flat and slope upward, because an impure substance changes state over a RANGE of temperatures instead of one sharp value.

1 mark -- melting point would be lower because impurities depress melting point 1 mark -- boiling point would be higher because impurities elevate boiling point 1 mark -- plateaus would slope upward because impure substances change state over a range of temperatures

(e) 2 marks

Explain why evaporation can occur at temperatures below the boiling point, using kinetic particle theory. Include reference to the distribution of particle energies.

Model Answer - Q3(e)

In a liquid, not all particles have the same energy -- there is a range/distribution of kinetic energies. Some particles have much more energy than the average [1]

Particles at the surface with enough kinetic energy to overcome the intermolecular forces can escape from the liquid and become gas, even below the boiling point. This is evaporation [1]

⚠ If you missed marks here: The key idea students miss is that not all particles have the same energy – there is a DISTRIBUTION, so some particles have well above the average (the question even told you to mention it). The second mark needs "at the surface" and "overcome intermolecular forces": writing "the fastest particles escape" without saying where from, or what they overcome, is incomplete.

1 mark -- particles have a range or distribution of kinetic energies and some have more energy than average 1 mark -- surface particles with enough energy overcome intermolecular forces and escape as gas

Question 4: Gas Laws -- Quantitative and Qualitative

Total: 12 marks

Gas behaviour is investigated in both school laboratories and industrial settings. The following questions explore how gases respond to changes in pressure, volume, and temperature.

(a) 3 marks

A gas occupies 500 cm³ at a pressure of 100 kPa. Calculate the volume when the pressure is increased to 250 kPa at constant temperature. Show your working.

Model Answer - Q4(a)

Use Boyle's Law: P₁V₁ = P₂V₂ (at constant temperature) [1]

100 × 500 = 250 × V₂ [1]

V₂ = 50 000 / 250 = 200 cm³ [1]

⚠ If you missed marks here: If you got 1250 cm³ you multiplied by 250/100 instead of dividing – sanity-check first: pressure went UP (100 → 250 kPa), so the volume MUST come out smaller than 500 cm³. Set out P₁V₁ = P₂V₂, substitute 100 × 500 = 250 × V₂, then V₂ = 50 000 ÷ 250 = 200 cm³ – showing the substitution line earns a mark even if the arithmetic slips.

1 mark -- Boyle's law stated or P1V1 equals P2V2 1 mark -- correct substitution 100 times 500 equals 250 times V2 1 mark -- correct answer 200 cm cubed

(b) 2 marks

The same gas is now heated from 27 °C to 327 °C at constant pressure. Explain qualitatively what happens to the volume.

Model Answer - Q4(b)

When the gas is heated, the particles gain kinetic energy and move faster, colliding with the walls of the container more frequently and with greater force [1]

At constant pressure, the gas must expand (volume increases) so that the increased particle speed is offset by a larger container volume, maintaining the same pressure. The volume doubles because the absolute temperature doubles (from 300 K to 600 K) [1]

⚠ If you missed marks here: "The gas expands when heated" on its own is not an explanation – you need the particle story: faster particles hit the walls more often and harder, so at CONSTANT pressure the volume must grow to compensate. And if you tried to quantify it, 327 ÷ 27 ≈ 12× is the classic Celsius trap: convert to kelvin first (300 K → 600 K), so the volume only doubles.

1 mark -- particles gain kinetic energy move faster and collide more with walls 1 mark -- volume increases or gas expands to maintain constant pressure

A deep-sea diver ascends from 40 m depth. The total pressure at 40 m is 500 kPa and at the surface is 100 kPa. A bubble of gas has volume 2.0 cm³ at 40 m. Calculate its volume at the surface. Assume constant temperature.

Model Answer - Q4(c)

P₁V₁ = P₂V₂ [1]

500 × 2.0 = 100 × V₂ [1]

V₂ = 1000 / 100 = 10.0 cm³ [1]

⚠ If you missed marks here: If you got 0.4 cm³ you flipped the ratio – think physically: the bubble rises to LOWER pressure (500 → 100 kPa), so it must EXPAND, and since pressure drops to one fifth the volume grows five-fold: 2.0 × 5 = 10.0 cm³. Write the substitution 500 × 2.0 = 100 × V₂ to lock in the method mark.

