CAPF AC 2020 Q22: Science & Technology – Physics

CAPF AC 2020, Paper I (General Ability and Intelligence) · Science & Technology

Latent heat corresponds to the change in heat at constant

Answer: (a) temperature only

Explanation

Latent heat

How to think: recall melting ice.

The thermometer stays at 0 °C until all the ice has melted, even though heat is being given.

Latent heat is the heat taken in or given out when a substance changes its state.

During the change the temperature does not rise.

The heat is used to break the bonds between particles, so it seems hidden.

Latent means hidden.

Quantity during a change of state	Stays constant?
Temperature	Yes
Volume	No; water expands about 1,600 times on turning to steam

Two kinds of latent heat (for water):

Type	Change	Value
Latent heat of fusion	Ice to water at 0 °C	About 334 joules per gram
Latent heat of vaporisation	Water to steam at 100 °C	About 2,260 joules per gram

Answer: (a) temperature only.

(a) CORRECT. A change of state takes place at a constant temperature.
(b) Incorrect. The volume changes a great deal, especially from water to steam.
(c) Incorrect. The defining feature is constant temperature; pressure alone does not define it.
(d) Incorrect. All three cannot stay constant, since the volume changes.

Latent heat = heat for a change of state at constant temperature. Fusion of ice: about 334 J/g. Vaporisation of water: about 2,260 J/g.

Latent heat = change of state at constant temperature.

Thinking the temperature keeps rising while ice melts or water boils.
Mixing up latent heat with specific heat, which is about a rise in temperature.
Forgetting that steam burns more than boiling water because it carries latent heat.

Melting ice stays at 0 °C; boiling water stays at 100 °C. That flat temperature is latent heat.