the increase in pressure) before boiling begins again. Thus boiling point increases with increase in pressure. Let us now remove the burner. Allow water to cool to about ° C.
Remove the thermometer and steam outlet. Close the flask with the airtight cork. Keep the flask turned upside down on the stand. Pour ice-cold water on the flask.
Water vapours in the flask condense reducing the pressure on the water surface inside the flask. Water begins to boil again, now at a lower temperature. Thus boiling point decreases with decrease in pressure. This explains why cooking is difficult on hills.
At high altitudes, atmospheric pressure is lower, reducing the boiling point of water as compared to that at sea level. On the other hand, boiling point is increased inside a pressure cooker by increasing the pressure. Hence cooking is faster. The boiling point of a substance at standard atmospheric pressure is called its normal boiling point .
However, all substances do not pass through the three states: solid-liquid-gas. There are certain substances which normally pass from the solid to the vapour state directly and vice versa. The change from solid state to vapour state without passing through the liquid state is called sublimation , and the substance is said to sublime. Dry ice (solid CO ) sublimes, so also iodine.
During the sublimation process both the solid and vapour states of a substance coexist in thermal equilibrium. . . Latent Heat In Section .
, we have learnt that certain amount of heat energy is transferred between a substance and its surroundings when it undergoes a change of state. The amount of heat per unit mass transferred during change of state of the substance is called latent heat of the substance for the process. For example, if heat is added to a given quantity of ice at – ° C, the temperature of ice increases until it reaches its melting point ( ° C). At this temperature, the addition of more heat does not increase the temperature but causes the ice to melt, or changes its state.
Once the entire