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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
⚠ If you missed marks here: A gas has NO fixed volume as well as no fixed shape – it spreads out to fill whatever container it is in, so writing "fixed" for gas volume (because the jar is a fixed size) loses that mark. For the liquid, "can be compressed" is wrong: its particles are already touching, so it hardly compresses at all, just like the solid.
(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]
⚠ If you missed marks here: "The particles are close together" does not explain a fixed shape – liquid particles are close together too, yet a liquid flows. The marks are for strong forces holding the particles in FIXED positions and for saying they only VIBRATE about those positions; "the particles do not move" is wrong, because they do vibrate.
(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]
⚠ If you missed marks here: Compressing a gas does not squash the particles themselves – it pushes them into the large empty spaces between them, so "the gas particles get smaller" scores nothing. For the liquid, the reason is that its particles are already touching with almost no space left; saying they are "held in fixed positions" describes a solid, not a liquid.
(c)[2]
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]
⚠ If you missed marks here: −39°C is BELOW room temperature, so at 25°C the substance has already melted but is still far below its boiling point: it is a liquid. Answering "solid" because −39 looks like a cold number, or "liquid" at 400°C because you only checked the melting point, are the usual slips – always place the temperature between the melting and boiling points.
(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]
⚠ If you missed marks here: Gas particles are NOT lighter or smaller than solid particles – for the same substance they are identical, so answers built on "gas particles weigh less" score nothing. The density is lower because the particles are far apart, so the same volume holds far FEWER particles and therefore less mass; the third mark needs that "fewer particles in the same volume" link.
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]
⚠ If you missed marks here: The mark most often lost is the liquid: draw its particles still TOUCHING one another but in an irregular jumble, not spread out with gaps like steam. Keep every particle the same size in all three drawings – only the arrangement and spacing change, not the particles themselves.
(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]
⚠ If you missed marks here: Each state needs BOTH its arrangement and spacing AND its movement, so describing only how the particles are arranged caps you at three of the six marks. The usual slips are saying solid particles "do not move" (they vibrate about fixed positions) and giving the liquid big gaps like a gas – liquid particles are still close together but slide over one another.
(c)[3]
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]
⚠ If you missed marks here: "The particles expand when heated" is wrong and scores nothing – the particles stay the same size; it is their kinetic energy that rises. The three marks follow a chain: energy is transferred to the particles, their kinetic energy increases, so they move (or vibrate) faster – skipping the kinetic-energy step loses the middle mark.
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]
⚠ If you missed marks here: A definition must name a TEMPERATURE and the change of state: "the point where it melts" just repeats the word being defined. For boiling point, "the temperature at which a liquid evaporates" is wrong – evaporation happens at any temperature, whereas boiling is the change to gas throughout the liquid at one fixed temperature.
(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.
(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]
⚠ If you missed marks here: B and D each contain TWO states (solid + liquid while melting, liquid + gas while boiling), so labelling B simply "liquid" or D "gas" costs the state marks. On the flat sections heat is still being supplied – "no energy is taken in" is wrong; the energy is used to overcome the forces of attraction between particles instead of raising their kinetic energy.
(c)[3]
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]
⚠ If you missed marks here: Saying only that evaporation is "slower" earns one mark at most – the examiner wants WHERE (evaporation only at the surface; boiling throughout the liquid, with bubbles) and WHEN (evaporation at any temperature; boiling only at the boiling point). "Boiling happens above the boiling point" is wrong: a liquid boils AT its boiling point, and its temperature stays there until it has all turned to gas.
(d)[2]
(i) Name the process by which a gas changes directly into a liquid.
(ii) In a foundry, molten iron at 1600°C is poured into a mould and left until it has set solid. Name the change of state that takes place as the iron sets.
Model Answer -- 3(d)
(i) Condensation [1]
(ii) Freezing (accept solidifying / solidification): liquid iron turning into solid iron is freezing, even though it happens at over 1500°C [1]
⚠ If you missed marks here: Gas → liquid is condensation; "evaporation" is the reverse change and scores nothing. Liquid → solid is FREEZING even for iron, which freezes at over 1500°C: a change of state is named by the two states it goes between, not by whether it feels cold, so "cooling", "setting" or "condensing" do not score.
(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]
⚠ If you missed marks here: "The liquid cannot get any hotter" or "the heat escapes as steam" describes what happens, not why, and scores nothing. The mark needs the energy being used to overcome the forces of attraction between the particles so they can separate into a gas, rather than to increase their kinetic energy (and so the 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]
⚠ If you missed marks here: This is a COOLING curve, so the line must fall from 100°C, run flat at 80°C and then fall again – a rising heating curve scores nothing for shape. Put the plateau at 80°C (a pure substance freezes at its melting point), mark 80 on the temperature axis and label both axes; a plateau drawn at any other temperature loses the second mark.
