Understanding Cooling Curves in BTEC Applied Science

In BTEC Applied Science Unit 2, Learning Aim B, you’ll explore thermal physics — including how substances change state and how energy is transferred during heating and cooling. A key concept here is the cooling curve.

Let’s break it down.

What Is a Cooling Curve?

A cooling curve is a graph that shows how the temperature of a substance decreases over time as it loses heat energy.

• The x-axis usually represents time.

• The y-axis shows the temperature of the substance.

  
  

As a substance cools, the temperature doesn’t always fall smoothly. Instead, you’ll see flat sections (plateaus) where the temperature stays constant for a while.

Why Do Flat Sections Appear?

When a substance changes state (for example, from liquid to solid), energy is still being transferred — but instead of lowering the temperature, the energy is used to rearrange particles.

• During freezing: the substance cools until it reaches its freezing point, then stays at that temperature until all the liquid becomes solid.

• Only after the change of state is complete will the temperature continue to fall.

  
  

This is why a cooling curve typically has sloping sections (temperature falling) and flat sections (change of state).

Example: Cooling Curve for Water

Imagine you take liquid water at 80 °C and allow it to cool:

1️⃣ From 80 °C down to 0 °C, the temperature falls steadily.

2️⃣ At 0 °C, the graph flattens — the water is freezing. The temperature stays at 0 °C until all water has turned to ice.

3️⃣ Once frozen, the temperature of the ice continues to fall below 0 °C.

Key Terms to Remember

• Freezing Point – the temperature where liquid turns to solid.

• Latent Heat – the “hidden” energy needed for a change of state, without a change in temperature.

• Plateau – the flat part of the cooling curve where the state change happens.

  
  

Why Cooling Curves Matter in Science

In Unit 2, Learning Aim B, you’re expected to show that you understand energy transfers during state changes. Cooling curves help you to:

• Interpret experimental data on heating and cooling.

• Explain why temperature stays constant during melting/freezing.

• Link observations to particle theory and latent heat.

  
  

Common Mistakes to Avoid

⚠️ Thinking temperature always drops steadily — remember, it levels off at a state change.

⚠️ Forgetting that energy is still being lost during the plateau (it’s just being used to change the state).

⚠️ Mixing up freezing and melting — both happen at the same temperature (e.g. 0 °C for water).

How to Revise Cooling Curves

• Draw labelled diagrams of typical cooling curves.

• Label the 3 points along the graph and calculate the gradient at each point

• Explain what is happening to the particles at each point in terms of movement, kinetic energy and intermolecular forces

• Practise explaining why the line is flat during state changes.

• Use keywords: latent heat, freezing point, plateau, energy transfer.

  
  

Real-World Applications of Cooling Curves

Understanding cooling curves is not just an academic exercise; it has practical implications in our daily lives. For instance, when you freeze food, the cooling curve illustrates how the temperature of the food drops until it reaches the freezing point, where it remains constant until fully frozen. This knowledge can help you better understand food preservation and safety.

In industries, cooling curves are crucial for processes like metal cooling. Engineers must consider how quickly materials cool to ensure they maintain their desired properties.

Final Thought

A cooling curve is more than just a graph — it’s a way of showing how energy, temperature, and particles are all linked during changes of state. Mastering this will not only help with Unit 2 assignments and assessments but also give you a stronger understanding of real-world applications, from freezing food to cooling metals in industry.

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