PHYSICS [2 Mark Question Answer]

An iron rim, whose diameter is made slightly smaller than the wooden wheel, is heated. It expands on heating. The hot iron rim is then placed around the wooden wheel to fit easily. Finally, cold water is poured on the wheel, which causes the metal rim to contract and grip the wheel tightly.

The following activity shows that gases expand on heating. Materials required : A metal can, a balloon, a Bunsen burner and a tripod stand
Procedure : Fix the balloon around the neck of the empty metal can. Heat the can by using the burner. Observation : The balloon grows bigger in size as the air inside it expands on heating.
Conclusion : Gases expand on heatig

When a solid is heated, the kinetic energy of its particles increases and the particles start moving more vigorously. As a result, the distance between the particles increases and the substance expands. But, as the particles are closely packed and are held together firmly, so, solids expand very little on heating.

Evaporation occurs only at the surface of the liquid. This is because in a liquid, the molecules are in constant random motion. The speed of these molecules keep changing due to their collisions with other molecules. If molecules near the surface of the liquid gain sufficient energy, they escape and become vapour.

A glass tumbler cracks on heating. This happens because glass is a poor conductor of heat. Heating a thick glass tumbler results in the uneven expansion, the expansion of the inner surface being more than the outer one. As a result, a strain builds up in the glass due to which it cracks.

Railway tracks are made of steel, which expands in summer when temperature rises. If the rails are fixed tightly next to each other, leaving no space for expansion, they will bend outward, and may cause derailment of a train. Hence, small gaps are left between successive rail tracks to allow them to expand in summer without any change in their shapes.

In solids, the particles are closely packed and are held together firmly. So, solids expand very little on heating. In liquids, particles are loosely packed and have more free space. On heating, the distance between the particles increases. So, liquids expand more than solids. In gases, particles are free to move and the force of attraction between them is almost zero. So, gases expand the most and solids expand the least.

Evaporation is accompanied by cooling because the heat energy required for the surface molecules to escape is provided by the surface the liquid is in contact with.

A thick glass tumbler could crack when ice is placed in it. This happens because glass is a bad conductor of heat. This results in the uneven expansion, the contraction of the inner surface being more than the outer one. As a result, a strain builds up in the glass due to which it cracks.