(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]
⚠ If you missed marks here: At the plateau the naphthalene is still losing heat to the room – "it stops cooling" or "no energy is lost" misses the point. The temperature holds because, as the particles are pulled together into fixed positions, forces of attraction form and RELEASE energy that replaces the heat lost; saying energy is "absorbed" to form the solid is backwards.
(c)[2]
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]
⚠ If you missed marks here: Impurities LOWER the freezing point, so "it would freeze at a higher temperature" is the wrong way round. The second mark is for the shape: an impure sample freezes over a range of temperatures, so the flat section becomes a downward slope – "it takes longer to freeze" is not the same idea and does not score.
(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]
⚠ If you missed marks here: Heating goes solid → liquid → gas, so the FIRST plateau (44°C) is the melting point and the SECOND (118°C) is the boiling point – swapping them loses both marks. Do not answer with 3 or 4 minutes: those are how LONG each change of state took, not the temperature at which it happened.
(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]
⚠ If you missed marks here: The freezing time is the LENGTH of the plateau, 10 − 4 = 6 minutes; giving 10 (when freezing ends) or 14 (adding the two times) loses the mark. For (ii), "because the freezing point is fixed" just repeats the question: the energy released as the particles take up fixed positions has to be lost to the surroundings gradually, so freezing every particle takes minutes.
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]
⚠ If you missed marks here: Heating a gas at constant pressure makes it EXPAND (volume increases); squeezing it to a higher pressure at constant temperature makes its volume DECREASE. The usual slip is reversing one of the two – check against the syringe: warm it and the plunger moves out; push the plunger in and the gas takes up less room.
(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]
⚠ If you missed marks here: Heating does not make the gas particles bigger or create more of them, so answers built on "the particles expand" score nothing. Full marks need both effects of the faster particles – they hit the walls MORE OFTEN and with MORE FORCE – so mentioning only "more collisions" loses the final mark.
(c)[3]
The syringe and the sealed flask hold a fixed amount of air: no air can get in or out. The student pulls the plunger out a short distance, holds it there for a moment and then lets go. The temperature of the air stays the same throughout. The plunger slides back in to its starting position.
Using the kinetic particle theory, explain why the plunger moves back in. In your answer, state what happens to the speed of the air particles inside the syringe when the plunger is pulled out.
Model Answer -- 5(c)
Pulling the plunger out gives the same number of air particles a larger volume, so they are further apart [1]
The temperature is constant, so the particles keep the same speed (the same kinetic energy); but because they are spread out they hit the plunger and walls less often, so the pressure inside falls below the atmospheric pressure outside [1]
Air particles outside now hit the plunger more often than those inside, so the greater pressure outside pushes the plunger back in until the pressures inside and outside are equal again, at the starting volume [1]
⚠ If you missed marks here: The temperature stays the same, so the air particles do NOT slow down when the plunger is pulled out – answers built on "the particles lose energy" score nothing. They keep their speed but spread into a larger volume, so they hit each part of the plunger less often and the pressure inside drops below the pressure of the air outside. That difference in pressure pushes the plunger back until the two pressures are equal again; "the plunger is sucked back" gives no particle explanation.
(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]
⚠ If you missed marks here: "The gas expands and bursts the container" misses the point: in a sealed container the volume cannot change, so it is the PRESSURE that rises. Both marks need the chain – faster particles hit the walls harder and more often, so the pressure rises until it is more than the container can withstand.
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]
⚠ If you missed marks here: The direction must be from HIGHER to LOWER concentration – writing it the other way round loses the second mark. "Particles spreading out" is too vague for the first: say NET movement of particles, caused by their 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]
⚠ If you missed marks here: Bromine (Br2, Mr 160) is much heavier than the nitrogen and oxygen in air, so "the bromine rises because it is light" is wrong – it spreads upward only because its particles move randomly in all directions. Finish the observation as well: the colour spreads until BOTH jars are the same colour, which is when the concentration is even.
(c)[2]
Explain why diffusion is faster in gases than in liquids.
