C7: States of Matter
States of matter and changes of state
📋 Key Definitions
Solid: A state of matter where particles are arranged in a regular pattern and vibrate about fixed positions. There are strong forces of attraction between particles.
Liquid: A state of matter where particles can move past each other randomly but are still close together. There are moderate forces of attraction between particles.
Gas: A state of matter where particles move very fast in all directions and are far apart. There are very weak forces of attraction between particles.
Sublimation: A change of state directly from solid to gas (or gas to solid) without going through the liquid state.
🧊 The Three States of Matter
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Arrangement of particles | Regular, close together | Random, close together | Random, far apart |
| Movement of particles | Vibrate about fixed positions | Move randomly around each other | Move very fast in all directions |
| Forces between particles | Strong (hold particles in place) | Moderate (keep particles close but allow movement) | Very weak (negligible) |
| Shape | Fixed shape | Takes shape of container | Fills container completely |
| Volume | Fixed volume | Fixed volume | Fills container (variable volume) |
| Compressibility | Cannot be compressed | Cannot be compressed | Can be compressed |
| Density | High | Medium | Low |
Examples of States
- Solid: Ice (solid water) - particles vibrate in a fixed arrangement
- Liquid: Water - particles move past each other but stay close
- Gas: Water vapour (steam) - particles move rapidly and are far apart
- Solid: Iron at room temperature - strong metallic bonds hold atoms in place
- Gas: Oxygen at room temperature - molecules move freely with lots of space between them
🔄 Changes of State
When a substance changes state, its particles gain or lose energy. The arrangement and movement of particles change, but the chemical identity of the substance does NOT change - it is a physical change, not a chemical change.
Solid → Liquid: Melting (energy absorbed)
Liquid → Gas: Boiling / Evaporation (energy absorbed)
Gas → Liquid: Condensing (energy released)
Liquid → Solid: Freezing (energy released)
Solid → Gas: Sublimation (energy absorbed)
Gas → Solid: Deposition (energy released)
Conservation of mass: When a substance changes state, the number and type of particles stays the same. The mass does not change. Only the arrangement and movement of particles changes.
Worked Example - Predicting State at a Given Temperature
The melting point of chlorine is −101°C and the boiling point is −34°C. What is the state of chlorine at: (a) −150°C, (b) −50°C, (c) 20°C?
(a) −150°C is below the melting point (−101°C), so chlorine is a solid.
(b) −50°C is between the melting point and boiling point, so chlorine is a liquid.
(c) 20°C is above the boiling point (−34°C), so chlorine is a gas.
Worked Example - Using State Symbols
When water boils: H₂O(l) → H₂O(g)
When iron melts: Fe(s) → Fe(l)
When carbon dioxide sublimes: CO₂(s) → CO₂(g)
When steam condenses: H₂O(g) → H₂O(l)
🌡️ State Symbols Reminder
| State Symbol | Meaning | Example |
|---|---|---|
| (s) | Solid | Fe(s), NaCl(s) |
| (l) | Liquid | H₂O(l), Br₂(l) |
| (g) | Gas | O₂(g), CO₂(g) |
| (aq) | Aqueous (dissolved in water) | NaCl(aq), HCl(aq) |
⚠️ Limitations of the Simple Particle Model
Limitations: The simple particle model assumes particles are solid, inelastic spheres. It does not show the forces between particles. It does not show that particles are not all the same size. It does not show the gaps between particles.
- Particles are drawn as solid spheres, but real atoms and molecules are not solid spheres
- The model does not show intermolecular forces - these are very important for explaining properties
- The model does not represent the different sizes of different particles
- The model does not show that particles vibrate even in a solid
- In reality, there is empty space between particles, especially in gases
❓ Practice Questions
Q1: Foundation Describe the arrangement and movement of particles in a solid, a liquid and a gas.
Q2: Foundation Bromine has a melting point of −7°C and a boiling point of 59°C. State the physical state of bromine at: (a) −20°C, (b) 25°C, (c) 80°C.
Q3: Foundation Name the changes of state that occur when: (a) ice becomes water, (b) water becomes steam, (c) steam becomes water, (d) water becomes ice.
Q4: Higher Explain why the mass of a substance does not change when it changes state. Why is this a physical change rather than a chemical change?
