Class 9 EXTRA QUESTIONS
GIVE REASONS:
1. The temperature remains constant during melting or during conversion of solid into liquid.
Ans: the heat energy is actually used up in overcoming the forces of attraction between the particles of a solid during the change of state from solid to liquid without increasing their kinetic energy.
- Since the kinetic energy of the particles does not increase, the temperature of the substance does not rise during the change of state. Heat is hidden within the particles.
2. Give reasons:
A. We sprinkle water on the roof or open ground in summer:
During summer, water evaporates by absorbing heat from the ground and the surrounding air. By losing heat, the ground becomes cool and people feel comfortable.
B. we see water droplets on the outer surface of a glass tumbler containing ice cold water. - refer book
C. We wear cotton clothes in summers. - refer book
3. What will happen if acetone is poured on your palm?
When acetone (nail polish remover) is poured on my palm I feel cool, because the energy needed for evaporation is taken from the palm. By losing heat, palm feels cooling.
4. Why do surgeons often spray some ether on skin before performing minor surgery?
Ether has low boiling point (308K) so it evaporates rapidly. The heat energy needed for evaporation is taken from the skin. As a result, the temperature of the skin becomes so low that it almost becomes numb. Due to this numbness, the patient doesn't feel much pain when a minor cut is made in the skin in order to perform surgery.
5. Why is ice rubbed on a burnt part of the skin?
Due to burning, the temperature of the injured skin increases. When ice is rubbed, the excess heat from the skin is taken by large latent heat of fusion of water. As a result, the temperature of the injured skin decreases and we feel less pain.
6. The states of matter are inter convertible. Discuss.
This inter convention can be carried out either by changing temperature or pressure or both.
For e.g.
Ice (solid) on heating becomes water (liquid)
Liquid on heating changes to gas.
A gas (LPG) on applying pressure, changes to liquid.
Carbon dioxide gas can be. Converted into dry ice (solid) by cooling under pressure.
7. Pressure and temperature determine the state of matter.
If at a given temperature and pressure, a particular substance is a solid, then on heating, it can be converted first into liquid and then into vapours. Conversely, vapours on cooling first give liquid and then solids.
If,however, at a particular temperature and pressure, a substance is a gas, then on cooling or by compressing or by both , the inter particle distance can be decreased and inter particle force of attraction can be increased.
As a result, the gas gets converted into first into a liquid and finally into a solid.
Thus, pressure and temperature determine the state of matter.
8. Define latent heat of fusion and latent heat of vapourisation.
9. Why does water become cold if we add some ice cubes to it?
When ice melts, it needs energy equal to latent heat of fusion which it takes from water. As a result, water becomes cold.
10. Two cubes of ice are pressed hard between the palms, when the pressure is released, the two ice cubes joined together. Explain.
When pressure is applied on ice cubes, the freezing point of ice decreases. As a result, some of the ice in between the cubes mets. The energy (latent heat of fusion) needed for melting is taken from the surroundings, however, when pressure is released, energy equal to latent heat of fusion is given out and the water in between the two cubes again freezes to join the two cubes together.
11. A student spilled a bottle of ammonia in one corner of the lab. Soon the whole lab is filled with pungent irritating smell. The students opened the windows and doors. After sometime, students got relief. Explain what did happen?
Due to random motion, the particles of ammonia gas readily moved into the spaces between the particles of air present in the lab.
As a result of this diffusion, the entire lab was filled with ammonia.
When the doors were opened, fresh air was drawn into the lab. This slowly helped to diffuse ammonia out of the lab.
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