IGCSE Physics 2.1 Kinetic Molecular Model MCQs – 30 Practice Questions with Answers (Cambridge 0625)
Revise Cambridge IGCSE Physics 0625 Section 2.1, the Simple Kinetic Molecular Model of Matter, with 30 original practice MCQs covering solids, liquids and gases, Brownian motion, diffusion, evaporation, and the behaviour of gases. Every question has a worked answer explanation, and Supplement-only questions are tagged [Supplement].
Formula and ideas recap: the temperature of a substance depends on the average kinetic energy of its particles, and gas pressure is caused by particles colliding with the walls of the container. For a fixed mass of gas at constant temperature, p1V1 = p2V2. Keep the full IGCSE Physics Formula Sheet open while you revise.
Solids, Liquids & Gases – the Particle Model
Q1. Which row best describes the particles in a solid?
- (A) Widely separated, moving randomly at high speed
- (B) Close together, sliding freely past each other
- (C) Close together in a regular pattern, vibrating about fixed positions
- (D) Widely separated in a regular pattern, not moving
Show Answer
Answer: C – In a solid the particles are packed closely in a regular arrangement. They cannot change places; they only vibrate about fixed positions, which is why a solid has a definite shape and volume.
Q2. Which description matches the particles in a liquid?
- (A) Close together, but free to move around and slide past one another
- (B) Locked in fixed positions in a regular lattice
- (C) Very far apart, with no forces between them at all
- (D) Completely still, with no movement of any kind
Show Answer
Answer: A – Liquid particles are close together, so a liquid has a fixed volume, but they are not in fixed positions. They can slide past one another, so the liquid flows and takes the shape of its container.
Q3. Compared with the particles in a liquid, the particles in a gas are much further apart. The forces between gas particles are:
- (A) Stronger than the forces in a liquid
- (B) About the same strength as in a liquid
- (C) Strong enough to hold the particles in fixed positions
- (D) Very weak, so the particles are free to move throughout the container
Show Answer
Answer: D – In a gas the particles are far apart and the forces between them are very weak (almost negligible). That is why a gas has no fixed shape or volume and expands to fill its container.
Q4. A metal block keeps its own shape and does not flow. This is because:
- (A) Its particles do not move at all
- (B) Strong forces hold its particles in fixed positions
- (C) Its particles can slide over each other easily
- (D) Its particles are spread far apart
Show Answer
Answer: B – The particles in a solid do vibrate, so (A) is wrong. Strong attractive forces between the particles keep them in fixed positions, so the block keeps a definite shape.
Q5. Water poured into a glass takes the shape of the glass but keeps the same volume. This happens because the particles in a liquid:
- (A) Are close together but not in fixed positions, so they can slide past each other
- (B) Are far apart and move at very high speeds
- (C) Have no forces between them at all
- (D) Sit in a regular pattern that can bend
Show Answer
Answer: A – The particles are close together (fixed volume) but can move around each other (no fixed shape), so the liquid flows into the shape of the glass.
Q6. Air pumped into a bicycle tyre spreads out to fill the whole tyre. A gas fills its container because its particles:
- (A) Get bigger until they touch the walls
- (B) Are pulled towards the walls by strong forces
- (C) Move randomly at high speed, and the forces between them are very weak
- (D) Stop moving and settle out evenly
Show Answer
Answer: C – Gas particles travel in straight lines at high speed in random directions. With only very weak forces between them, nothing holds them together, so they spread through all the available space.
Q7. For most substances the solid is the densest state and the gas is the least dense. Which is a well-known exception to this pattern?
- (A) Solid aluminium is less dense than liquid aluminium
- (B) Ice is less dense than liquid water, so ice floats on water
- (C) Steam is denser than liquid water
- (D) Solid iron floats on liquid iron
Show Answer
Answer: B – Water is unusual: when it freezes, its particles form an open structure with more space between them, so ice (about 920 kg/m3) is less dense than liquid water (1000 kg/m3) and floats.
