Specific latent heat - Teacher Slides.pptx

AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Specific latent heat

Particle model of matter

Lesson 4 of 6

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Warm-up

Particle model of matter

Lesson 4 of 6

Answer each one, then check.

1

What happens to temperature during melting?

2

What is the unit of energy?

3

What is the equation for a temperature change?

4

Name the change from liquid to gas.

5

What is internal energy?

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AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Warm-up - Answers

Particle model of matter

Lesson 4 of 6

1

What happens to temperature during melting?

It stays constant

2

What is the unit of energy?

Joule

3

What is the equation for a temperature change?

ΔE = mcΔθ

4

Name the change from liquid to gas.

Boiling / evaporating

5

What is internal energy?

Total kinetic and potential energy of the particles

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AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Learning Objectives

Particle model of matter

Lesson 4 of 6

1

Define specific latent heat and distinguish fusion from vaporisation.

2

Apply E = mL.

3

Interpret heating and cooling graphs that include changes of state.

4

Distinguish specific heat capacity from specific latent heat.

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AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Particle model of matter

Lesson 4 of 6

SPECIFIC LATENT HEAT

The specific latent heat of a substance is the energy needed to change the state of 1 kg of the substance with no change in temperature.

E = mL is given on the equation sheet. Specific latent heat of fusion: solid to liquid. Specific latent heat of vaporisation: liquid to gas. L is in J/kg.

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A Heating Curve

Particle model of matter

Lesson 4 of 6

Flat sections show a change of state.

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Two Different Ideas

Particle model of matter

Lesson 4 of 6

SPECIFIC HEAT CAPACITY

• Energy to raise the temperature of 1 kg by 1 °C.

• The temperature changes.

• ΔE = mcΔθ; J/kg °C.

SPECIFIC LATENT HEAT

• Energy to change the state of 1 kg with no temperature change.

• The temperature stays the same.

• E = mL; J/kg.

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Melting Ice

Particle model of matter

Lesson 4 of 6

Calculate the energy needed to melt 0.20 kg of ice at 0 °C. Specific latent heat of fusion of water = 334 000 J/kg.

1

Write the equation

E = mL

2

Substitute

E = 0.20 × 334 000

3

Answer

E = 66 800 J

ANSWER

66 800 J

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Boiling Water

Particle model of matter

Lesson 4 of 6

Calculate the energy to change 0.50 kg of water at 100 °C into steam. Specific latent heat of vaporisation = 2 260 000 J/kg.

1

Substitute

E = 0.50 × 2 260 000

2

Answer

E = 1 130 000 J

ANSWER

1 130 000 J (1.13 MJ)

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Reading the Graph

Particle model of matter

Lesson 4 of 6

Explain why the temperature stays constant during section B of the heating curve.

1

Energy is still supplied

The particles are gaining energy

2

Where it goes

It increases the potential energy of the particles as the bonds are broken

3

So

The average kinetic energy, and the temperature, stay the same

ANSWER

The energy supplied breaks the bonds (changing potential energy) so the temperature does not rise.

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Why Steam Burns More

Particle model of matter

Lesson 4 of 6

A favourite exam idea.

Condensing

Steam at 100 °C condenses to water at 100 °C and releases a lot of latent heat.

Then

The water then cools and releases more energy.

So

Steam transfers more energy to skin than boiling water at the same temperature.

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Key Terms

Particle model of matter

Lesson 4 of 6

Specific latent heat

Energy to change the state of 1 kg with no temperature change.

Latent heat of fusion

For solid to liquid.

Latent heat of vaporisation

For liquid to gas.

Heating curve

A temperature-time graph for a heated substance.

Constant temperature

Temperature that does not change during a change of state.

Potential energy

Energy stored due to the forces between particles.

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AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER

Particle model of matter

Lesson 4 of 6

YOUR TASK

Read the Curve

12 minutes

On a heating curve for water there are flat sections at 0 °C and 100 °C. Explain what is happening in each and why the second is longer.

1

Name each change.

2

Compare the latent heats.

WHAT A GOOD ANSWER SHOWS

At 0 °C the ice is melting; at 100 °C the water is boiling. The boiling section is longer because the latent heat of vaporisation is much greater than the latent heat of fusion.

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Can I...?

Particle model of matter

Lesson 4 of 6

State what specific latent heat is.

