C8: States of Matter
Particle model for solids, liquids and gases, state changes, state symbols and limitations of the simple particle model
📋 Key Definitions
Solid: A state of matter where particles are arranged in a regular, fixed pattern and vibrate about fixed positions. Solids have a fixed shape and volume and cannot be compressed.
Liquid: A state of matter where particles are close together but can move past each other randomly. Liquids have a fixed volume but can flow to take the shape of their container and cannot be compressed easily.
Gas: A state of matter where particles are far apart and move rapidly in random directions. Gases have no fixed shape or volume and can be compressed.
Melting: The change of state from solid to liquid. This occurs at the melting point, when enough energy is supplied to overcome the forces holding the particles in fixed positions.
Boiling: The change of state from liquid to gas. This occurs at the boiling point, when enough energy is supplied to overcome the forces between the particles entirely.
Sublimation: The change of state directly from solid to gas, without passing through the liquid state. Examples include iodine and carbon dioxide (dry ice).
🔬 The Particle Model
The particle model explains the properties of solids, liquids and gases in terms of the arrangement and movement of particles.
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Arrangement of particles | Regular, fixed pattern (lattice) | Close together, random arrangement | Far apart, random arrangement |
| Movement of particles | Vibrate about fixed positions | Move around each other randomly | Move rapidly in all directions |
| Energy of particles | Lowest | Medium | Highest |
| Forces between particles | Strong | Medium (weaker than in solids) | Very weak (negligible) |
| Spacing between particles | Very close (touching) | Close (touching) | Far apart |
| Shape | Fixed shape | Takes shape of container | Fills container |
| Volume | Fixed volume | Fixed volume | Variable volume (compressible) |
| Can it flow? | No | Yes | Yes |
| Density | High | Medium to high | Very low |
Particle spacing: In solids and liquids, the particles are very close together and the substance is difficult to compress. In gases, the particles are far apart with large gaps between them, so gases are easy to compress. The difference in density between solids/liquids and gases is very large.
🔄 State Changes
State changes are physical changes - no new substances are formed and the chemical composition of the substance does not change. The same particles are present before and after the change. Energy must be supplied to overcome the forces between particles.
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)
Evaporation vs boiling: Evaporation can happen at any temperature and only occurs at the surface of a liquid. Boiling happens at a specific temperature (the boiling point) and occurs throughout the liquid, producing bubbles of gas.
Predicting the State of a Substance
To predict the state of a substance at a given temperature, compare the temperature with the substance's melting point and boiling point.
Example: Water (melting point = 0°C, boiling point = 100°C)
- Below 0°C: solid (ice)
- Between 0°C and 100°C: liquid (water)
- Above 100°C: gas (steam)
Example: Oxygen (melting point = −218°C, boiling point = −183°C)
- Below −218°C: solid
- Between −218°C and −183°C: liquid
- Above −183°C: gas (oxygen is a gas at room temperature, 25°C)
Example: Iron (melting point = 1538°C, boiling point = 2862°C)
- Below 1538°C: solid (iron is a solid at room temperature)
- Between 1538°C and 2862°C: liquid
- Above 2862°C: gas
🔤 State Symbols
State symbols are used in chemical equations to show the physical state of each substance:
| State Symbol | Meaning | Example |
|---|---|---|
| (s) | Solid | NaCl(s), Fe(s) |
| (l) | Liquid (pure substance, not solution) | H₂O(l), Br₂(l) |
| (g) | Gas | CO₂(g), O₂(g) |
| (aq) | Aqueous (dissolved in water) | NaCl(aq), HCl(aq) |
Important distinction: (l) means the substance is a pure liquid, while (aq) means the substance is dissolved in water. For example, HCl(g) is hydrogen chloride gas but HCl(aq) is hydrochloric acid (dissolved in water).
Writing Equations with State Symbols
When magnesium reacts with hydrochloric acid:
Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
When sodium hydroxide reacts with sulfuric acid:
2NaOH(aq) + H₂SO₄(aq) → Na₂SO₄(aq) + 2H₂O(l)
📈 Heating and Cooling Curves
A heating curve shows how the temperature of a substance changes as it is heated at a constant rate. A cooling curve shows the reverse process.
Flat sections on a heating curve: During melting and boiling, the temperature stays constant even though energy is still being supplied. This energy is being used to overcome the forces between particles rather than increase their kinetic energy. The energy absorbed during these flat sections is called latent heat.
Reading a Heating Curve for Water
A heating curve for water has five distinct sections:
- Section 1 (rising): Ice is warming from below 0°C to 0°C. Particles vibrate more vigorously.
- Section 2 (flat at 0°C): Ice is melting. Energy is being used to overcome intermolecular forces between water molecules in the solid lattice. Temperature stays at 0°C until all ice has melted.
- Section 3 (rising): Liquid water is warming from 0°C to 100°C. Particles move faster.