Q8. On a cold day, water vapour in the air turns into liquid droplets on a window. This change of state is called:
- (A) Evaporation
- (B) Melting
- (C) Sublimation
- (D) Condensation
Show Answer
Answer: D – Condensation is the change from a gas (or vapour) to a liquid. The vapour particles lose energy to the cold window, slow down, and are held together as liquid droplets.
Q9. Dry ice is solid carbon dioxide. Left out in air, it turns directly into a gas without becoming a liquid first. This change of state is called:
- (A) Sublimation
- (B) Condensation
- (C) Evaporation
- (D) Freezing
Show Answer
Answer: A – Sublimation is when a solid changes directly into a gas, missing out the liquid stage. Its particles gain enough energy to escape from the solid surface completely.
Q10. A solid is heated at a steady rate. While it is melting, its temperature stays constant even though energy is still being supplied. During melting, the supplied energy is used to:
- (A) Increase the average kinetic energy of the particles
- (B) Make the particles vibrate faster and faster
- (C) Break the particles free from their fixed positions in the solid
- (D) Make each particle expand in size
Show Answer
Answer: C – During a change of state the energy goes into separating the particles (increasing their potential energy), not into making them move faster. The average kinetic energy, and so the temperature, stays constant until melting is complete.
Evidence & Explanations
Q11. In the Brownian motion experiment, smoke is trapped in a small cell, lit from the side, and viewed through a microscope. What is observed?
- (A) Smoke particles drifting smoothly in one direction
- (B) Bright specks moving randomly in a jerky, zigzag way
- (C) Individual air molecules moving in straight lines
- (D) Smoke particles that stay perfectly still
Show Answer
Answer: B – The smoke particles are seen as tiny bright specks. They move continuously in a random, jerky fashion – never smoothly and never stopping – which is the visible sign of Brownian motion.
Q12. Brownian motion is evidence for the kinetic molecular model because the movement of the smoke particles is caused by:
- (A) The smoke particles being alive
- (B) Heat from the microscope lamp pushing them round in circles
- (C) The smoke particles attracting each other
- (D) Uneven collisions with fast-moving air molecules that are too small to be seen
Show Answer
Answer: D – Air molecules are constantly moving at high speed but are far too small to see. At any instant they strike a smoke particle unevenly from different sides, giving it a push that constantly changes – hence the random zigzag path.
Q13. In the smoke cell experiment, the particles that keep colliding with the smoke specks and making them jiggle are:
- (A) Air molecules, which are far too small to be seen in the microscope
- (B) Only the other smoke particles
- (C) Large dust grains floating inside the cell
- (D) The glass walls of the cell vibrating
Show Answer
Answer: A – A smoke particle is huge compared with an air molecule, yet it is light enough to be knocked about by the uneven bombardment of the invisible, fast-moving air molecules around it.
Q14. A bottle of perfume is opened in one corner of a still room. A few minutes later the smell can be detected on the other side of the room. The perfume vapour spreads because its particles:
- (A) Are carried across the room by sound waves
- (B) Expand in size until they fill the room
- (C) Move randomly from a region of high concentration to regions of lower concentration
- (D) Are attracted towards the walls of the room
Show Answer
Answer: C – This is diffusion. The vapour particles move randomly at high speed and gradually spread out from where they are crowded (high concentration) until they are evenly spread through the air.
Q15. Diffusion takes place much more quickly in gases than in liquids. This is mainly because in a gas:
- (A) The particles are held in fixed positions
- (B) The particles move faster and the forces between them are weaker
- (C) The particles are heavier
- (D) The particles never collide with each other
Show Answer
Answer: B – Gas particles move at high speeds with very weak forces between them and large gaps to move through, so they mix rapidly. Liquid particles move more slowly and are held close together, so diffusion in liquids is slow.
