🔵 Describe the arrangement and movement of particles in solids, liquids and gases
🔴 Explain properties of each state of matter in terms of particle model
🟢 Name and describe all six changes of state: melting, freezing, boiling, condensing, sublimation, deposition
🟠 Interpret heating and cooling curves, identifying flat sections at melting and boiling points
🟣 Explain why temperature stays constant during a change of state in terms of energy and bonds
⭐ Apply knowledge of specific latent heat to changes of state calculations
🔵 The Three States of Matter — Particle Model
All matter is made of tiny particles (atoms or molecules). The state of a substance — solid, liquid or gas — depends on how much energy those particles have and how strongly they are held together by intermolecular forces.
Particle Model: A simplified model that describes matter as tiny spheres with forces between them. It explains physical properties and changes of state.
Solids
In a solid, particles are arranged in a regular, close-packed lattice. The intermolecular forces are very strong, so particles can only vibrate about fixed positions — they cannot flow or move past each other. This gives solids a fixed shape and fixed volume. Solids are virtually incompressible because the particles are already touching each other.
Liquids
In a liquid, particles are still close together but arranged randomly. The intermolecular forces are weaker than in a solid, so particles can move past each other and flow. Liquids have a fixed volume but no fixed shape — they take the shape of their container. Liquids are also nearly incompressible because particles remain close together.
Gases
In a gas, particles are far apart and move rapidly in random directions. Intermolecular forces are negligible (effectively zero). Gases have no fixed shape and no fixed volume — they expand to fill any container. Gases are highly compressible because there is a large amount of empty space between particles.
Property
Solid
Liquid
Gas
Particle arrangement
Regular lattice
Random, close
Random, far apart
Particle movement
Vibrate only
Flow past each other
Move rapidly, freely
Intermolecular forces
Very strong
Moderate
Negligible
Fixed shape?
Yes
No
No
Fixed volume?
Yes
Yes
No
Compressible?
No
No
Yes
💡 The key difference between states is the energy of the particles. Adding energy increases particle motion; removing energy reduces it. This drives changes of state.
🔴 Changes of State
A change of state occurs when a substance moves from one state to another. During a change of state, energy is transferred to or from the particles, changing the strength of the intermolecular bonds but not the temperature.
Change of State: A physical change (not a chemical change) in which matter transitions between solid, liquid and gas. The process is reversible — no new substance is created.
The Six Changes of State
Change
Direction
Energy Change
Melting
Solid → Liquid
Energy absorbed (endothermic)
Freezing
Liquid → Solid
Energy released (exothermic)
Boiling / Vaporisation
Liquid → Gas
Energy absorbed (endothermic)
Condensing
Gas → Liquid
Energy released (exothermic)
Sublimation
Solid → Gas
Energy absorbed (endothermic)
Deposition
Gas → Solid
Energy released (exothermic)
What Happens at a Particle Level?
When a substance melts, energy is supplied to the particles. This energy goes into breaking intermolecular bonds (overcoming the forces holding particles in their fixed positions), not into increasing kinetic energy. Because kinetic energy doesn't increase, the temperature stays constant.
When a substance boils, energy is again used to completely overcome the intermolecular forces so that particles can escape into the gas phase. Again, temperature remains constant during this process.
The reverse is true for freezing and condensing — energy is released as new intermolecular bonds form.
💡 Changes of state are physical changes, not chemical changes. The substance remains chemically the same — only the arrangement of particles changes.
🟢 Heating Curves
A heating curve shows how the temperature of a substance changes over time when it is heated at a constant rate. It is one of the most important graphs in this topic.
Shape of a Heating Curve
When you heat a solid, the temperature rises as particles gain kinetic energy and vibrate faster. When the melting point is reached, the temperature stays constant — this creates a flat (horizontal) section on the graph. All the energy being supplied is used to break intermolecular bonds, not to raise the temperature. Once all the solid has melted, the temperature rises again through the liquid phase. When the boiling point is reached, there is another flat section as energy breaks the remaining intermolecular bonds.
Heating Curve Summary:
Segment 1 (rising): Solid heating up → KE of particles increases
Segment 2 (flat): Melting point → bonds breaking, temperature constant
Segment 3 (rising): Liquid heating up → KE of particles increases
Segment 4 (flat): Boiling point → bonds breaking, temperature constant
Segment 5 (rising): Gas heating up → KE of particles increases
Cooling Curves
A cooling curve is the reverse. As a gas cools, it eventually reaches the condensing point (= boiling point) where a flat section appears. Then the liquid cools until it reaches the freezing point (= melting point), where another flat section appears as bonds form and energy is released.
💡 The melting point and freezing point of a substance are the same temperature. Similarly, the boiling point and condensing point are the same temperature — just approached from different directions.
🟠 Specific Latent Heat
The energy needed to change the state of a substance without changing its temperature is called the latent heat. The word "latent" means "hidden" — the energy is hidden because it doesn't show up as a temperature change.
Specific Latent Heat (L): The energy needed to change the state of 1 kg of a substance without changing its temperature. Units: J/kg (joules per kilogram).
There are two types:
Specific latent heat of fusion (Lf) — energy for melting/freezing
Specific latent heat of vaporisation (Lv) — energy for boiling/condensing
Latent Heat Equation:
Q = m × L
Q = energy transferred (J)
m = mass (kg)
L = specific latent heat (J/kg)
Symbol
Quantity
SI Unit
Q
Energy transferred
Joules (J)
m
Mass
Kilograms (kg)
L
Specific latent heat
J/kg
Typical Values
Substance
Lf (J/kg)
Lv (J/kg)
Water (ice/steam)
334,000
2,260,000
Ethanol
108,000
841,000
💡 Notice that Lv for water is much larger than Lf. This is because all intermolecular bonds must be fully broken during boiling, whereas only some are broken during melting.
