NCERT Solutions for Class 9 Science Chapter 1

Substance undergo change in physical state because both inter-particle spaces and inter-particle forces can be changed by changing the condition of temperature and pressure.

This is because of the presence of inter particle space or empty spaces. Particles or molecules of water can fill the empty space in the particles or molecules of sugar and vice versa. That is why there is hardly any change in the volume as a result of dissolution of sugar in water.

A substance is said to be in the gaseous state if under normal pressure, its boiling point is below the room temperature.

Increase in pressure helps in the liquefication of a gas. The particles or molecules of a gas come closer and closer as the pressure is being increased gradually. They ultimately condense and as a result, the gas liquefies or changes into the liquid state.

In the solids, the particle are very closely packed. As a result, the number of particles per unit volume is quite large. Therefore, the solids are normally quite heavy. In the gases, the particles are loosely packed. The number of particle per unit volume is completely small. Therefore, gases are light.

Naphthalene shows the property of sublimation. Evaporation of naphthalene takes place easily and so it disappears during course of time without leaving a solid.

Perfumes vaporize very fast and its vapours diffuse into air easily. That is why we can smell perfume sitting several meters away.

Water at room temperature is a liquid because it has fluidity and has definite volume but no definite shape.

An iron almirah is a solid at room temperature because it is rigid and has a definite shape.

Ice at 273 K is less energetic than water. It is because of the difference in the latent heat of fusion which is present in water at the same temperature in the form of extra energy.

Steam produces more severe burns than boiling water. This is because steam has more energy than boiling water, present in it in the form of latent heat of vaporization.