Chemistry

ything that occupies space and has mass is called matter. Matter can exist in three physical states—solid, liquid, and gaseous. Chair and almond are forms of matter in the solid state. Cold drink is a liquid state of matter. Air is a gaseous state of matter.

Solids diffuse at a very slow rate. But, if the temperature of the solid is increased, then the rate of diffusion of the solid particles into air increases. This is due to an increase in the kinetic energy of solid particles. Hence, the smell of hot sizzling food reaches us even at a distance, but to get the smell from cold food we have to go close.

The ability of a diver to cut through water in a swimming pool shows that the particles of matter have intermolecular spaces. The intermolecular spaces in liquids is fair enough to let the diver pass through it.

The characteristics of particles of matter are: (i) Particles of matter have spaces between them. (ii) Particles of matter are continuously moving. (iii) Particles of mater attract each other.

The given substances in the increasing order of their densities can be represented as: Air < Exhaust from chimney < Cotton < Water < Honey < Chalk < Iron

a) The differences in the characteristics of states of matter are given in the following table. S. No. Solid state Liquid state Gaseous state 1. Definite shape and volume. No definite shape. Liquids attain the shape of the vessel in which they are kept. Gases have neither a definite shape nor a definite volume. 2. Incompressible Compressible to a small extent. Highly compressible 3. There is little space between the particles of a solid. These particles have a greater space between them. The space between gas particles is the greatest. 4. These particles attract each other very strongly. The force of attraction between liquid particles is less than solid particles. The force of attraction is least between gaseous particles. 5. Particles of solid cannot move freely. These particles move freely. Gaseous particles are in a continuous, random motion. (b) Rigidity can be expressed as the tendency of matter to resist a change in shape. Compressibility is the ability to be reduced to a lower volume when force is applied. Fluidity is the ability to flow. By filling a gas container we mean the attainment of shape of the container by gas. Shape defines a definite boundary. Kinetic energy is the energy possessed by a particle due to its motion. Density is mass per unit volume.

(a) There is little attraction between particles of gas. Thus, gas particles movefreely in all directions. Therefore, gas completely fills the vessel in which it is kept. (b) Particles of gas move randomly in all directions at high speed. As a result, theparticles hit each other and also hit the walls of the container with a force. Therefore, gas exerts pressure on the walls of the container. (c) A wooden table has a definite shape and volume. It is very rigid and cannot be compressed i.e., it has the characteristics of a solid. Hence, a wooden table should be called a solid. (d) Particles of air have large spaces between them. On the other hand, wood has little space between its particles. Also, it is rigid. For this reason, we can easily move our hands in air, but to do the same through a solid block of wood, we need a karate expert.

The mass per unit volume of a substance is called density (density = mass/volume). As the volume of a substance increases, its density decreases. Though ice is a solid, it has large number of empty spaces between its particles. These spaces are larger as compared to the spaces present between the particles of water. Thus, the volume of ice is greater than that of water. Hence, the density of ice is less than that of water. A substance with lower density than water can float on water. Therefore, ice floats on water.

(a) 300 K = (300 − 273)°C = 27°C (b)573 K = (573 − 273)°C = 300°C

a) Water at 250°C exists in gaseous state. (b) At 100°C, water can exist in both liquid and gaseous form. At this temperature, after getting the heat equal to the latent heat of vaporization, water starts changing from liquid state to gaseous state.

During a change of state, the temperature remains constant. This is because all the heat supplied to increase the temperature is utilised in changing the state by overcoming the forces of attraction between the particles. Therefore, this heat does not contribute in increasing the temperature of the substance.

By applying pressure and reducing the temperature, atmospheric gases can be liquefied.

When a liquid evaporates, the particles of the liquid absorb energy from the surroundings to compensate the loss of energy during evaporation. This makes the surroundings cool. In a desert cooler, the water inside it is made to evaporate. This leads to absorption of energy from the surroundings, thereby cooling the surroundings. Again, we know that evaporation depends on the amount of water vapour present in air (humidity). If the amount of water vapour present in air is less, then evaporation is more. On a hot dry day, the amount of water vapour present in air is less. Thus, water present inside the desert cooler evaporates more, thereby cooling the surroundings more. That is why a desert cooler cools better on a hot dry day.

There are some pores in an earthen pot through which the liquid inside the pot evaporates. This evaporation makes the water inside the pot cool. In this way, water kept in an earthen pot becomes cool during summers.

When we put some acetone or petrol or perfume on our palm, it evaporates. During evaporation, particles of the liquid absorb energy from the surrounding or the surface of the palm to compensate for the loss of energy, making the surroundings cool. Hence, our palm feels cold when we put some acetone or petrol or perfume on it.