Model Answer -- 6(c)
In a gas the particles are far apart with almost no forces of attraction between them, so each particle travels a long way in a straight line before it hits another particle [1]
In a liquid the particles are close together and attract one another, so each particle collides with its neighbours all the time and keeps changing direction; it moves only a short distance in a given time, so it spreads much more slowly [1]
Note: do not say that gas particles have more kinetic energy – at the same temperature the particles in a gas and in a liquid have the same average kinetic energy. The difference is spacing, forces and freedom to move.
⚠ If you missed marks here: "Gas particles are smaller" is not the reason, and nor is "gas particles have more kinetic energy" – at the same temperature the particles in a gas and in a liquid have the same average kinetic energy. The difference is spacing, forces and freedom to move: gas particles are far apart with almost no attraction between them, so they travel a long way before hitting another particle, while in a liquid every particle is constantly bumping into close neighbours that attract it, so it spreads slowly.
(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]
⚠ If you missed marks here: The cotton wool holds solutions, but it is the GASES (ammonia and hydrogen chloride) given off from them that diffuse along the tube – saying the liquids soak or flow along it misses the first mark. The white ring is a new SOLID, ammonium chloride, made where the two gases meet and react; "the gases mix and turn white" gives no reaction and no product.
(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 (Mr = 17) than HCl (Mr = 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]
⚠ If you missed marks here: The ring forms where the gases MEET, so it sits nearer the end of the SLOWER gas: HCl (Mr 36.5) is heavier and moves more slowly than NH3 (Mr 17), so ammonia covers more of the tube in the same time. Saying the heavier gas "travels further", or quoting the Mr values without linking lighter to faster, loses the marks.
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]
⚠ If you missed marks here: The relationship runs the opposite way to what many write: the HIGHER the Mr, the SLOWER the gas diffuses. The second mark needs the reason – at the same temperature heavier particles move more slowly – not "heavier particles have more energy", which is wrong.
(b)[3]
The student sets up the apparatus shown. An unglazed porous pot full of air is closed with a bung. Gas particles can pass in both directions through the tiny holes in the wall of the pot. A glass tube runs from the bung down into a beaker of water.
A large beaker full of hydrogen is held upside down over the pot. Bubbles of gas come out of the bottom of the glass tube.
Explain why bubbles come out of the glass tube. Use relative molecular masses in your answer.
(Ar: H = 1. The average Mr of the gases in air is 29.)
Model Answer -- 7(b)
Hydrogen, H2, has Mr = 2, far lower than the average Mr of air (29), so hydrogen molecules move faster at the same temperature [1]
So hydrogen diffuses into the pot through the pores faster than the air diffuses out [1]
The number of gas particles inside the pot increases, so the pressure inside rises above atmospheric pressure and gas is pushed out through the glass tube as bubbles [1]
⚠ If you missed marks here: Gas passes through the pores in BOTH directions, so "hydrogen gets into the pot" is not enough. The bubbles appear because hydrogen (Mr 2) diffuses IN faster than the much heavier air (average Mr 29) diffuses OUT: particles build up inside the pot, the pressure rises and gas is driven out of the tube. Saying the pot "heats up" or that hydrogen "pushes" the air out gives no reason for the pressure rise.
(c)[2]
The hydrogen is taken away and the pot is left until it is full of air again. The pot is then surrounded by argon gas instead (Ar of argon = 40).
Predict what happens to the water in the glass tube. Explain your answer.
Model Answer -- 7(c)
Argon is monatomic, so its Mr is 40, more than the average Mr of air (29): argon particles move more slowly, so air diffuses out of the pot faster than argon diffuses in [1]
The number of gas particles inside the pot falls, so the pressure inside drops below atmospheric pressure and the water is pushed up the glass tube (no bubbles) [1]
⚠ If you missed marks here: Argon is a noble gas, so its particles are single atoms and its Mr is 40 (not 80). That is HEAVIER than air (29), so this time the air escapes from the pot faster than argon gets in – the opposite of the hydrogen experiment. Particles are lost from the pot, its pressure falls below atmospheric pressure and the water is pushed UP the tube; predicting bubbles again, or "nothing happens", loses both marks.
(d)[3]
Two gases, X (Mr = 28) and Y (Mr = 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 (Mr = 28) could be nitrogen (N2) or carbon monoxide (CO). Gas Y (Mr = 44) could be carbon dioxide (CO2) or propane (C3H8) [1]
⚠ If you missed marks here: Gas X (Mr 28) arrives first because lighter particles move faster at the same temperature – "Y, because heavier particles push through faster" is wrong. For the identities, work from the Mr: 28 is N2 (2 × 14) or CO and 44 is CO2 (12 + 32); writing "N" for X forgets that nitrogen gas is diatomic.
Self-Assessment
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