1 mark -- Boyle's law or P1V1 equals P2V2 applied 1 mark -- correct substitution 500 times 2.0 equals 100 times V2 1 mark -- correct answer 10.0 cm cubed

(d) 2 marks

Explain why the diver must ascend slowly, linking your answer to the gas law calculation above.

Model Answer - Q4(d)

As the diver ascends, the pressure decreases and dissolved gases (especially nitrogen) in the blood expand / form bubbles, as shown by the gas bubble volume increasing from 2.0 to 10.0 cm³ [1]

If the diver ascends too quickly, these expanding gas bubbles in the blood and tissues can block blood vessels and cause decompression sickness (the bends), which can be fatal [1]

⚠ If you missed marks here: Vague answers like "it's dangerous to come up fast" or "the diver's lungs would burst" don't score – the question said LINK to the calculation, so quote it: gas in the blood expands 5× (2.0 → 10.0 cm³) as pressure falls from 500 to 100 kPa. Then name the consequence: expanding bubbles block blood vessels, causing decompression sickness (the bends).

1 mark -- pressure decreases and dissolved gases expand or form bubbles in the blood 1 mark -- rapid ascent causes decompression sickness or the bends from expanding gas bubbles

(e) 2 marks

A car tyre contains gas at 200 kPa. After a long drive, the tyre pressure increases to 220 kPa. Explain this observation using kinetic particle theory.

Model Answer - Q4(e)

Friction between the tyre and road surface generates heat, which increases the temperature of the gas inside the tyre. The gas particles gain kinetic energy and move faster [1]

The volume of the tyre is approximately constant (rigid walls), so the faster-moving particles collide with the tyre walls more frequently and with greater force, increasing the pressure from 200 to 220 kPa [1]

⚠ If you missed marks here: Two things get skipped: WHERE the heat comes from (friction with the road – "the tyre gets hot" alone is too vague) and the constant-volume link. Saying "the air expands" is the giveaway error – the tyre's volume is roughly fixed, which is exactly why faster, harder, more frequent wall collisions show up as a pressure rise (200 → 220 kPa) instead of an expansion.

1 mark -- friction heats the tyre and gas particles gain kinetic energy and move faster 1 mark -- constant volume so more frequent and forceful collisions increase pressure

Question 5: Advanced Diffusion Analysis

Total: 10 marks

Forensic scientists at a crime scene investigate a gas leak inside a sealed room. Understanding diffusion is critical for predicting how quickly hazardous gases spread.

(a) 3 marks

A sealed room contains a gas leak. Using kinetic particle theory, explain why the gas eventually spreads uniformly throughout the room.

Model Answer - Q5(a)

Gas particles are in constant random motion and move at high speeds in all directions [1]

The leaked gas particles collide with air molecules and with each other, changing direction after each collision. This causes them to spread out from areas of high concentration to areas of low concentration (diffusion) [1]

Eventually, through these random collisions, the gas particles become evenly distributed throughout the room, reaching a uniform concentration (equilibrium) [1]

⚠ If you missed marks here: "The gas spreads out" just restates the question – the marks are for the mechanism: constant RANDOM motion, collisions with air molecules changing particle directions, and net movement from high to low concentration. Avoid saying the gas "wants" or "tries" to spread, or that it flows like wind – diffusion is purely the statistical result of random collisions, ending in uniform concentration.

1 mark -- gas particles in constant random motion at high speeds 1 mark -- particles collide and spread from high concentration to low concentration 1 mark -- eventually evenly distributed reaching uniform concentration or equilibrium

(b) 3 marks

The gas has a relative molecular mass (M_r) of 44. Another gas with M_r = 2 is released at the same time from the same point. The rate of diffusion is inversely proportional to the square root of the relative molecular mass. Calculate how many times faster the lighter gas diffuses compared to the heavier gas.

