Subject: Physics | Level: GCSE | Exam Board: WJEC
This guide provides a comprehensive, exam-focused breakdown of the WJEC GCSE Physics topic: Particle Model of Matter (1.2). It covers the states of matter, internal energy, density, and gas pressure, complete with multi-modal resources to secure top marks.
Revision Notes & Key Concepts
Revision Podcast Transcript
Hello and welcome! I'm so glad you've tuned in today, because we're diving into one of the most fundamental topics in GCSE Physics — the Particle Model of Matter. This is topic 1.2 on the WJEC specification, and I promise you, by the end of this episode, you're going to feel genuinely confident about it. Whether you're sitting Foundation or Higher tier, this topic comes up every single year, and the marks are very much there for the taking if you know exactly what examiners are looking for. So let's get into it. I'm going to take you through the core concepts clearly and carefully, then we'll do exam tips and common mistakes, a quick-fire quiz to test your recall, and finish with a summary. Grab a pen — you might want to jot a few things down. --- SECTION ONE: CORE CONCEPTS Let's start at the very beginning. The particle model of matter is exactly what it sounds like — it's a model that describes all matter as being made up of tiny particles. These particles are atoms or molecules, and the way they're arranged and how they move explains everything we observe about solids, liquids, and gases. So let's go through the three states. In a SOLID, particles are packed tightly together in a regular, fixed arrangement — think of a neat grid. They don't move around freely; instead, they vibrate about their fixed positions. The forces between particles in a solid are very strong, which is why solids hold their shape and can't be compressed easily. In a LIQUID, particles are still close together, but they're no longer in a fixed arrangement. They can slide past each other and move around within the liquid. The forces between them are weaker than in a solid, which is why liquids can flow and take the shape of their container. But they still can't be compressed much, because the particles are still touching. In a GAS, particles are far apart — much further apart than in a solid or liquid. They move rapidly and randomly in all directions. The forces between gas particles are negligible — essentially zero. This is why gases can be compressed, and why they fill any container they're put in. Now, here's something really important that examiners test every year: what happens to the particles when you heat a substance? The key idea is KINETIC ENERGY. Temperature is a measure of the average kinetic energy of the particles. So when you heat something up, you're giving the particles more kinetic energy — they move faster. When you cool something down, they slow down. But — and this is crucial — when a substance is changing state, the temperature does NOT change, even though you're still adding energy. Let me say that again: during a change of state, temperature stays constant. Why? Because the energy you're adding is being used to break or overcome the intermolecular bonds between particles — not to speed them up. So the potential energy of the particles increases, but their kinetic energy stays the same. That's why the temperature on a heating curve goes flat during melting and boiling. This brings us to INTERNAL ENERGY. Internal energy is the total energy stored by all the particles in a system. It has two components: the kinetic energy of the particles — linked to temperature — and the potential energy of the particles — linked to the state and the forces between them. When you heat a substance, you increase its internal energy. But whether that increase goes into kinetic energy or potential energy depends on whether the substance is changing state or not. Now let's talk about DENSITY. Density is defined as mass per unit volume. The formula is: rho equals m divided by V, where rho is density in kilograms per cubic metre, m is mass in kilograms, and V is volume in cubic metres. Solids are generally denser than liquids, which are denser than gases. Why? Because in a solid, the particles are packed closely together, so there's a lot of mass in a small volume. In a gas, the particles are spread far apart, so the same number of particles takes up a much larger volume — giving a much lower density. For the required practical on density, you need to know how to measure the density of both regular and irregular solids. For a regular solid like a cuboid or cylinder, you measure its dimensions using a ruler or vernier callipers, calculate the volume using the appropriate formula, then measure the mass on a balance. For an irregular solid, you use a displacement method — you submerge the object in water using an eureka can, collect the displaced water in a measuring cylinder, and the volume of water displaced equals the volume of the object. Now let's talk about GAS PRESSURE. This is a Higher-tier concept but it's worth understanding clearly. Gas pressure is caused by the particles of a gas colliding with the walls of their container. Each collision exerts a tiny force on the wall, and the total effect of billions of these collisions per second is what we measure as pressure. Pressure is defined as force per unit area. If you increase the temperature of a gas in a fixed container, the particles move faster, so they hit the walls more often and with greater force — pressure increases. If you decrease the volume of a gas at constant temperature — by pushing a piston in, for example — the particles have less space to move around in, so they hit the walls more frequently — pressure increases again. This gives us Boyle's Law: at constant temperature, pressure multiplied by volume is constant. So p times V equals a constant. If pressure doubles, volume