NDA (I) 2017 General Ability Test · Question 83 of 149

Correct — C, latent heat of vaporization. The word latent means hidden, and it is used because this heat produces no rise in temperature — a thermometer in boiling water sits stubbornly at 100 degrees Celsius while the water goes on absorbing energy and turning to steam. The energy is spent breaking the attractions that hold molecules together in the liquid rather than speeding them up. For water the specific latent heat of vaporisation is about 2260 kilojoules per kilogram, far more than the 334 kilojoules per kilogram needed to melt ice, and it is exactly why steam at 100 degrees Celsius scalds far worse than boiling water at the same temperature.

• (a)specific heat capacity — Specific heat capacity applies precisely when the temperature does change — it is the heat needed to raise unit mass by one degree. The question specifies no change of temperature, which rules this out at once.
• (b)mechanical equivalent of heat — This is Joule's constant, the factor relating a unit of mechanical work to a unit of heat. It belongs to the history of thermodynamics and has nothing to do with a change of state.
• (d)quenching — Quenching is the metallurgical practice of cooling a hot metal rapidly, usually in water or oil, to harden it. It is a process, not a quantity of heat, and it goes in the opposite direction.

Heat supplied to a substance either raises its temperature or changes its state, never both at the same instant. During melting or boiling the temperature holds steady while the added energy overcomes intermolecular forces, and the heat absorbed per unit mass in that stage is the latent heat — of fusion for solid to liquid, of vaporisation for liquid to gas. The same energy is released again when the vapour condenses or the liquid freezes.

The phrase 'without any change in temperature' in the stem is the entire clue, because it eliminates specific heat capacity immediately and leaves only one option that names a quantity of heat tied to a change of state. If a heating curve is drawn for water, the two flat portions are exactly where latent heat is being supplied — this same idea returns as an image-based question in later NDA papers.

• Latent heat is absorbed or released at constant temperature during a change of state.
• The specific latent heat of vaporisation of water is about 2260 kilojoules per kilogram.
• The specific latent heat of fusion of ice is about 334 kilojoules per kilogram.
• Steam at 100 degrees Celsius carries more energy than water at 100 degrees Celsius by exactly this latent heat.

• Choosing specific heat capacity because it is the more familiar term; it applies only when the temperature actually changes.
• Assuming steam and boiling water at 100 degrees Celsius carry the same energy — the latent heat is the whole difference.

NDA either asks for the term that fits a definition or gives a heating curve and asks what the flat stretch represents, so learn the words and the graph together.

• practice — not a real PYQ

  While ice at zero degrees Celsius is melting into water at zero degrees Celsius, the heat supplied is used to

  • (a)raise the temperature of the ice
  • (b)overcome the forces holding the molecules in the solid state
  • (c)compress the water formed
  • (d)increase the pressure of the surroundings

  Answer(b) overcome the forces holding the molecules in the solid state — this is the latent heat of fusion.

• practice — not a real PYQ

  A burn from steam at 100 degrees Celsius is more severe than a burn from water at 100 degrees Celsius because steam

  • (a)is at a higher temperature
  • (b)has a higher specific heat capacity
  • (c)gives out its latent heat of vaporisation on condensing
  • (d)conducts heat faster than water

  Answer(c) gives out its latent heat of vaporisation on condensing — about 2260 kJ per kg extra.