question_answer

  (a) Why do solids have fixed shape and fixed volume? (b) Why is air dense at the sea level? (c) On melting of ice, there is decrease involume instead of increase. Why? (d) What is the binding force between molecules if a substance is a gas under ordinary conditions of temperature and pressure? (e) Why are average kinetic energies of hydrogen, carbon dioxide and ethane the same at the same temperature? Or (a) When we light an incense stick (agarbatti) in a corner of our room, why does its fragrance spread in the whole room quickly? (b) Liquid A has higher vapour pressure than liquid B. Which liquid out of A and B will have lower boiling point? (c) Name the five states of matter, which the scientists are now talking of? (d) What do you understand by plasma? (e) Name the Indian physicist, who worked for a fifth state of matter.

Answer:

  (a) In solids, the particles are closely packed and their positions are fixed due to strong forces of attraction existing between them. So, solids have fixed shape. Since, the space between the particles are also fixed, the solids have fixed volume.                                                                                                                                                    (1)                 (b) Air at sea level is compressed by the mass of air above it. Hence, air is more dense at sea level than atan altitude.                                                                                                                                                                                               (1)                 (c) There are empty spaces in the packing of water molecules in ice. On heating, different strings of hydrogen bonded water molecules break and water molecules come closer to each other. Hence, the volume decreases on melting of ice.                                                                                                                                                   (1)                 (d) The binding force between the molecules of a gas is van der Waals' force.                                                     (1)                 (e) Average kinetic energy of any gas is directly proportional to its absolute temperature. Since, all the gases are at the same temperature, their average kinetic energies will also be the same.                                                       (1) Or                 (a) The fragrance of 'agarbatti' spreads all around due to diffusion of its smoke into the air. The particles of gas produced on burning of incense stick move rapidly inall the directions. They mix with air and reach the whole room very quickly.                                                                                                                                                       (1)                 (b) The boiling point of a liquid is the temperature at which its vapour pressure becomes equal to the atmospheric pressure. The vapour pressure of liquid A is higher than liquid B. So, vapour pressure of A will become equal to atmospheric pressure at alower temperature than liquid B. Consequently boiling point of liquid A is lower than that of liquid B.                                                                                                                           (1)                 (c) The five states of matter are solid, liquid, gas, plasma and Bose-Einstein condensate.                                               (1)                 (d) Plasma is the state, which consists of super energetic and super excited particles in the form of ionised gases. The non-sign bulbs and the fluorescent tube consist of plasma. The plasma glows with a special colour depending on the nature of the gas, when electric energy flows through it. The sun and stars glow due to the presence of plasma in them at very high temperature.                                                                                                            (1)                 (e) In 1920, Indian physicist Satyendra Nath Bose did some calculations for the fifth state of matter.        (1)