The variation of vapour pressure of different liquids with temperature is shown in the figure.
$(i)$ Calculate graphically the 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 does liquid $D$ boil?
(iv) $A$ pressure cooker is used for cooking food at hill stations. Explain in terms of vapour pressure why this is so.

(N/A) $(i)$ Boiling point is the temperature at which vapour pressure equals atmospheric pressure $(760 \ mm \ Hg)$. From the graph,for liquid $A$,the boiling point is approximately $315 \ K$,and for liquid $B$,it is approximately $345 \ K$.
(ii) Liquid $C$ will not boil in a closed vessel because as the temperature increases,the vapour pressure increases,but the pressure inside the vessel also increases simultaneously,preventing the liquid from reaching its boiling point.
(iii) At $60 \ mm \ Hg$,by observing the graph for liquid $D$,the corresponding temperature is approximately $313 \ K$.
(iv) $A$ liquid boils when its vapour pressure equals the external atmospheric pressure. At high altitudes,atmospheric pressure is low,so water boils at a lower temperature,which is insufficient for cooking food properly. $A$ pressure cooker increases the internal pressure,which in turn increases the boiling point of water,allowing food to cook faster at a higher temperature.