question_answer 1)

  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? (iv) Pressure cooker is used for cooking food at hill station. Explain in terms of vapour pressure why is it so?  

Answer:

  (i) The temperature at which the vapour pressure of a liquid becomes equal to atmospheric pressure (760 mm Hg) is called its boiling point. Then, boiling point of liquid (A) and (B) will be respectively 315 K and 345 K. (ii) Liquid will not boiled in closed vessel. (iii) At 313 K, vapour pressure of liquid (D) will be atmospheric pressure (60 mm Hg) hence it will boil at 313 K. (iv) On high altitude, the atmospheric pressure is low hence water boils at low temperature on hills. We know that, a liquid boils when its vapour pressure becomes equal to the atmospheric pressure. Pressure cooker increases the boiling point of water and its heat content hence food is cooked early.