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Gases are less dense than solids because their particles are spread far apart, so less mass fits in the same volume. Free GCSE Physics notes and 13 FAQs.
States of Matter and Density is part of Changes of state and the particle model in GCSE AQA Higher Physics.
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GCSE AQA Higher
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Gases are less dense than solids because their particles are spread far apart, so less mass fits in the same volume. Free GCSE Physics notes and 13 FAQs.
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Use this lesson to stay inside Changes of state and the particle model while connecting the detail back to the wider Particle model of matter topic.
Read the core explanations from States of Matter and Density before testing yourself from memory.
Ice, water, and steam are all the same substance - H₂O. Yet ice is solid, water flows, and steam fills a room. What's happening at the level we can't see?
Everything around you is made of tiny particles in constant motion. These particles are so small that millions could fit on the full stop at the end of this sentence. The way these particles move and arrange themselves determines whether something is solid, liquid, or gas.
Think of particles like people at a concert. In the seated area (solid), everyone stays in their place but can still wiggle. In the standing area (liquid), people can move around but stay close together. Outside in the car park (gas), people spread out and move freely in all directions.
Picture looking through the world's most powerful microscope at a piece of ice. You'd see water molecules locked in a rigid pattern, vibrating on the spot like dancers frozen mid-move. This is a solid - particles packed tightly in fixed positions.
Now watch that ice melt. The particles gain energy and break free from their fixed positions. They can slide past each other but stay close together, like people in a crowded room. This is a liquid - particles close but mobile.
Heat the water further and the particles gain so much energy they break apart completely. They zoom around at high speed, filling any container and spreading as far apart as possible. This is a gas - particles widely separated and moving rapidly.
Examiners love asking you to draw particle diagrams. Here's the secret: it's not about artistic skill, it's about showing the key differences between states. Use circles to represent particles and focus on three things: spacing, arrangement, and implied movement.
For solids, draw circles touching each other in a regular pattern - imagine stacking oranges in a box. For liquids, keep circles close but in an irregular, jumbled arrangement. For gases, spread circles far apart with lots of empty space between them.
The key mistake students make is drawing liquid particles too far apart or gas particles too close together. Remember: liquids are nearly as dense as solids, while gases have huge gaps between particles.
Density is mass per unit volume - how much stuff is crammed into a given space. It's measured in kg/m³ or g/cm³. Think of it as how tightly packed the particles are in a material.
The particle model explains why density changes between states. In solids, particles are tightly packed, giving high density. In liquids, particles are still close but less organised, so density is slightly lower. In gases, particles are spread far apart, giving very low density.
This is why ice floats on water - a rare exception where the solid is less dense than the liquid. When water freezes, it forms a crystal structure with gaps, making ice about 10% less dense than liquid water.
Worked example: Calculating Density
A block of wood has a mass of 200g and a volume of 250 cm³. Calculate its density.
Answer: 0.8 g/cm³
When you heat a solid, you're giving its particles more kinetic energy. They vibrate faster and faster until they have enough energy to break free from their fixed positions - the solid melts. Continue heating and the particles gain even more energy, eventually escaping as a gas.
The reverse happens when you cool a gas. Particles lose kinetic energy, slow down, and eventually get close enough for attractive forces to pull them together into a liquid, then a solid. Temperature is really a measure of average particle kinetic energy.
During state changes, temperature stays constant even though you're adding energy. This energy goes into breaking or forming bonds between particles, not increasing their speed. That's why ice stays at 0°C while melting.
Exam questions on states of matter follow predictable patterns. You'll be asked to describe particle arrangements, explain density differences, or draw particle diagrams. The key is using precise scientific language and the particle model consistently.
When describing states, always mention three things: particle spacing, particle arrangement, and particle movement. For example: 'In a gas, particles are far apart, randomly arranged, and moving rapidly in all directions.' This hits all the mark scheme points.
For density explanations, link particle arrangement to mass per unit volume. Don't just say 'solids are denser' - explain that 'solids have higher density because particles are packed more closely together, so more mass fits in the same volume.'
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Density is measured in kg/m³ or g/cm³. It is mass per unit volume, calculated with density = mass ÷ volume, so the units are a mass unit divided by a volume unit.
Particles in a liquid are close together but in an irregular, jumbled arrangement, and they can slide past each other. Liquids are nearly as dense as solids because their particles are still close.
Ice floats on water because it is less dense than liquid water, a rare exception where the solid is less dense than the liquid. When water freezes, it forms a crystal structure with gaps, making ice about 10% less dense than liquid water.
When a solid is heated, its particles gain kinetic energy and vibrate faster until they have enough energy to break free from their fixed positions, so the solid melts. Further heating gives the particles even more energy, until they escape as a gas.
Wood floats on water because its density is less than water's. A block of wood with a mass of 200 g and a volume of 250 cm³ has a density of 0.8 g/cm³, while water has a density of 1.0 g/cm³.
Gases are less dense than solids because their particles are spread far apart, so less mass fits in the same volume. Solids have higher density because particles are packed more closely together, while liquids are nearly as dense as solids.
Use circles for particles: for solids, draw circles touching in a regular pattern; for liquids, keep circles close but in an irregular, jumbled arrangement; for gases, spread circles far apart with lots of empty space. A common mistake is drawing liquid particles too far apart.
A block of wood with a mass of 200 g and a volume of 250 cm³ has a density of 0.8 g/cm³. Using density = mass ÷ volume, 200 ÷ 250 = 0.8 g/cm³, which is less than water at 1.0 g/cm³, so the wood floats.
Ice, water and steam are all the same substance, H₂O, with different particle arrangements. In ice the molecules are locked in a rigid pattern, vibrating on the spot; in water they slide past each other but stay close; in steam they are widely separated and moving rapidly.
Temperature is a measure of the average kinetic energy of particles. A higher temperature means particles have more kinetic energy and move faster. During a change of state, though, the temperature stays constant, as with ice at 0°C while melting.
When a gas cools, its particles lose kinetic energy and slow down. Eventually they get close enough for attractive forces to pull them together into a liquid, and then into a solid.
A state of matter is described using particle spacing, particle arrangement and particle movement. For example, in a gas, particles are far apart, randomly arranged and moving rapidly in all directions.
Solids are denser than gases because their particles are packed more closely together, so more mass fits in the same volume. In gases, particles are spread far apart with huge gaps, giving very low density.