ICSE Class 8 Physics Chapter 04 More About Solids Liquids and Gases

More About Solids, Liquids and Gases

All matter is made up of tiny particles, which may be molecules or atoms (metals, for example, are made up of atoms). For the sake of simplicity, we will call the particles of matter 'molecules' in this chapter. The characteristics of a substance are determined by the type of molecules it is made up of. For example, a substance made up of large (heavy) molecules is heavier (has a greater density) than one made up of smaller molecules.

Molecules of matter exert attractive forces on each other. This means any two molecules of a substance that are close to each other attract each other with some force. This intermolecular force is called cohesive force in the case of molecules of the same substance. To 'cohere' means to stick together. The tendency of molecules to stick together due to the attractive force between them is called cohesion.

General Properties

The general properties of solids, liquids and gases and the temperatures at which they melt, boil, etc., depend largely on the cohesive forces between their molecules.

Solids

The cohesive force that binds the molecules of a solid is very strong. Hence, the molecules can neither move away from each other nor change their positions relative to each other. This is why the shape and size (volume) of a solid do not change easily. Solids are said to be rigid, which means they retain their shape even when a force is applied. This is particularly useful in the construction of buildings, bridges, and so on.

The molecules of matter are in constant motion. In a solid, the molecules only vibrate about the same position, somewhat the way we jog in place. When a solid is heated, the molecules vibrate with greater energy, which shows as a rise in temperature. The molecules also gain enough energy to move apart a little, which results in expansion.

The more a solid is heated, the more energetically its molecules move. In other words, the more its temperature rises. Then there comes a point when the molecules become energetic enough to change their positions relative to each other. That is when a solid melts. The temperature at which this occurs depends on the strength of the cohesive force that binds the molecules of a solid. In general, the cohesive force that binds a metal is stronger than that which binds a nonmetal, so metals have a higher melting point than nonmetals.

Liquids

The cohesive force between the molecules of a liquid is not as strong as that between the molecules of a solid. Hence, the molecules can change their relative positions, though they cannot move away from each other. This is why a liquid can flow and change its shape, though its volume remains the same. The volume of a liquid does not change even when it is under pressure because its molecules cannot move closer together. This is why a liquid is incompressible.

The fact that the volume of a liquid does not change under pressure means that its density also does not change with pressure. However, liquids expand considerably when they are heated. Hence, the density of a liquid decreases when it is heated. This, as you may remember, helps to set up convection currents in a liquid. The warmer, lighter part of a liquid rises, while the colder, denser part of a liquid sinks.

Gases

The cohesive force between the molecules of a gas is almost negligible. Hence, the molecules are free to move as they please. In other words, the distances between them as well as their relative positions can change. This means the volume as well as the shape of a gas can change. The molecules of a gas move freely and fill up any space (volume) that is available to them. They can also be compressed into a small space. Thus, only the mass of a gas remains constant. Its volume and density depend on its container. Its pressure too depends on the size of its container, as we will discuss in the section on pressure.

Surface of a Liquid

You have learnt that the molecules of a liquid attract each other with a force called cohesive force. This makes the surface of a liquid behave in an interesting manner. Any molecule inside a liquid is attracted equally on all sides by neighbouring molecules, so the net force on it is zero. In other words, molecules inside a liquid are not pulled in any particular direction. Molecules on the surface of a liquid, however, experience a net downwards pull, since there are no molecules pulling them upwards (Figure 4.4). This makes the surface of a liquid tend to contract in a manner similar to a stretched rubber sheet. Just as a stretched rubber sheet is in a state of tension, we assume that the surface of a liquid is in a state of tension. This tension acting on the surface of a liquid is called surface tension. It is defined as the force acting per unit length of an imaginary line on a liquid surface. Its SI unit is newton per metre. As an example, the surface tension of water is about 0.07 newton per metre.

Smear a sheet of glass or any glass surface with Vaseline. Use a dropper or toothbrush to put small drops of water on the surface. You will see that the water forms spherical drops instead of spreading out. The smaller the drops, the more spherical they will be. The reason why this happens is surface tension. Surface tension makes a small amount of liquid contract or draw itself into a small ball.

The curved surface of a liquid in a container is called a meniscus (a Greek word that means crescent). It forms because the attraction (adhesive force) between the molecules of the liquid and the molecules of the container pulls up the part of the liquid surface that is in contact with the container.

Meniscus

So far we have discussed only cohesive force, or the force of attraction between the molecules of a liquid. However, this is not the only force experienced by the molecules of a liquid. They are also attracted by the molecules of the container (or any surface) they are in contact with. This force, which acts between the molecules of a liquid and those of the surface it is in contact with, is called adhesive force. 'Adhesion' means sticking. Adhesive force makes a liquid stick to the surface, or wet the surface, it is in contact with. You may have noticed that water sticks to any surface it comes into contact with, including your skin.

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