The variation of vapour pressure of different liquids with temperature is shown in Fig. 5.6. (i) Calculate graphically boiling points of liquids A and B. (ii) If we take liquid C in a closed vessel and heat it continuously. At what temperature will it boil? (iii) At high altitude, atmospheric pressure is low (say 60 mm Hg). At what temperature liquid D boils? Pressure cooker is used for cooking food at hill station. Explain in terms of vapour pressure why is it so? (iv) Pressure cooker is used for cooking food at hill station. Explain in terms of vapour pressure why is it so?

Question

Philoid · Accepted Answer

i) Boiling point of liquid A is: 315 K (approx.) and the boiling point of B is: 345 K (approx.)

ii) In the closed vessel the liquid C will not boil because the pressure is kept on increasing in the vessel because the vessel is closed and the vapours generated by heating of liquid C keeps on increasing due to which there is no external atmospheric pressure which is needed to be exceeded to boil off the liquid.

iii) Since at high altitude, atmospheric pressure is low say = 60mm of hg (given) so as according to the graph Temperature corresponding to 60mm of hg = 313 K. So liquid D will boil at 313 K.

iv) At hill stations, the normal atmospheric pressure is less than the pressure at plains or at sea level. Due to this reason it takes longer time to cook food on hill stations. So, with the use of pressure cooker, the pressure of water is increased due to which water boils at even low temperature within a short period of time and the food doesn’t take long time to get cooked.