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Latent heat

Your guide: Sir HamzaBelieves every formula has a story, and units never lie.

Heat that hides in a change of state

Latent heat is the heat absorbed or released when a substance changes state without changing its temperature. The energy breaks or forms the bonds between molecules instead of speeding them up.

Latent heat of fusion Lf is the heat to melt 1 kg of a solid at its melting point. For ice, Lf = 3.36 × 105 J kg−1, and Q = m Lf.

Latent heat of vaporisation Lv is the heat to turn 1 kg of a liquid into vapour at its boiling point. For water, Lv = 2.26 × 106 J kg−1, and Q = m Lv.

What the experiments show (Fig. 10.12, 10.13)

Heat crushed ice steadily: the temperature stays at 0 °C until all the ice has melted, then rises.

Heat water to boiling: the temperature stays at 100 °C while it boils away.

The flat parts of the graph are where latent heat is being absorbed.

During melting or boiling the thermometer stops rising, but heat is still going in. It goes into breaking bonds, not into speed.

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Notes, short questions and MCQs

Read the full notes: key terms, model answers and MCQs with answers

Heat that hides in a change of state

Latent heat is the heat absorbed or released when a substance changes state without changing its temperature. The energy breaks or forms the bonds between molecules instead of speeding them up.

Latent heat of fusion Lf is the heat to melt 1 kg of a solid at its melting point. For ice, Lf = 3.36 × 105 J kg−1, and Q = m Lf.

Latent heat of vaporisation Lv is the heat to turn 1 kg of a liquid into vapour at its boiling point. For water, Lv = 2.26 × 106 J kg−1, and Q = m Lv.

What the experiments show (Fig. 10.12, 10.13)

Heat crushed ice steadily: the temperature stays at 0 °C until all the ice has melted, then rises.

Heat water to boiling: the temperature stays at 100 °C while it boils away.

The flat parts of the graph are where latent heat is being absorbed.

During melting or boiling the thermometer stops rising, but heat is still going in. It goes into breaking bonds, not into speed.

Key terms

Latent heat
Heat absorbed or released during a change of state.
Latent heat of fusion (Lf)
Heat to melt 1 kg of a solid at its melting point.
Latent heat of vaporisation (Lv)
Heat to turn 1 kg of a liquid to vapour at its boiling point.
Temperature–time graph
Shows plateaus where latent heat is absorbed.

Short questions with model answers

  1. Q1. Example 10.2 (p. 13): how much heat melts 600 g (0.600 kg) of ice at 0 °C? Lf = 3.36 × 105 J kg−1.

    • Q = m Lf
    • Q = 0.600 × 336000 = 201600 J

    201,600 J, about 201.6 kJ.

  2. Q2. Numerical 10.7 (p. 18): how much heat turns 1 kg of water at 100 °C into steam? Lv = 2.26 × 106 J kg−1.

    • Q = m Lv
    • Q = 1 × 2.26 × 106 J

    2.26 × 106 J = 2.26 MJ, the book's answer.

Common mistakes

  • ✗ While ice melts, its temperature keeps rising.

    ✓ It stays at 0 °C until all the ice has melted (Fig. 10.12).

  • ✗ Use 500 g directly in Q = m Lf.

    ✓ Lf is per kilogram, so use 0.5 kg.

  • ✗ Use Lf to boil water.

    ✓ Lf is for melting; Lv is for boiling, and it is much larger.

MCQs

  1. 1. During melting, the supplied heat mainly:

    1. (a) Raises the temperature
    2. (b) Breaks the bonds between molecules
    3. (c) Is lost to the air
    4. (d) Makes the molecules heavier
    Show answer

    (b) p. 13: latent heat breaks or forms intermolecular bonds.

  2. 2. The latent heat of fusion of ice is:

    1. (a) 2.26 × 106 J kg−1
    2. (b) 4,200 J kg−1 K−1
    3. (c) 810 J kg−1 K−1
    4. (d) 3.36 × 105 J kg−1
    Show answer

    (d) p. 13. 2.26 × 106 J kg−1 is Lv of water.

  3. 3. Heat needed to melt 2 kg of ice at 0 °C:

    1. (a) 672 kJ
    2. (b) 336 kJ
    3. (c) 168 kJ
    4. (d) 4.52 MJ
    Show answer

    (a) Q = 2 × 3.36 × 105 = 6.72 × 105 J = 672 kJ.

  4. 4. On the temperature–time graph for heating ice, the flat parts show:

    1. (a) Heating stopped
    2. (b) The thermometer broke
    3. (c) A change of state
    4. (d) Cooling
    Show answer

    (c) Fig. 10.12: latent heat is absorbed at constant temperature.

  5. 5. Which needs more energy for 1 kg of water?

    1. (a) Melting it from ice
    2. (b) Boiling it into steam
    3. (c) Both need the same
    4. (d) Neither needs energy
    Show answer

    (b) Lv (2.26 × 106) is almost seven times Lf (3.36 × 105).

Quick revision

  • Latent heat changes the state, not the temperature; it breaks or forms bonds.
  • Q = m Lf for melting; Lf of ice is 3.36 × 105 J kg−1.
  • Q = m Lv for boiling; Lv of water is 2.26 × 106 J kg−1.

Before you start

What is Class 10 Physics Latent heat?
Latent heat is the open Class 10 Physics lesson for Matric Part 2, Punjab board. It is a step-by-step lesson, not a PDF of the book.
Which boards is this Class 10 Physics lesson for?
The same Punjab textbook: Lahore, Rawalpindi, Gujranwala, Faisalabad, Multan, Sargodha, Bahawalpur, Dera Ghazi Khan, Sahiwal.
Can I study this in Urdu?
Yes. English, Urdu or Roman Urdu. Science terms stay as the exam writes them.
Is this a notes PDF?
No. The book stays with you. This page is the lesson and the check.

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