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47
MCQ (Single Correct Answer)

Assertion (A) Gases do not liquefy above their critical temperature, even on applying high pressure.

Reason (R) Above critical temperature, the molecular speed is high and intermolecular attractions cannot hold the molecules together because they escape because of high speed.

A
Both $A$ and $R$ are true and $R$ is the correct explanation of $A$
B
Both $A$ and $R$ are true but $R$ is not the correct explanation of $A$
C
$A$ is true but $R$ is false
D
$A$ is false but $R$ is true
48
MCQ (Single Correct Answer)

Assertion (A) At critical temperature liquid passes into gaseous state imperceptibly and continuously.

Reason (R) The density of liquid and gaseous phase is equal to critical temperature.

A
Both $A$ and $R$ are true and $R$ is the correct explanation of $A$
B
Both $A$ and $R$ are true but $R$ is not the correct explanation of $A$
C
$A$ is true but $R$ is false
D
$A$ is false but $R$ is true
49
MCQ (Single Correct Answer)

Assertion (A) Liquids tend to have maximum number of molecules at their surface.

Reason (R) Small liquid drops have spherical shape.

A
Both $A$ and $R$ are true and $R$ is the correct explanation of $A$
B
Both $A$ and $R$ are true but $R$ is not the correct explanation of $A$
C
$A$ is true but $R$ is false
D
$A$ is false but $R$ is true
50
Subjective

Isotherms of carbon dioxide at various temperatures are represented in figure. Answer the following questions based on this figure.

(i) In which state will $\mathrm{CO}_2$ exist between the points $a$ and $b$ at temperature $T_1$ ?

(ii) At what point will $\mathrm{CO}_2$ start liquefying when temperature is $T_1$ ?

(iii) At what point will $\mathrm{CO}_2$ be completely liquefied when temperature is $T_2$ ?

(iv) Will condensation take place when the temperature is $T_3$ ?

(v) What portion of the isotherm at $T_1$ represent liquid and gaseous $\mathrm{CO}_2$ at equilibrium?

Explanation

(i) In gaseous state, $\mathrm{CO}_2$ will exist between the points $a$ and $b$ at temperature $T_1$.

(ii) At point $b$, the plot becomes linear, this shows the phase transition, i.e., liquification of $\mathrm{CO}_2$ starts and at point $c$, it gets completely liquified.

(iii) Similarly, at temperature $T_2, g$ is the point at which $\mathrm{CO}_2$ will be completely liquified.

(iv) Condensation will not take place at $T_3$ temperature because $T_3>T_{\mathrm{C}}$ (critical temperature). (v) Between $b$ and $c$, liquid and gaseous $\mathrm{CO}_2$ are in equilibrium.

51
Subjective

The variation of vapour pressure of different liquids with temperature is shown in figure

(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?

Explanation

(i) Boiling point of

$A=$ approximately 315 K ,

$B=$ approximately 345 K .

(ii) In a closed vessel, liquid $C$ will not boil because pressure inside keeps on increasing.

(iii) Temperature corresponding to $60 \mathrm{~mm}=313 \mathrm{~K}$.

(iv) A liquid boils when vapour pressure becomes equal to the atmospheric pressure. However at high altitudes i.e., on, hills, water boils at low temperature due to low atmospheric pressure. But when pressure cooker is used, the vapour pressure of water is increased due to which water boils at even lower temperature within a short period of time

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