In ionic compounds, there is electrostatic forces between oppositely charged ions. Covalent bonds are present in polar as well as non-polar molecules. Polar molecules have dipole-dipole attraction. Polar molecules with non-polar molecules have dipole-induced dipole attraction, non-polar molecules have induced dipole-induced dipole, dispersion forces (London forces). H-bonds are present in some polar molecules. Ion-dipole attraction exist between ions and polar molecules intermolecular forces are negligible in gaseous state with large intermolecular space. Interdependence of some observable properties pressure, volume, temperature and mass lead to different gas laws, Boyle's law, Charle's law, Gay-Lussac's law, Avogadro's law and Dalton's law of partial pressure. Ideal gas equation pV = nRT is followed by ideal gases. At high pressure and low temerature intermolecular forces start operatin~ strongly and molecules come close to each other. They show positive and negative deviation from ideal gas behaviour and follow van der Waals' equation. Gases can be liquefied at low temperature and high pressure. Liquids have more intermolecular forces of attraction. Some properties of liquids, e.g., surface tension and viscosity are due to strong attractive forces.
Molecules of gases are in constant random motion. Their velocity can be expressed as average velocity, root mean square velocity and most portable velocity.
(a) Under what conditions real gases show ideal gas behaviour?
(b) What type of forces of attraction exist between NO₃^- and water?
(c) How are different types of velocities related to molar mass?
(d) A gas at 350 K and 15 bar has molar volume 20% less than that for an ideal gas under the same conditions. What is value of 'Z'?
(e) Why is HF liquid, HCI, HBr, HI are gases?
(f) How is pressure of dry gas calculated?
(g) What is Boyle's temperature?

Observe the graph A, B and C and answer the questions that follow based on the graph and related studied concepts.

(a) Which law is depicted by graph A?
(b) Give relationship between T₁,T₂ and T₃ .
(c) Which law is depicted by graph B?
(d) Give mathematical formula of Charle's law
(e) Why are straight lines in graph C called isochore?
(f) What is relationship between V₁, V₂, V₃ and V₄ ?
(g) What is mathematical formula for Gay Lussac's law.

Table shows critical temperatures of some substances. Study the table and answer the questions that follow:
Critical Constants for Some Substances

(a) Why is Helium most difficult to liquefy?
(b) Out of CO₂ and NH₃ which will liquefy first?
(c) Critical temperature of CH₄ and CO₂ are -81.9°C and 31.1°C respectively. Which of these have stronger intermolecular forces of attraction?
(d) What is critical temperature?
(e) What is critical volume? How is it related to 'b' van der Waals' constant?

The pressure exerted by vapours over the liquid is called vapour pressure of liquid in a closed container. When liquid and vapour phase are in equilibrium, the vapour pressure of this stage is known as equilibrium vapour pressure or saturated vapour pressure. Vapour pressure depends upon temperature. Boiling point is temperature at which vapour pressure of liquid becomes equal to atmospheric pressure.
(a) Why does water boil below 100°C at hill stations?
(b) Why does vapour pressure of sea water is lower than distilled water?
(c) Two liqurds 'X' and 'V' boil at 110°Cand 130°C respectively. Which of them has higher vapour pressure at 50°C?
(d) What is boiling point of water inside the pressure cooker?
(e) How is vapour pressure related to temperature?