halves. If pressure halves, volume doubles. Always state that temperature must be constant when using this relationship — examiners specifically credit this condition. And finally, SPECIFIC LATENT HEAT. This is the energy required to change the state of one kilogram of a substance without changing its temperature. The formula is: E equals m times L, where E is energy in joules, m is mass in kilograms, and L is specific latent heat in joules per kilogram. There are two types: specific latent heat of fusion — for melting and freezing — and specific latent heat of vaporisation — for boiling and condensing. Vaporisation values are always much larger than fusion values, because you have to completely separate the particles from each other, not just loosen them. --- SECTION TWO: EXAM TIPS AND COMMON MISTAKES Right, let's talk about how to actually get the marks. Tip number one: the most common mistake in this topic — and I see it every year — is candidates writing that particles expand when heated. This is WRONG. Individual particles do not expand. What happens is that the SPACE BETWEEN the particles increases. Make sure you say that. Tip number two: don't confuse specific heat capacity with specific latent heat. Specific heat capacity — using the formula E equals m c delta T — is for temperature changes. Specific latent heat — using E equals m L — is for state changes. If the question mentions a change of state, use latent heat. If it mentions a temperature change, use specific heat capacity. Read the question carefully. Tip number three: for gas pressure questions, candidates often say pressure is caused by particles colliding with each other. This is wrong. Pressure is caused by particles colliding with the CONTAINER WALLS. This distinction earns or loses marks every year. Tip number four: unit conversions for density. This is a classic mark-loser. If your volume is in cubic centimetres, you must divide by one million — that's 1,000,000 — to convert to cubic metres. One cubic metre equals one million cubic centimetres. Many candidates divide by 1,000 by mistake, confusing it with the conversion for litres. Write it out: 1 m³ = 1,000,000 cm³. Stick that in your memory right now. Tip number five: for QER questions — that's Quality of Extended Response — about the density practical, name your instruments specifically. Don't just say "a measuring device" — say "vernier callipers" for small dimensions, "a metre ruler" for larger ones, and "a eureka can" for irregular solids. Examiners award credit for naming specific apparatus. Tip number six: on heating curve questions, when you describe the flat sections, always use the words "potential energy" and "kinetic energy" explicitly. Say: "During melting, the kinetic energy of the particles remains constant, so temperature does not change. The energy supplied increases the potential energy of the particles as intermolecular bonds are broken." That's a full-mark answer. Tip number seven: timing. Allow roughly one minute per mark. A six-mark QER question should take about six to seven minutes. Don't rush it — these extended questions are where you can really demonstrate your understanding and pick up multiple marks. --- SECTION THREE: QUICK-FIRE RECALL QUIZ Right, I'm going to fire some questions at you. Pause the audio after each one and try to answer before I give you the answer. Ready? Question one: What is internal energy? Pause now. Answer: Internal energy is the sum of the kinetic and potential energies of all the particles in a system. Question two: Why does temperature remain constant during a change of state? Pause now. Answer: Because the energy supplied is used to break or overcome intermolecular bonds — increasing potential energy — rather than increasing the kinetic energy of the particles. Question three: What causes gas pressure? Pause now. Answer: Gas pressure is caused by the force exerted per unit area due to collisions of gas particles with the container walls. Question four: A metal block has a mass of 540 grams and a volume of 200 cubic centimetres. Calculate its density in kilograms per cubic metre. Pause now. Answer: First, convert units. 540 grams equals 0.54 kilograms. 200 cubic centimetres equals 200 divided by 1,000,000 equals 0.0002 cubic metres. Density equals 0.54 divided by 0.0002 equals 2700 kilograms per cubic metre. That's aluminium, by the way. Question five: State Boyle's Law. Pause now. Answer: At constant temperature, the pressure of a fixed mass of gas is inversely proportional to its volume. Or: pressure times volume equals a constant, provided temperature is constant. --- SECTION FOUR: SUMMARY AND SIGN-OFF Let's bring it all together. The five things you absolutely must know for this topic are: One: The three states of matter differ in particle arrangement, motion, and the strength of forces between particles. Two: Internal energy equals kinetic energy plus potential energy of all particles. Temperature is linked to kinetic energy; state changes are linked to potential energy. Three: During a change of state, temperature is constant because energy goes into breaking intermolecular bonds — increasing potential energy — not into increasing kinetic energy. Four: Density equals mass divided by volume. Watch your units — convert cubic centimetres to cubic metres by dividing by one million. Five: Gas pressure is caused by particle collisions with container walls. At constant temperature, pressure times volume is constant — Boyle's Law. You've got this. The Particle Model of Matter is one of those topics where understanding the physics deeply — really picturing what the particles are doing — makes everything else fall into place. Don't just memorise the formulas; understand the story behind them. Good luck with your revision, and I'll see you in the next episode. Keep going — you're doing brilliantly.