Distinguish fusion from vaporisation.

Use E = mL.

Read a heating curve.

Explain flat sections.

Distinguish c from L.

Explain steam burns.

Give units of L.

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Summary & Exam Focus

E = mL.

Temperature is constant during a change of state.

Heating curves have flat sections at melting and boiling points.

Distinguish c and L.

EXAM FOCUS

Calculate the energy needed to melt 0.35 kg of ice at 0 °C. Specific latent heat of fusion = 334 000 J/kg. (2 marks)

Multiply mass by L.

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Exam Practice: Specific latent heat

Particle model of matter

Lesson 4 of 6

Answer all questions. Use the mark allocation as a guide to how much to write. · 15 minutes

Question 1 · 2 marks · Calculate

Calculate the energy needed to melt 0.35 kg of ice at 0 °C. The specific latent heat of fusion of water is 334 000 J/kg. Use the equation…

Question 2 · 4 marks · Use the graph

The graph shows how the temperature of a solid changes as it is heated at a steady rate until it is a gas. (a) State the melting point. (b)…

Question 3 · 2 marks · Distinguish

Describe the difference between specific heat capacity and specific latent heat.

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Exam Practice: Specific latent heat (cont.)

Particle model of matter

Lesson 4 of 6

Question 4 · 3 marks · Calculate

Calculate the energy needed to change 0.50 kg of water at 100 °C into steam. The specific latent heat of vaporisation of water is 2 260 000…

Question 5 · 3 marks · Explain

Steam at 100 °C causes a more serious burn than boiling water at 100 °C. Explain why.

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Question 1 · 2 marks · Calculate

Particle model of matter

Lesson 4 of 6

“

Calculate the energy needed to melt 0.35 kg of ice at 0 °C. The specific latent heat of fusion of water is 334 000 J/kg. Use the equation: energy for a change of state = mass × specific latent heat

HOW TO ANSWER IT

Command word: Calculate. Worth 2 marks, so plan before writing.

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Question 1 · mark scheme

Particle model of matter

Lesson 4 of 6

2 marks available. Award a mark for each point made.

Correct substitution

1 mark

116 900 J

1 mark

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Question 2 · 4 marks · Use the graph

Particle model of matter

Lesson 4 of 6

The graph shows how the temperature of a solid changes as it is heated at a steady rate until it is a gas. (a) State the melting point. (b) What is happening between 2 and 8 minutes? (c) Explain why the temperature stays constant during this time. (4 marks)

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Question 2 · mark scheme

Particle model of matter

Lesson 4 of 6

4 marks available. Award a mark for each point made.

0 °C

1 mark

Melting

1 mark

Energy used to break bonds or change potential energy

1 mark

Kinetic energy or temperature does not increase

1 mark

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Question 3 · 2 marks · Distinguish

Particle model of matter

Lesson 4 of 6

“

Describe the difference between specific heat capacity and specific latent heat.

HOW TO ANSWER IT

Command word: Distinguish. Worth 2 marks, so plan before writing.

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Question 3 · mark scheme

Particle model of matter

Lesson 4 of 6

2 marks available. Award a mark for each point made.

Specific heat capacity: temperature change

1 mark

Specific latent heat: change of state, no temperature change

1 mark

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Question 4 · 3 marks · Calculate

Particle model of matter

Lesson 4 of 6

“

Calculate the energy needed to change 0.50 kg of water at 100 °C into steam. The specific latent heat of vaporisation of water is 2 260 000 J/kg.

HOW TO ANSWER IT

Command word: Calculate. Worth 3 marks, so plan before writing.

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Question 4 · mark scheme

Particle model of matter

Lesson 4 of 6

3 marks available. Award a mark for each point made.

Correct substitution

1 mark

1 130 000 J

1 mark

Unit J

1 mark

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Question 5 · 3 marks · Explain

Particle model of matter

Lesson 4 of 6

“

Steam at 100 °C causes a more serious burn than boiling water at 100 °C. Explain why.

HOW TO ANSWER IT

Command word: Explain. Worth 3 marks, so plan before writing.

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Question 5 · mark scheme

Particle model of matter

Lesson 4 of 6

3 marks available. Award a mark for each point made.

Steam condenses on the skin

1 mark

Releases latent heat

1 mark

So more energy is transferred than from the water alone

1 mark

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