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The variation of vapour pressure of different liquids with temperature is shown in Fig.
(i) Calculate graphically boiling points of liquids $\mathrm{A}$ and B.
(ii) If we take liquid $\mathrm{C}$ in a closed vessel and heat it continuously. At what temperature will it boil?
(iii) At high altitude, atmospheric pressure is low (say $60 \mathrm{~mm} \mathrm{~Hg}$). At what temperature liquid $\mathrm{D}$ boils?
(iv) Pressure cooker is used for cooking food at hill station. Explain in terms of vapour pressure why is it so?
(i) Calculate graphically boiling points of liquids $\mathrm{A}$ and B.
(ii) If we take liquid $\mathrm{C}$ in a closed vessel and heat it continuously. At what temperature will it boil?
(iii) At high altitude, atmospheric pressure is low (say $60 \mathrm{~mm} \mathrm{~Hg}$). At what temperature liquid $\mathrm{D}$ boils?
(iv) Pressure cooker is used for cooking food at hill station. Explain in terms of vapour pressure why is it so?
Solution:
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(i) The boiling point of liquid $\mathrm{A}$ is: $315 \mathrm{~K}$ (approx) and the boiling point of $\mathrm{B}$ is: $345 \mathrm{~K}$ (approx.)
(ii) In the closed vessel the liquid $\mathrm{C}$ will not boil because the pressure is kept on increasing in the vessel.
(iii) According to the graph Temperature corresponding to $60 \mathrm{~mm}$ of $\mathrm{hg}=313 \mathrm{~K}$. So liquid $\mathrm{D}$ will boil at 313 $\mathrm{K}$.
So liquid $\mathrm{D}$ will boil at $313 \mathrm{~K}$.
(iv) Since the atmospheric pressure is low at high altitudes, water boils at low temperatures on hills. We already know that a liquid boils when its vapour pressure equals atmospheric pressure. Because a pressure cooker raises the boiling point of water and its heat content, food cooks faster.
(ii) In the closed vessel the liquid $\mathrm{C}$ will not boil because the pressure is kept on increasing in the vessel.
(iii) According to the graph Temperature corresponding to $60 \mathrm{~mm}$ of $\mathrm{hg}=313 \mathrm{~K}$. So liquid $\mathrm{D}$ will boil at 313 $\mathrm{K}$.
So liquid $\mathrm{D}$ will boil at $313 \mathrm{~K}$.
(iv) Since the atmospheric pressure is low at high altitudes, water boils at low temperatures on hills. We already know that a liquid boils when its vapour pressure equals atmospheric pressure. Because a pressure cooker raises the boiling point of water and its heat content, food cooks faster.
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