- Section 4 (flat at 100°C): Water is boiling. Energy is being used to completely overcome the remaining intermolecular forces so molecules can escape as gas. Temperature stays at 100°C until all water has evaporated.
- Section 5 (rising): Steam is being heated above 100°C. Gas particles move even faster.
When describing a heating or cooling curve, always explain why the flat sections are flat: the energy is being used to overcome or form intermolecular forces (or bonds), not to change the temperature. The substance is changing state during these flat sections.
⚠️ Limitations of the Simple Particle Model
The simple particle model represents particles as solid spheres with no forces between them. This is a useful simplification but has important limitations:
Limitations of the simple particle model: It assumes particles are solid, inelastic spheres with no forces between them. In reality, particles are atoms, ions or molecules that can have complex shapes, and there are forces of attraction between them. The model also does not show that particles in a solid vibrate about fixed positions.
- The model does not show the forces between particles - in reality, there are attractive forces that hold particles together, especially in solids and liquids
- Particles are not actually solid spheres - they are atoms, ions or molecules with internal structure
- The model does not show that particles vibrate in solids - it can suggest they are completely stationary
- It does not account for the different sizes of different particles
- It does not show that some particles (molecules) contain more than one atom bonded together
When asked about limitations of the particle model, focus on what the model does not show rather than simply saying "it is too simple." Specific points about missing forces, particle shapes, and vibration score more marks.
❓ Practice Questions
Q1: Foundation Describe the arrangement and movement of particles in a solid, a liquid and a gas.
Solid: particles arranged in a regular, fixed pattern (lattice); they vibrate about fixed positions and cannot change places. Liquid: particles are close together in a random arrangement; they can move past each other randomly and flow. Gas: particles are far apart in a random arrangement; they move rapidly in all directions and have lots of kinetic energy.
Q2: Foundation Explain why the temperature stays constant during melting, even though energy is still being supplied.
During melting, the energy supplied is being used to overcome the forces of attraction between the particles in the solid lattice. This energy allows particles to break free from their fixed positions and move around as a liquid. The energy goes into overcoming these intermolecular forces rather than increasing the kinetic energy of the particles, so the temperature remains constant until all the solid has melted.
Q3: Higher 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. Give a reason for each answer.
(a) Solid - because −20°C is below the melting point of −7°C, so the particles are held in fixed positions. (b) Liquid - because 25°C is between the melting point (−7°C) and boiling point (59°C), so the particles are close together but can move past each other. (c) Gas - because 80°C is above the boiling point of 59°C, so the particles have enough energy to overcome the forces between them and move freely far apart.
Q4: Higher State two limitations of the simple particle model and explain why each is a limitation.
Limitation 1: The model represents particles as solid spheres with no forces between them. In reality, there are forces of attraction between particles, which is why solids and liquids hold together and why energy is needed to overcome these forces during state changes. Limitation 2: The model does not show that particles in solids vibrate about fixed positions. This can lead to the misconception that solid particles are completely still. Limitation 3: The model shows all particles as the same size and shape, but real particles (atoms, ions, molecules) vary in size and can have complex shapes and internal bonding.
🎯 Exam Tips
State changes are physical changes, not chemical changes. No new substances are formed. The same particles are present - they just have different amounts of energy and move differently.
When describing state changes, always name the process correctly: melting (solid→liquid), boiling (liquid→gas), condensing (gas→liquid), freezing (liquid→solid). Sublimation is solid→gas directly - remember iodine and carbon dioxide as examples.
On heating/cooling curves, the flat sections indicate state changes. The substance exists as a mixture of both states during the flat section (e.g. solid + liquid during melting). Energy goes into breaking/forming forces between particles, not changing temperature.
Don't confuse (l) and (aq) state symbols. (l) is for a pure liquid substance; (aq) means dissolved in water. This is a common mistake in equations.
🧮 Maths Skills
Interpreting Heating and Cooling Curves
Reading coordinates from a graph: On a heating curve, the flat sections show state changes. The y-value of the flat section gives the melting point or boiling point. The x-axis (time) tells you how long the state change takes. E.g. if the first flat section is at 0°C from t = 2 min to t = 5 min, melting takes 3 minutes.
Calculating energy during state changes: Energy = mass × specific latent heat. E.g. to melt 2 kg of ice at 0°C: energy = 2 × 334 000 = 668 000 J = 668 kJ. During the flat section of a heating curve, energy goes into overcoming intermolecular forces, not raising temperature.
Calculating energy during temperature change: Energy = mass × specific heat capacity × temperature change. E.g. to heat 0.5 kg of water from 20°C to 100°C: energy = 0.5 × 4180 × 80 = 167 200 J = 167.2 kJ. During the rising sections of a heating curve, energy goes into increasing kinetic energy of particles.
