(a) Distinguish between temperature and heat. State the units in which they are measured (ii) State two physical properties used for measuring temperature. ...

(a)(i) Temperature and heat

(a)(ii) Two physical properties used in thermometry are:

1. change in volume of a liquid with temperature;
2. change in electrical resistance of a conductor with temperature.

(b)(i) Determination of the upper fixed point, or steam point

The apparatus is arranged as shown below.

Pure water is boiled in a hypsometer. The bulb of the mercury-in-glass thermometer is surrounded by dry steam above the boiling water. The pressure of the steam is measured with the manometer and is adjusted to standard atmospheric pressure, 76 cm Hg (760 mm Hg). When the mercury thread becomes steady, its level is marked on the stem. This mark is the upper fixed point, equal to 100 b0C.

Precaution: Ensure that the thermometer bulb is completely surrounded by steam and does not touch the boiling water or the wall of the vessel.

(b)(ii)

• Advantage: A constant-volume gas thermometer gives very accurate measurements over a wide temperature range, since the expansion of the containing glass is negligible compared with the change in pressure of the gas. It is therefore suitable as a standard thermometer for calibration.
• Reason it is seldom used routinely: It is bulky, delicate and inconvenient to handle, and it gives readings slowly.

(c) Why evaporation causes cooling

Liquid molecules are in continual random motion and have different kinetic energies. At the surface, the fastest molecules, having sufficiently high kinetic energy, escape from the liquid as vapour. These molecules carry away more than the average kinetic energy. The average kinetic energy of the molecules left in the liquid therefore decreases. Since temperature is proportional to the average kinetic energy of the molecules, the temperature of the remaining liquid falls, causing cooling.