Q16. A drop of ink spreads through a beaker of warm water faster than through cold water. This is because in the warm water:
- (A) The particles have more kinetic energy and move faster
- (B) The ink particles become much smaller
- (C) The water particles stop moving
- (D) The particles lose all their kinetic energy
Show Answer
Answer: A – Raising the temperature increases the average kinetic energy of the particles. Faster-moving particles diffuse more quickly, so the ink spreads through warm water in less time.
Q17. Which statement about evaporation is correct?
- (A) It takes place throughout the whole liquid
- (B) It only takes place at the boiling point of the liquid
- (C) It only takes place when the liquid is heated strongly
- (D) It takes place at the surface of the liquid, at any temperature
Show Answer
Answer: D – Evaporation is a surface process: only particles at the surface with enough energy can escape. It happens at any temperature, unlike boiling, which happens throughout the liquid at one fixed temperature.
Q18. Which statement describes boiling correctly?
- (A) Bubbles form only at the surface of the liquid
- (B) It is the same process as evaporation, but slower
- (C) Bubbles of vapour form throughout the liquid, at a fixed temperature
- (D) The temperature of the liquid keeps rising above its boiling point
Show Answer
Answer: C – At the boiling point, vapour bubbles form inside the liquid as well as at the surface, and the temperature stays at the boiling point while boiling continues. This is the key contrast with evaporation.
Q19. Sweating helps to cool the body. Evaporation of a liquid causes cooling because:
- (A) Cold molecules from the air enter the liquid
- (B) The most energetic molecules escape from the surface, so the average kinetic energy of the molecules left behind falls
- (C) Every molecule in the liquid loses all of its kinetic energy
- (D) The molecules left behind become heavier
Show Answer
Answer: B – Only the fastest, most energetic surface molecules can escape during evaporation. Removing them lowers the average kinetic energy of the rest, and since temperature measures average kinetic energy, the liquid (or skin) cools down.
Q20. A puddle dries up on a cool day, far below the boiling point of water. Evaporation can happen at any temperature because:
- (A) All the molecules in a liquid have exactly the same kinetic energy
- (B) Only the molecules at the bottom of the liquid can escape
- (C) The surface of the liquid secretly reaches boiling point
- (D) The molecules have a range of kinetic energies, so a few at the surface always have enough energy to escape
Show Answer
Answer: D – At any temperature the molecules have a spread of energies. Most are too slow to escape, but a few surface molecules are always fast enough to break free – so evaporation happens even in a cold liquid.
Temperature, Pressure & the Gas Laws (qualitative + simple calc)
Q21. The temperature of a substance is a measure of:
- (A) The average kinetic energy of its particles
- (B) The total kinetic energy of all its particles added together
- (C) The average potential energy of its particles only
- (D) The speed of the single fastest particle
Show Answer
Answer: A – Temperature measures the average kinetic energy per particle. The total kinetic energy also depends on how much substance there is, so a large lukewarm bath can store more total energy than a small cup of hot water.
Q22. A fixed mass of gas is trapped in a strong sealed container of constant volume and is heated. The pressure increases because the molecules:
- (A) Expand and take up more room
- (B) Split into a larger number of smaller molecules
- (C) Move faster, so they hit the walls harder and more often
- (D) Stick to the walls of the container
Show Answer
Answer: C – Heating raises the average kinetic energy of the molecules. They collide with the walls more frequently and with greater force, and pressure is force per unit area, so the pressure rises.
Q23. A gas is slowly compressed to half its original volume while its temperature stays constant. The pressure increases because the molecules:
- (A) Move faster than before
- (B) Are closer together, so they collide with the walls more frequently
- (C) Grow larger as the volume falls
- (D) Gain kinetic energy from being compressed
Show Answer
Answer: B – At constant temperature the average speed and kinetic energy of the molecules are unchanged. In the smaller space, each molecule has less distance to travel between wall collisions, so collisions happen more often and the pressure is higher.