Key Terms & Definitions
- Internal Energy
- The sum of the kinetic and potential energies of all the particles (atoms and molecules) within a system.
- Specific Latent Heat of Fusion
- The energy required to change the state of 1 kg of a substance from solid to liquid with no change in temperature.
- Specific Latent Heat of Vaporisation
- The energy required to change the state of 1 kg of a substance from liquid to gas with no change in temperature.
- Density
- The mass per unit volume of a substance.
- Pressure
- The force exerted per unit area.
- Kinetic Energy (in this context)
- The energy a particle has due to its motion. Temperature is a measure of the average kinetic energy of the particles.
Worked Examples
Worked Example
Question: A student wants to determine the density of a small, irregular-shaped stone. Describe a method they could use to find its density. (6 marks)
Solution: Step 1: Measure the mass of the stone using a top-pan balance and record the result in kilograms. Step 2: Fill a Eureka can with water until the water level is just below the spout. Step 3: Place an empty measuring cylinder under the spout of the Eureka can. Step 4: Carefully lower the stone into the Eureka can, ensuring it is fully submerged and no water is splashed out. Step 5: Collect the water that is displaced from the can into the measuring cylinder. The volume of this water is equal to the volume of the stone. Record this volume in m³. Step 6: Calculate the density of the stone by using the formula ρ = m / V, substituting the measured mass and volume. The final answer should be given in kg/m³.
Worked Example
Question: A block of ice has a mass of 0.50 kg. The specific latent heat of fusion for water is 3.34 x 10⁵ J/kg. Calculate the energy required to melt the ice at 0°C. (3 marks)
Solution: Step 1: State the correct formula: Energy = mass × specific latent heat (E = mL). Step 2: Substitute the given values into the formula: E = 0.50 kg × 3.34 x 10⁵ J/kg. Step 3: Calculate the final answer: E = 167,000 J or 1.67 x 10⁵ J. Final answer: 167,000 J
Worked Example
Question: Explain, in terms of particles, why the pressure of a gas in a sealed container increases when its temperature is increased. (4 marks)
Solution: Step 1: Increasing the temperature of the gas increases the average kinetic energy of the gas particles. Step 2: This means the particles move faster and more randomly. Step 3: As a result, the particles collide with the walls of the container more frequently (more often). Step 4: The collisions are also more forceful (have a greater change in momentum). Since pressure is the force per unit area, this increased force and frequency of collisions results in a higher pressure.
Practice Questions
Question: Compare the arrangement and motion of particles in a liquid and a gas. (3 marks)
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Question: A student heats 200g of water from 20°C to its boiling point of 100°C. The specific heat capacity of water is 4200 J/kg°C. They continue to heat it until all the water has turned to steam. The specific latent heat of vaporisation of water is 2.26 x 10⁶ J/kg. Calculate the total energy supplied to the water. (6 marks)
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Question: A sealed bottle of air is left in the sun. Explain what happens to the pressure of the air inside the bottle. (3 marks)
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Question: A cube of aluminium has a side length of 5.0 cm and a mass of 337.5 g. Calculate its density in g/cm³. (3 marks)
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Question: A gas has a volume of 2.5 m³ at a pressure of 100,000 Pa. The gas is compressed at a constant temperature until its pressure is 500,000 Pa. Calculate the new volume of the gas. (3 marks)
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