Gradient interpretation: The gradient of the rising sections on a heating curve = rate of temperature increase. A steeper gradient means the substance heats faster (lower specific heat capacity or higher heating rate). Different substances have different gradients.
Graph description skills: When describing a heating curve, use precise language: "the temperature increases linearly from X°C to Y°C" for rising sections, and "the temperature remains constant at Z°C" for flat sections. State which state change occurs at each flat section. The longer the flat section, the more energy is needed for that state change.
❌ Common Misconceptions
Misconceptions About States of Matter
Wrong: Particles expand when heated — the particles themselves get bigger
Correct: Particles stay the same size when heated. It is the gaps between the particles that increase. In a solid, the particles vibrate more vigorously and push further apart. In a liquid or gas, the particles move faster and spread out more. The expansion of a material when heated is due to increased spacing, not larger particles.
Wrong: Boiling and evaporation are the same process
Correct: Evaporation happens at any temperature and only at the surface of a liquid — the most energetic surface molecules escape. Boiling happens at a specific temperature (the boiling point) throughout the entire liquid — bubbles of gas form within the liquid and rise to the surface. All boiling is evaporation, but not all evaporation is boiling.
Wrong: When a substance melts, the bonds between atoms are broken
Correct: When a simple molecular substance melts, only the weak intermolecular forces between molecules are overcome. The covalent bonds within the molecules are NOT broken. For ionic and metallic substances, the forces between ions or atoms are partially overcome to allow movement.
✍️ 6-Mark Extended Question
Question
Explain, using the particle model, what happens to a substance as it is heated from a solid to a gas. Include changes in internal energy.
Heating the solid: As the solid is heated, energy is transferred to the particles. The particles vibrate more vigorously about their fixed positions in the lattice. The internal energy (the total kinetic energy and potential energy of the particles) increases. The temperature rises because the average kinetic energy of the particles increases [1 mark].
Melting (solid to liquid): At the melting point, the temperature stops rising even though energy is still being supplied. This energy is being used to overcome the forces of attraction between the particles in the solid lattice, allowing them to break free from their fixed positions. The internal energy increases but the kinetic energy stays the same, so the temperature remains constant. The substance exists as a mixture of solid and liquid during this flat section [2 marks].
Heating the liquid: Once all the solid has melted, the liquid particles move faster as more energy is supplied. The temperature rises again as the average kinetic energy of the particles increases. The particles are close together but can move past each other randomly [1 mark].
Boiling (liquid to gas): At the boiling point, the temperature again stays constant. Energy is being used to completely overcome the remaining intermolecular forces so the particles can separate and move freely as a gas. The internal energy increases but kinetic energy stays constant, so the temperature does not change. Bubbles of gas form throughout the liquid [1 mark].
Heating the gas: Once all the liquid has boiled, the gas particles move even faster. The temperature rises as kinetic energy increases. The particles are far apart and move rapidly in all directions [1 mark].
Mark scheme: 1 mark for solid heating (vibration increases, KE increases); 2 marks for melting (forces overcome, temperature constant, internal energy increases); 1 mark for liquid heating (particles move faster, KE increases); 1 mark for boiling (forces completely overcome, temperature constant); 1 mark for gas heating (particles move rapidly, KE increases). Must mention internal energy for full marks.🔍 AO3: Analyse and Evaluate
Predicting and Explaining a Heating Curve
Scenario: A student heats a pure substance and records its temperature every 30 seconds. The data shows: the temperature rises from 25°C to 114°C, then stays at 114°C for 3 minutes, then rises again to 184°C, then stays at 184°C for 4 minutes, then rises above 184°C. Sketch the heating curve and explain each section.
Analysis: The heating curve has two flat sections, meaning the substance undergoes two state changes. Section 1 (rising from 25°C to 114°C): the substance is a solid warming up — particles vibrate more vigorously, KE increases. Section 2 (flat at 114°C for 3 min): the substance is melting at 114°C — energy overcomes forces between particles in the solid lattice, temperature stays constant. Section 3 (rising from 114°C to 184°C): the substance is now a liquid warming up — particles move faster. Section 4 (flat at 184°C for 4 min): the substance is boiling at 184°C — energy overcomes remaining intermolecular forces completely. Section 5 (rising above 184°C): the substance is now a gas, particles move even faster.
Prediction: The substance has a melting point of 114°C and a boiling point of 184°C. At room temperature (25°C), it is a solid. The longer flat section at 184°C (4 min vs 3 min at 114°C) suggests that more energy is needed to completely separate the particles into a gas than to partially overcome the forces during melting. This substance is likely ionic or metallic given the high melting and boiling points.
Evaluation of missing data: If a section of the heating curve were missing (e.g. the thermometer broke during the experiment), the student could still predict the shape: the curve must have two flat sections corresponding to the two state changes. The melting and boiling points would be at the same temperatures if the substance is pure, allowing the missing section to be reconstructed.
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