Model Answer - Q5(b)

Rate₁ / Rate₂ = √(M₂ / M₁) (Graham's Law stated or applied) [1]

Ratio = √(44 / 2) = √22 [1]

√22 ≈ 4.7 times faster [1]

⚠ If you missed marks here: If your answer was 22, you forgot the square root – the rate is inversely proportional to √Mₛ, so the ratio is √(44 ÷ 2) = √22 ≈ 4.7, not 44 ÷ 2 = 22. If you got 0.21 you inverted the fraction: the LIGHTER gas is faster, so the answer must be bigger than 1.

1 mark -- Graham's law or rate ratio equals square root of mass ratio 1 mark -- correct substitution square root of 44 divided by 2 equals square root of 22 1 mark -- correct answer approximately 4.7 times faster

Explain why diffusion in liquids is much slower than in gases, with specific reference to particle spacing and collisions.

Model Answer - Q5(c)

In liquids, particles are much closer together than in gases, so there is less free space for particles to move into [1]

Liquid particles collide far more frequently with neighbouring particles due to the closer spacing, so they cannot travel as far between collisions, and their net movement in any one direction is much slower [1]

⚠ If you missed marks here: Saying only "particles are closer together in a liquid" gets 1 mark, not 2 – you must complete the chain: closer spacing → far more frequent collisions → particles travel only a tiny distance between collisions → slow net movement. A common wrong answer is "liquid particles move slower" – their speed isn't the key point; it's how often they get knocked off course by neighbours.

1 mark -- liquid particles are closer together with less free space to move into 1 mark -- more frequent collisions so particles cannot travel as far between collisions

(d) 2 marks

In a hospital, oxygen (M_r = 32) and an anaesthetic gas (M_r = 142) are stored in adjacent rooms. If both leak simultaneously, which reaches a detector placed at equal distance first? Explain your answer.

Model Answer - Q5(d)

Oxygen reaches the detector first because it has a lower relative molecular mass (32 vs 142) [1]

Lighter molecules move faster at the same temperature (higher average speed) and therefore diffuse faster than heavier molecules [1]

⚠ If you missed marks here: Naming oxygen without a reason only gets the first mark – the explanation mark needs the link: at the SAME temperature, lighter molecules (Mₛ = 32 vs 142) have a higher average speed, so they diffuse faster. Don't say oxygen "is smaller" or "less dense" – it's the lower relative molecular MASS that matters.

1 mark -- oxygen reaches detector first because it has lower molecular mass 1 mark -- lighter molecules move faster at same temperature so diffuse faster

Question 6: Experimental Design and Evaluation

Total: 12 marks

Planning and evaluating experiments is a key skill in chemistry. The following questions test your ability to design practical investigations and analyse results.

(a) 4 marks

Design an experiment to determine the boiling point of an unknown liquid. Include:

the apparatus needed
the method (step by step)
how to ensure accuracy of the result

Model Answer - Q6(a)

Apparatus: beaker (water bath), boiling tube containing the liquid, thermometer (-10 to 110 °C or suitable range), Bunsen burner or electric heater, clamp and stand, stopwatch [1]

Method: Place the unknown liquid in a boiling tube. Heat the liquid gently using a water bath. Record the temperature at regular intervals (e.g. every 30 seconds). Note the temperature at which the liquid boils (constant temperature with vigorous bubbling throughout the liquid, not just at the surface) [1]

The boiling point is the temperature at which the reading remains constant while the liquid changes to gas [1]

Accuracy: Use a water bath for even heating (not direct flame); ensure the thermometer bulb is in the liquid but not touching the glass; repeat the experiment and take an average; read the thermometer at eye level to avoid parallax error [1]

⚠ If you missed marks here: Design questions are marked against the bullet list, so answer all three: apparatus, step-by-step method, AND accuracy – the accuracy mark is the one most often left out entirely. The other frequent gap is not defining WHEN you've found the boiling point: it's the temperature that stays CONSTANT while the liquid boils, not just "when it bubbles". "Be careful" is not an accuracy measure – name a specific one (water bath, repeats and average, eye-level reading).

1 mark -- suitable apparatus including thermometer and beaker or water bath and heat source 1 mark -- method describes heating liquid and recording temperature at intervals 1 mark -- boiling point identified as constant temperature during state change 1 mark -- accuracy measures such as water bath or repeat experiment or eye level reading

(b) 3 marks

A student heats water in an open beaker and records the following data:

Time / min	0	2	4	6	8	10	12	14
Temperature / °C	20	40	60	80	100	100	100	100

Describe the expected graph shape and explain why the temperature levels off at 100 °C.

