Assertion (A) Liquids tend to have maximum number of molecules at their surface.
Reason (R) Small liquid drops have spherical shape.
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?
(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.
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?
(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
Why does the boundary between liquid phase and gaseous phase disappear on heating a liquid upto critical temperature in a closed vessel? In this situation what will be the state of the substance?
In a closed vessel, it is essential to know that below the critical point (i.e., critical temperature and critical pressure), the surface of separation between the liquid and its vapour is clearly visible. As we approach towards the critical point, the density of the liquid decreases while that of the vapour increases due to compression.
At the critical point, the densities of the liquid and that of the vapour become equal and the surface of separation disappears i.e., the liquid and the gaseous state become not separable. In other words, the meniscus is no longer visible. The fluid which is now a homogeneous mixture is called supercritical fluid. Hence, any fluid above its critical temperature and pressure is called a supercritical fluid. These supercritical fluids dissolve many organic substances. They are used for quick separation of a mixture into its components. e.g., $\mathrm{CO}_2$ above $31.1^{\circ} \mathrm{C}$ and above 600 bar pressure has a density of about $1 \mathrm{~g} / \mathrm{cm}^3$. It is used to dissolve out caffeine from coffee beans as it is a better substitute than chlorofluorocarbons which are harmful for the environment.
Why does sharp glass edge become smooth on heating it upto its melting point in a flame? Explain which property of liquids is responsible for this phenomenon.
Sharp glass edges are heated to make them smooth. Because on heating glass melts and the surface of the liquid tends to take the rounded shape at the edges which has minimum surface area. This is called fire polishing of glass.