Latent heat corresponds to the change in heat at constant

Correct — A, temperature only. Latent heat is the heat a substance takes in or gives out while changing phase, and the defining feature of that exchange is that the thermometer does not move. Energy going into melting ice at 0 °C is spent breaking the bonds that hold the lattice together rather than raising the kinetic energy of the molecules, so the temperature stays put until the last of the ice is gone. Volume changes sharply during a phase change, which rules out the options that include it.

• (b)volume only — Volume is one of the things that changes most. Water expands roughly 1,600 times on becoming steam at atmospheric pressure, and ice is famously less dense than the water it melts into.
• (c)pressure only — Latent heats are quoted at a stated pressure because the boiling point depends on it, but constant pressure is a condition of the measurement rather than what defines the quantity. Heat can be supplied at constant pressure with the temperature rising all the while, and that heat is not latent.
• (d)temperature, volume and pressure — This asks for all three at once, and volume plainly does not hold constant across a phase change.

Heat supplied to a substance does one of two things. Sensible heat raises the temperature and shows on a thermometer, and how much is needed depends on the specific heat capacity. Latent heat drives a change of state at a fixed temperature and shows on no thermometer at all; it is spent on the potential energy of the molecular arrangement rather than on molecular speed.

A heating curve makes the distinction visible. Warm ice from below zero and the temperature climbs; hold at 0 °C and the line goes flat while the ice melts; carry on and the water warms; hold flat again at 100 °C while it boils. The flat portions are the latent heats. The numbers are large — about 334 kilojoules per kilogram to melt ice and about 2,260 to boil water — which is why steam at 100 °C scalds far worse than water at the same temperature, and why sweating cools the body so effectively.

• Latent heat is absorbed or released at constant temperature during a change of state.
• The latent heat of fusion of ice is about 334 kJ/kg; the latent heat of vaporisation of water is about 2,260 kJ/kg.
• Volume changes markedly during a phase change — water becomes roughly 1,600 times its volume as steam at atmospheric pressure.
• Sensible heat, by contrast, changes the temperature and is governed by the specific heat capacity.
• Latent heats are quoted at a stated pressure because the melting and boiling temperatures themselves depend on pressure.

• Assuming the temperature must rise whenever heat is supplied.
• Treating constant pressure as the defining condition when the defining condition is constant temperature.
• Forgetting that a phase change involves a large change in volume.

A definition item in which each wrong option adds a quantity that in fact changes. Ask of each quantity whether it stays fixed while ice melts.

• practice — not a real PYQ

  Heat supplied to a solid at its melting point is used mainly to

  • (a)raise the temperature of the solid
  • (b)break the bonds holding the particles in the lattice
  • (c)increase the pressure of the surroundings
  • (d)reduce the volume of the solid

  Answer(b) break the bonds holding the particles in the lattice — which is why the temperature stays fixed until melting is complete.

• practice — not a real PYQ

  Steam at 100 °C causes more severe burns than water at 100 °C because steam

  • (a)is at a higher temperature
  • (b)carries an extra latent heat of vaporisation
  • (c)has a higher specific heat capacity
  • (d)conducts heat better

  Answer(b) carries an extra latent heat of vaporisation — about 2,260 kJ/kg is given up as the steam condenses on the skin.