Q24. The pressure exerted by a gas inside a container is caused by:
- (A) The weight of the gas pressing downwards only
- (B) Strong forces attracting the molecules to the walls
- (C) The molecules swelling until they push the walls outwards
- (D) The molecules colliding with the walls, exerting a force over the area of the walls
Show Answer
Answer: D – Gas pressure comes from molecular collisions with the container walls. Billions of tiny impacts per second add up to a steady force spread over the wall area – that force per unit area is the pressure, and it acts in all directions, not just downwards.
Q25. A fixed mass of gas has a pressure of 100 kPa and a volume of 200 cm3. It is compressed to a volume of 50 cm3 at constant temperature. What is the new pressure? [Supplement]
- (A) 25 kPa
- (B) 100 kPa
- (C) 400 kPa
- (D) 800 kPa
Show Answer
Answer: C – For a fixed mass of gas at constant temperature, pV = constant, so p1V1 = p2V2. p2 = (100 × 200) / 50 = 20 000 / 50 = 400 kPa. The volume fell to a quarter, so the pressure rose to four times the original.
Q26. A gas at a pressure of 300 kPa occupies a volume of 60 cm3. It is allowed to expand to a volume of 180 cm3 at constant temperature. What is the new pressure? [Supplement]
- (A) 100 kPa
- (B) 300 kPa
- (C) 600 kPa
- (D) 900 kPa
Show Answer
Answer: A – Using p1V1 = p2V2: p2 = (300 × 60) / 180 = 18 000 / 180 = 100 kPa. The volume trebled, so the pressure fell to one third.
Q27. Absolute zero (0 K) is the temperature at which: [Supplement]
- (A) Water freezes
- (B) The particles of a substance have the minimum possible kinetic energy
- (C) Every gas turns into a liquid
- (D) All thermometers read −100°C
Show Answer
Answer: B – Absolute zero is 0 K, which is −273°C. At this lowest possible temperature the particles have their minimum average kinetic energy – they effectively have no thermal motion. Water freezes at 273 K, not at 0 K.
Q28. A gas is at a temperature of 27°C. What is this temperature on the Kelvin scale? [Supplement]
- (A) 27 K
- (B) 246 K
- (C) 273 K
- (D) 300 K
Show Answer
Answer: D – To convert from Celsius to Kelvin, add 273: T = 27 + 273 = 300 K. Kelvin temperatures are always 273 higher than the Celsius value for the same temperature.
Q29. A fixed mass of gas in a sealed container of constant volume has a pressure of 120 kPa at a temperature of 300 K. It is heated to 600 K. What is the new pressure? [Supplement]
- (A) 60 kPa
- (B) 120 kPa
- (C) 240 kPa
- (D) 480 kPa
Show Answer
Answer: C – At constant volume, pressure is proportional to the Kelvin temperature. The temperature doubled (300 K to 600 K), so the pressure doubles: p2 = 120 × (600 / 300) = 120 × 2 = 240 kPa. Remember to use Kelvin, not Celsius, in this ratio.
Q30. A sealed flask of gas is placed in a freezer. The volume of the gas stays the same. What happens to the pressure of the gas, and why?
- (A) It decreases, because the molecules move more slowly and collide with the walls less often and less hard
- (B) It increases, because the molecules move more slowly
- (C) It stays the same, because the volume has not changed
- (D) It decreases, because the molecules shrink in size
Show Answer
Answer: A – Cooling lowers the average kinetic energy of the molecules. Slower molecules hit the walls less frequently and with less force, so the pressure falls – the molecules themselves do not change size.
Keep practising: find MCQ pages for every Physics board and topic in the Physics MCQs hub, and revise the key equations for this topic on the IGCSE Physics Formula Sheet.
All questions on this page are original practice questions written for QuaidianPoint revision. They are not Cambridge past-paper questions, and this page is not affiliated with Cambridge Assessment International Education.