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Thermodynamics

Measuring specific latent heat

  • 1 thing to know
  • Papers 2 and 3

Overview

Recall questions on the funnel-and-heater method for ice, why a second control funnel is used, the boiling-liquid method, and reading a heating curve.

What you need to know

Answer each one before you reveal it.

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Process

Why are readings taken only after the liquid has been boiling steadily for some time?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    How can the specific latent heat of fusion of ice be found using an electrical heater?

    Model answer

    Place an immersion heater of known power in a funnel packed with melting ice, collect and weigh the melt water produced in a measured time, then use L = Pt/Δm.

  2. 2

    Why is a second identical funnel of ice, with no heater running, set up alongside the first?

    Model answer

    It measures the mass of ice melted by heat gained from the surroundings in the same time, which is subtracted so that only the electrically melted mass is used.

  3. 3

    A 50 W heater is run for 300 s and melts an extra 45 g of ice compared with the control. What is the specific latent heat of fusion of ice?

    Model answer

    L = (50 × 300)/0.045 = 3.3 × 10⁵ J kg⁻¹.

  4. 4

    How can the specific latent heat of vaporisation of a liquid be found using a heater and a balance?

    Model answer

    Bring the liquid to a steady boil with a heater of known power, record the mass lost over a measured time from the balance reading, and use L = Pt/Δm.

  5. 5

    Why are readings taken only after the liquid has been boiling steadily for some time?

    Model answer

    Once boiling steadily the temperature is constant, so all the electrical energy supplied goes into vaporisation rather than raising the temperature, and heat losses are steady.

  6. 6

    A substance is heated at a constant rate and its temperature-time graph shows a horizontal plateau. What is happening during the plateau?

    Model answer

    The substance is changing state, so the energy supplied is increasing molecular potential energy instead of raising the temperature.

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