Question Answers

Thermal Expansion

1. Explain the significance of anomalous expansion of water with an example observed in nature.

Generally substance expands on heating but water shows peculiar behavior on heating. When water is heated from 0° C, its volume decreases until its temperature reaches to 4° C but above 4° C, its volume increases. This behavior shown by water is known as anomalous expansion of water.

This signifies that water at temperature 4°C has highest density. During winter, when the temperature of water falls, water at 4° C remains at the bottom. And ice having density less density floats on the surface. This makes the survival of aquatic animals possible even at low temperature of the surrounding.

2. Frozen water pipes often burst. Will an alcohol thermometer break if the temperature drops below the freezing point of alcohol?

No, alcohol thermometer will not break. Frozen water pipes often burst due to anomalous expansion of water with decrease in temperature below 4°C. Due to expansion an outward pressure is exerted on the wall of the pipe which burst the pipe. But in case of alcohol, its volume decreases with decrease in temperature below the freezing point. Hence, it will not exert any outward pressure on the capillary of thermometer. So, it will not break.

3. A hole is drilled in a flat metal sheet. What happens to the diameter of the hole as the metal sheet is heated to higher temperature?

When a flat metal sheet with a hole drilled in it is heated to higher temperature, the metal expands. Each segment of the metal expands lengthwise as well as thickness wise. Since the length increases more and more with rise in temperature, it makes the increment of the diameter of the hole.

4. Does the cubical expansivity of a liquid depend on its original volume? Explain.

No, the cubical expansivity of a liquid does not depend on its original volume. It is the proportionality constant that depends on the nature of the material.

5. When a metallic block with hole in it is heated, why does not the material around the hole expand into hole and make it smaller?

When a metal block with a hole drilled in it is heated to higher temperature, the metal expands. Each segment of the metal expands lengthwise as well as thicknesswise. Since the length increases more and more with rise in temperature, it makes the increment of the diameter of the hole. Thus, the hole expands and does not make it smaller.

6. Why are glass windows possible to be cracked in very cold region?

In the cold region, the temperature outside the room is very low as compared to the temperature inside the room. Due to low thermal conductivity of glass, there is unequal expansion of the outer and inner sides of the glass. When the temperature difference is very high, the glass may break.

7. Why does a thick glass tumbler crack when boiling water is poured on it?

When boiling water is poured it thick glass tumbler, the inner surface expands. Due to low thermal conductivity of glass, the expansion of the outer surface is very small. Thus there is unequal expansion of outer and inner surface of the tumbler and hence it breaks.

8. Why a column of mercury in a thermometer first descends slightly and then rises when placed in hot water? Explain

When a mercury thermometer is placed in hot water, at first, its bulb gets heated and expands. So, the mercury level descends slightly to fill up the increased volume of the bulb. Later on, heat is transferred to the mercury and it expands. Then, the mercury level rises. Since the expansivity of mercury is more than glass, the later mercury level rise is more than the initial descend of mercury level.

9. Fishes stay alive in frozen pond in winter. Explain.

During winter, the temperature of water in pond decreases. The water layer at 4°C sinks to the bottom due to high density and the water layer below 4°C rises up due to lower density. So, ice layer formed at 0°C lies at the top. Since, ice is a bad conductor of heat; it does not allow heat energy to escape out. Thus, water below lies unfrozen and hence fishes and aquatic animals stay alive.

10. Water level initially falls in a vessel when it is heated. Why?

When the vessel with water inside it is heated, at first, the vessel expands. So, the water level falls slightly to fill up the increased volume of the vessel. Later on, heat is transferred to the water and it expands. Then, the water level rises.

11. A few gap is left between two iron rails. Why?

In summer, the iron rails expand due to rise in temperature. Also, the heat produced due to friction with wheels of train causes expansion of iron rails. If no gap is left, the expansion will cause the rails to buckle. Hence, gap is left between two iron rails to allow space for expansion of the rails.