Model Answer - Q6(b)

The graph shows a straight line rising from 20 °C to 100 °C between 0-8 minutes, then a horizontal/flat line (plateau) at 100 °C from 8 minutes onwards [1]

At 100 °C, the water is boiling. The heat energy supplied is being used to break the intermolecular forces (bonds) between water molecules, not to increase their kinetic energy [1]

Since temperature measures the average kinetic energy of particles, the temperature does not rise while the change of state (liquid to gas) is occurring [1]

⚠ If you missed marks here: "Water can't get hotter than 100 °C" describes the plateau but doesn't explain it – the energy mark needs: the heat supplied is used to BREAK the intermolecular forces between water molecules, NOT to increase their kinetic energy. Careful with wording: it's intermolecular forces being overcome, not covalent bonds inside H₂O being broken – that's a different (wrong) claim.

1 mark -- graph rises linearly from 20 to 100 then levels off as horizontal plateau 1 mark -- heat energy used to break intermolecular forces not increase kinetic energy 1 mark -- temperature stays constant during change of state from liquid to gas

Another student claims that adding salt to water will raise its boiling point. Design a simple experiment to test this claim. Include: the independent variable, the dependent variable, and at least two control variables.

Model Answer - Q6(c)

Independent variable: the amount of salt added (e.g. 0 g, 5 g, 10 g, 15 g) OR whether salt is present or not [1]

Dependent variable: the boiling point of the water (measured with a thermometer) [1]

Control variables: volume of water (e.g. always use 200 cm³), same heat source at the same setting, same type of container, same type of salt, atmospheric pressure [1]

⚠ If you missed marks here: The usual mix-up: the INDEPENDENT variable is what you change (mass of salt added), the DEPENDENT is what you measure (the boiling point) – writing them the wrong way round loses both marks. For the third mark you need at least TWO named control variables (e.g. same volume of water, same heat setting); "keep everything else the same" is too vague to score.

1 mark -- independent variable is amount of salt added 1 mark -- dependent variable is boiling point measured with thermometer 1 mark -- at least two control variables such as volume of water and heat source

(d) 2 marks

Evaluate the limitations of the particle model. Give TWO examples of phenomena the simple particle model does not explain well.

Model Answer - Q6(d)

Limitation 1: The simple model treats particles as hard spheres with no internal structure, so it cannot explain why different substances have different melting/boiling points (requires understanding of different types and strengths of intermolecular forces) [1]

Limitation 2: The model does not explain why water expands on freezing (anomalous expansion of water requires understanding of hydrogen bonding and crystal structure), or why some substances are coloured, or why some conduct electricity [1]

⚠ If you missed marks here: "It's too simple" or "particles aren't really spheres" only scores if you say what the model then FAILS to explain – each mark = a named phenomenon, e.g. it can't explain why substances have different melting points (no forces between the hard spheres in the model), why water expands on freezing, why some substances conduct or are coloured. One limitation stated twice in different words counts once.

1 mark -- first valid limitation such as cannot explain different melting points or treats particles as identical spheres 1 mark -- second valid limitation such as cannot explain water expanding on freezing or colour or conductivity

Question 7: Synoptic -- Linking Concepts

Total: 10 marks

Real-world applications often require combining multiple concepts about states of matter and particle behaviour.

(a) 3 marks

An aerosol can contains liquid propellant and gas under pressure. When the nozzle is pressed, the liquid turns to gas and the spray feels cold on the skin. Explain these observations using kinetic particle theory.

Model Answer - Q7(a)

When the nozzle is pressed, the pressure inside the can drops, allowing the liquid propellant to boil/evaporate (change from liquid to gas) because the boiling point is lowered at reduced pressure [1]

To change from liquid to gas, the particles must overcome the intermolecular forces holding them together. This requires energy [1]

This energy is taken from the surroundings (the skin and the air nearby), causing the temperature of the surroundings to drop, which is why the spray feels cold. This is an endothermic process [1]

⚠ If you missed marks here: "The gas inside the can is cold" is the classic wrong answer – the propellant isn't cold to start with; evaporation MAKES things cold. The chain the examiner wants: pressure drops when the nozzle opens → liquid boils/evaporates → energy is needed to overcome intermolecular forces → that energy is taken FROM your skin (endothermic), which is why it feels cold.

