Welcome! In northern Canada, lorries drive across frozen lakes on roads made of ice. In this lesson you’ll learn about density and changes of state, then write a safety briefing for ice road truckers. It takes about one hour. Work through the steps in order.
Before you start
- You need: your exercise book or paper, a pen and a calculator.
- Write the title and date: Density and changes of state – Ice Road Truckers.
- The golden rule: write your answer first, then tap Show answer. Correct mistakes in a different colour.
- Look out for the blue 📝 boxes. Copy each one into your book neatly – these are your revision notes.
📝 Copy into your book
By the end of this lesson I will be able to:
1. calculate density and explain why ice floats
2. explain why temperature stays the same during a change of state
3. use specific heat capacity and specific latent heat in calculations.
Step 1: Starter (5 minutes)
Most substances are denser as solids than as liquids – a lump of solid wax sinks in melted wax. But ice floats on water. Why does this matter for ice road truckers? Write down your ideas.
Show answer
Because ice is less dense than water, it forms a layer on top of the lake instead of sinking. That layer is what the trucks drive on! If ice sank, lakes would freeze from the bottom up and there would be no ice road.
📝 Copy into your book
density = mass ÷ volume ρ = m ÷ V
ρ in kg/m³, m in kg, V in m³
An object floats if it is less dense than the liquid it is in.
Step 2: Read and write (8 minutes)
First, copy these three questions into your book. Then read the text below carefully and answer them in full sentences.
- Why are gases much less dense than solids?
- What happens to the temperature of ice while it is melting?
- What is the difference between specific heat capacity and specific latent heat?
📖 Read: Particles, density and changes of state
In a solid, particles are packed closely in a regular pattern and vibrate in fixed positions. In a liquid, they are still close together but can move past each other. In a gas, they are far apart and move quickly in random directions. Because gas particles are so spread out, gases have a much lower density than solids and liquids. Water is unusual: when it freezes, its particles arrange themselves slightly further apart, so ice (about 920 kg/m³) is less dense than liquid water (about 1000 kg/m³) and floats.
Heating a substance increases its internal energy. Usually this raises its temperature. The energy needed to raise the temperature of 1 kg of a substance by 1 °C is its specific heat capacity. For water it is 4200 J/kg °C.
But while a substance is changing state – melting, freezing, boiling or condensing – its temperature stays the same. The energy is being used to break (or form) the bonds between particles instead. The energy needed to change the state of 1 kg of a substance without changing its temperature is its specific latent heat. Melting 1 kg of ice needs 334 000 J. Changes of state are physical changes: the mass stays the same, and the change can be reversed.
Show answers
(a) The particles in a gas are much further apart, so there is much less mass in the same volume.
(b) It stays the same (at 0 °C) until all the ice has melted.
(c) Specific heat capacity is the energy to raise 1 kg by 1 °C. Specific latent heat is the energy to change the state of 1 kg without a change in temperature.
📝 Copy into your book
Changing temperature: E = m c Δθ (c = specific heat capacity, J/kg °C)
Changing state: E = m L (L = specific latent heat, J/kg)
During a change of state the temperature does NOT change – the energy breaks or forms bonds between particles.
Step 3: Calculations (10 minutes)
Use: specific heat capacity of water = 4200 J/kg °C; specific latent heat of fusion of ice = 334 000 J/kg.
Q1 A block of lake ice has a mass of 460 kg and a volume of 0.50 m³. Calculate its density.
Show answer
ρ = m ÷ V = 460 ÷ 0.50 = 920 kg/m³ – less than water’s 1000 kg/m³, so it floats.
Q2 How much energy is needed to melt 2.0 kg of ice at 0 °C?
Show answer
E = m L = 2.0 × 334 000 = 668 000 J
Q3 How much energy is needed to warm that 2.0 kg of water from 0 °C to 10 °C?
Show answer
E = m c Δθ = 2.0 × 4200 × 10 = 84 000 J
Q4 Compare your answers to Q2 and Q3. What do they tell you?
Show answer
Melting the ice takes about 8 times more energy than warming the water by 10 °C. That’s why ice takes so long to melt, even on a mild day – and why ice roads can last for weeks.
✅ Score out of 4.
Step 4: Main task – Ice Road Safety Briefing (14 minutes)
You work for the ice road company. New drivers must understand the science before they’re allowed on the ice. Write a safety briefing that answers these four questions using physics:
- Why is there ice on top of the lake at all? Use the word density.
- Why doesn’t the ice melt the moment the air gets above 0 °C? Use the words latent heat and bonds, and your answer to Q4.
- A thermometer in slushy ice reads 0 °C all afternoon, even though the sun is shining. Is the ice safe? Explain what’s happening to the energy.
- When the ice melts in spring, does the lake have more water, less water or the same mass? Explain.
Show a model briefing
1. Ice has a lower density (about 920 kg/m³) than water (1000 kg/m³), so it floats and forms a layer on top of the lake. That’s the road.
2. To melt, the ice must absorb its latent heat – 334 000 J for every kilogram – to break the bonds holding its particles in place. That’s about 8 times the energy needed to warm the same water by 10 °C, so melting happens slowly.
3. NOT safe. The temperature stays at 0 °C because the ice is melting – the energy from the sun is breaking bonds rather than raising the temperature. A steady 0 °C reading in slushy ice means it’s changing state and getting weaker.
4. The same mass. Melting is a physical change – the particles are rearranged, but none are created or destroyed, so mass is conserved.
✅ Check: did you use all the key words, and explain why each time?
Step 5: Exam practice (13 minutes)
Look at the marks – they tell you how many points to make. Answer in full sentences, then mark yourself.
Q1 (3 marks) A metal block has a mass of 2.7 kg and a volume of 0.001 m³. Calculate its density.
Show model answer
ρ = m ÷ V (1)
ρ = 2.7 ÷ 0.001 (1)
ρ = 2700 kg/m³ (1)
Q2 (2 marks) Explain why the temperature of water stays at 100 °C while it is boiling, even though it is still being heated.
Show model answer
The energy supplied is used to break the bonds between the water particles / change the state (1), so it increases the internal energy without increasing the temperature (the kinetic energy of the particles) (1).
Q3 (3 marks) The specific latent heat of vaporisation of water is 2 260 000 J/kg. Calculate the energy needed to boil away 0.50 kg of water at 100 °C.
Show model answer
E = m L (1)
E = 0.50 × 2 260 000 (1)
E = 1 130 000 J (1.13 × 106 J) (1)
✅ Add up your marks out of 8 and write the score in your book.
Step 6: Exit ticket (5 minutes)
- Ice floats on water because…
- While ice is melting, the energy it absorbs is used to…
- One thing I’m still not sure about is…
🎉 Well done – lesson complete! Your 📝 boxes are your revision notes for this topic. Want more? Try the Particle model mixed equation practice.