Example 1: Calculate the energy needed to melt 0.5 kg of ice at 0°C. The specific latent heat of fusion of water is 334,000 J/kg.
1Identify the equation: Q = m × L
2List the known values: m = 0.5 kg L = 334,000 J/kg
✅ Energy released = 565,000 J = 565 kJ This explains why steam burns are much more severe than boiling water burns — steam releases a huge amount of latent heat when it condenses on skin.
Example 3: A substance has a heating curve with a flat section at 80°C. It then has another flat section at 350°C. The substance is heated from –20°C. Describe what is happening at each stage of the curve and identify the state of the substance between 80°C and 350°C.
1Stage 1 (–20°C to 80°C, rising): The substance is a solid. Particles gain kinetic energy and vibrate more quickly. Temperature increases steadily.
2Stage 2 (flat at 80°C): Melting is occurring. 80°C is the melting point. Energy supplied breaks intermolecular bonds. Temperature stays constant at 80°C.
3Stage 3 (80°C to 350°C, rising): The substance is now a liquid. Particles move faster and flow more freely as temperature increases.
4Stage 4 (flat at 350°C): Boiling is occurring. 350°C is the boiling point. Energy completely breaks all intermolecular bonds. Temperature stays constant at 350°C.
5Stage 5 (above 350°C, rising): The substance is a gas. Particles move rapidly in all directions with increasing kinetic energy.
✅ Between 80°C and 350°C the substance is a liquid. The melting point is 80°C and the boiling point is 350°C.
Example 4: How much mass of a substance can be completely melted by 50,000 J of energy, if its specific latent heat of fusion is 200,000 J/kg?
1Write the equation: Q = m × L
2Rearrange for mass: m = Q ÷ L
3Substitute values: Q = 50,000 J L = 200,000 J/kg m = 50,000 ÷ 200,000
4Calculate: m = 0.25 kg
✅ Mass melted = 0.25 kg (250 g)
Question 1: Which statement correctly describes particles in a liquid?
Question 2: On a heating curve, what happens to the temperature of a substance during melting?
Question 3: Calculate the energy needed to melt 2 kg of ice. (Specific latent heat of fusion of water = 334,000 J/kg). Enter your answer in J.
Question 4: A substance changes directly from solid to gas without passing through the liquid state. This process is called:
Question 5: Steam at 100°C condenses to form liquid water. What happens to the energy during this process? Enter 1 for "energy is released" or 2 for "energy is absorbed".
Challenge 1 (4 marks): Explain, using the particle model, why the temperature of water remains at 100°C while it is boiling, even though a heater is continuously supplying energy.
Model Answer:
• During boiling, the heater continuously supplies energy to the water particles. (1)
• This energy is used to break the intermolecular bonds between water molecules, allowing them to escape into the gas phase. (1)
• The energy goes into increasing potential energy (overcoming bonds), not kinetic energy. (1)
• Since temperature is a measure of average kinetic energy of particles, and KE does not increase, the temperature remains constant at 100°C. (1)
Challenge 2 (5 marks): A block of ice of mass 1.5 kg is at –10°C. The specific heat capacity of ice is 2,100 J/kg°C and the specific latent heat of fusion of water is 334,000 J/kg. (a) Calculate the energy needed to warm the ice from –10°C to 0°C. (b) Calculate the energy needed to then completely melt the ice at 0°C. (c) Calculate the total energy needed for both processes.
Model Answer: (a) Heating ice from –10°C to 0°C:
Q = m × c × ΔT
Q = 1.5 × 2,100 × 10
Q = 31,500 J (1 mark for working, 1 mark for answer)
(b) Melting the ice at 0°C:
Q = m × L
Q = 1.5 × 334,000
Q = 501,000 J (1 mark for working, 1 mark for answer)
Challenge 3 (4 marks): The specific latent heat of vaporisation of water is 2,260,000 J/kg and the specific latent heat of fusion is 334,000 J/kg. Explain why Lv is so much larger than Lf, and state one real-world consequence of this difference.
Model Answer:
• During melting (fusion), only some intermolecular bonds are broken — enough to allow particles to flow past each other. (1)
• During boiling (vaporisation), all intermolecular bonds must be completely broken so particles can move freely as a gas. This requires far more energy. (1)
• Lv is approximately 6.8× larger than Lf because significantly more bonds must be broken. (1)
• Real-world consequence (any one of): steam burns are more severe than boiling water burns; sweating is an effective cooling mechanism because evaporation requires a large amount of energy; coastal areas have milder climates because the sea can absorb/release large amounts of energy. (1)
Challenge 4 — Extended Writing (6 marks): A student heats a 0.5 kg sample of an unknown substance from –50°C. The substance has a melting point of 0°C and a boiling point of 120°C. The specific latent heat of fusion is 180,000 J/kg and the specific latent heat of vaporisation is 900,000 J/kg. Sketch and describe the shape of the heating curve you would expect, and calculate the energy released when the same mass of the substance is cooled from gas at 120°C back to liquid at 120°C.
Model Answer: Heating curve description:
• Rising line from –50°C to 0°C: solid heating up, KE of particles increasing. (1)
• Flat section at 0°C: melting — energy breaks bonds, temperature constant. (1)
• Rising line from 0°C to 120°C: liquid heating up. (1)
• Flat section at 120°C: boiling — all bonds broken, temperature constant. (1)
• Rising line above 120°C: gas heating up. (1)
Energy released during condensing (gas → liquid at 120°C):
Q = m × Lv
Q = 0.5 × 900,000
Q = 450,000 J (1 mark)