Q5: Higher State two limitations of the simple particle model and explain why each is a limitation.
Q6: Foundation Dry ice (solid CO₂) turns directly into gas at −78°C. What is this change of state called? Write an equation with state symbols.
✅ Answers
- Solid: Particles are arranged in a regular pattern, close together, and vibrate about fixed positions. Liquid: Particles are close together but can move randomly past each other. Gas: Particles are far apart and move very fast in all directions.
- (a) −20°C is below the melting point (−7°C), so bromine is a solid. (b) 25°C is between the melting point (−7°C) and boiling point (59°C), so bromine is a liquid. (c) 80°C is above the boiling point (59°C), so bromine is a gas.
- (a) Melting. (b) Boiling / evaporation. (c) Condensing. (d) Freezing.
- The mass does not change because no particles are created or destroyed during a change of state - only the arrangement and movement of the particles changes. It is a physical change rather than a chemical change because no new substances are formed; the particles remain the same substance (e.g. H₂O remains H₂O whether solid, liquid or gas).
- Limitation 1: The model does not show the forces between particles. This is a limitation because intermolecular forces are crucial for explaining why substances have different melting points and states at different temperatures. Limitation 2: The model represents particles as solid spheres, but real atoms/molecules are not solid spheres. This is a limitation because real particles have internal structure (nucleus and electrons) and are mostly empty space.
- This change of state is called sublimation (solid directly to gas). Equation: CO₂(s) → CO₂(g).
🎯 Exam Tips
- When describing particle arrangement, always mention both arrangement AND movement for each state
- For predicting state at a given temperature: below melting point = solid, between mp and bp = liquid, above boiling point = gas
- Remember sublimation: solid → gas directly. Carbon dioxide and iodine are common examples
- Changes of state are physical changes - no new substances are formed, and mass is conserved
- For limitations of the particle model, focus on: no forces shown, particles are not really solid spheres, different sizes not shown
- State symbols: (aq) means dissolved in water - do not confuse with (l)
🔢 Maths Skills
Mathematical Skills
Interpreting heating curves: A heating curve plots temperature (y-axis) against time (x-axis). Flat sections show changes of state (energy is being used to overcome forces, not increase temperature). Diagonal sections show the substance heating up in one state.
Example: For water: 0°C flat = melting; 100°C flat = boiling. The length of the flat section indicates how much energy is needed for the state change.
⚠️ Common Misconceptions
Watch Out!
Particles expand when heated. Wrong: particles expand when heated Correct: particles themselves do not expand; the gaps between particles increase as they move faster and further apart
Boiling and evaporation are the same thing. Wrong: boiling = evaporation Correct: evaporation happens at any temperature from the surface only; boiling happens at a specific temperature throughout the liquid
✍️ 6-Mark Question
Extended Answer
6 marks: Explain changes of state using the particle model.
When a solid is heated, particles gain energy and vibrate more vigorously. At the melting point, particles have enough energy to break free from their fixed positions and the regular arrangement collapses — the solid becomes a liquid. When a liquid is heated, particles move faster. At the boiling point, particles have enough energy to overcome the intermolecular forces holding them together and they escape entirely — the liquid becomes a gas where particles move rapidly in all directions and are far apart. During each state change, the temperature stays constant because the energy is being used to overcome forces between particles rather than increase kinetic energy. No particles are created or destroyed — mass is conserved.
Mark scheme: 1 mark for particles gain energy; 1 mark for melting explanation (break fixed positions); 1 mark for boiling explanation (overcome intermolecular forces); 1 mark for constant temperature during state change; 1 mark for energy used to overcome forces; 1 mark for conservation of mass.
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A student heats a pure substance and records the temperature every minute. The graph shows the temperature rising from 20°C to 80°C, then staying at 80°C for 3 minutes, then rising again to 150°C.
Question: Sketch the heating curve. What is the melting point? Explain what is happening during the flat section.
Answer: Melting point = 80°C. During the flat section, the substance is changing state from solid to liquid. The temperature stays constant because the energy being supplied is used to overcome the forces holding the particles in their fixed positions (intermolecular forces / lattice), rather than increasing the kinetic energy of the particles.
📝 Exam Questions by Topic
🎬 Video Resources
Ready to ace your GCSE Combined Science exams?
Get the best revision books and guides to boost your grades.