1 mark -- pressure drops when nozzle pressed allowing liquid to boil or evaporate 1 mark -- energy needed to overcome intermolecular forces during change of state 1 mark -- energy taken from surroundings causing cooling or endothermic process

(b) 3 marks

Freeze-drying is used to preserve food. In this process, frozen food is placed in a vacuum and the ice turns directly to vapour. Name this change of state and explain why a vacuum is necessary.

Model Answer - Q7(b)

The change of state is sublimation (solid directly to gas without passing through the liquid state) [1]

A vacuum (very low pressure) is necessary because at normal atmospheric pressure, ice would melt to liquid water first before evaporating [1]

At very low pressures, the boiling point is reduced so much that the ice can change directly to water vapour without melting first, preserving the structure and nutrients of the food [1]

⚠ If you missed marks here: Calling it "evaporation" costs the first mark – solid straight to gas with no liquid stage is SUBLIMATION. For the vacuum marks, "the vacuum sucks the water out" earns nothing: the point is that at very low pressure the ice does not melt first (which it would at normal atmospheric pressure) but changes directly to vapour, keeping the food's structure intact.

1 mark -- sublimation named as the change of state from solid directly to gas 1 mark -- vacuum provides very low pressure which prevents normal melting 1 mark -- low pressure allows ice to change directly to vapour preserving food structure

On a humid day in Singapore, water droplets form on the outside of a cold drink can. On a dry day in Rajasthan, the same effect is not observed. Explain this difference using particle theory.

Model Answer - Q7(c)

In humid Singapore, the air contains a high concentration of water vapour molecules. When these fast-moving gas particles contact the cold can surface, they lose kinetic energy and slow down enough to be attracted to each other by intermolecular forces, condensing into liquid water droplets [1]

In dry Rajasthan, there are very few water vapour molecules in the air, so even though the can is cold, there are not enough water vapour particles to form visible droplets on the surface [1]

⚠ If you missed marks here: "The water comes from the drink leaking through the can" is the misconception to avoid – the droplets are water vapour from the AIR condensing. The particle-level detail is what earns the first mark: vapour molecules hit the cold surface, LOSE kinetic energy, slow down, and intermolecular forces pull them together into liquid. The second mark is simply that dry air has too few water vapour molecules for visible droplets.

1 mark -- humid air has high concentration of water vapour that loses kinetic energy on cold surface and condenses 1 mark -- dry air has very few water vapour molecules so not enough to form visible droplets

(d) 2 marks

A student sets up the NH₃/HCl diffusion tube experiment in a warm laboratory (30 °C) and then repeats it in a cold laboratory (10 °C). Predict and explain how the results would differ.

Model Answer - Q7(d)

In the warm laboratory (30 °C), the white ring of ammonium chloride would form more quickly because at higher temperatures, the gas particles have more kinetic energy and move faster, so both NH₃ and HCl diffuse more rapidly [1]

The position of the white ring would be the same relative distance from each end (closer to the HCl end) because the ratio of molecular masses has not changed, so the relative rates of diffusion remain the same. Only the overall speed of the process changes, not the position [1]

⚠ If you missed marks here: Most students get the first mark (ring forms faster at 30 °C because particles have more kinetic energy) but then guess that the ring MOVES – it doesn't. Both NH₃ (Mₛ = 17) and HCl (Mₛ = 36.5) speed up together, so their mass RATIO – and therefore the ring's position nearer the HCl end – is unchanged; only the time taken changes.

1 mark -- white ring forms faster in warm lab because particles have more kinetic energy and move faster 1 mark -- position of white ring stays the same because ratio of molecular masses unchanged

Exam Score Summary

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out of 80

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Grade

A* : 56+

A : 48-55

B : 40-47

C : 32-39

D : 24-31

E : 16-23

U : <16

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