(a) Distinguish between temperature and heat. State the units in which they are measured
(ii) State two physical properties used for measuring temperature.
(b) (i) Describe, with the aid of a diagram, how the upper fixed point is determined for a mecury-in-glass thermometer. State one precaution to ensure accurate results
(ii) State one advantage which a constant-volume gas thermometer has over other thermometers and one reason why it is seldom used as an everyday laboratory instrument.
(c) Using the kinetic theory of matter explain why evaporation causes cooling.
(a)(i) Temperature is a number which expresses how hot or cold a body is on scale such as centigrade or kelvin. S.I. unit is kelvin (K). Temperature decides which direction heat will flow while, heat is a form of energy S.I. is Joules (J).
(ii) Two physical properties used for measuring temperature are:- (1) Change in volume of liquid with temperature (2) Change in resistance. Others are: (3) Expansion of solids (4) Expansion of volume of gas.
(b)(i) Determination of the upper - fixed point for a mercury-in-glass thermometer.
To determine the upper fixed point, a hypsometer is used. The experiment is set up as shown above. The manometer is used to measure the pressure of the steam.
If it is not 76cm Hg, an adjustment is made to the upper fixed point. When the mercury level is constant, a mark is made on its glass to fix the point. One precaution that must be observed in this experiment is that the thermometer must not touch the water, because its temperature is affected by dissolved impurities in the water.
(ii) The constant - volume gas thermometer measures temperature more accurately than other thermometers because its glass expansion is negligible compared to the expansion of the gas. It is seldom used as everyday laboratory thermometer because it is not easily handled unlike the laboratory thermometer.
(c) In the kinetic theory, molecules are in random motion. They all have different velocities. Near the surface, some liquid molecules with high velocities in an upward direction may have sufficient kinetic energy to break through the surface. Then they become molecules of vapour. The average kinetic energy of the liquid remaining is therefore reduced, and so its temperature is lowered.
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