Unit 7 Properties Of Bulk Matter
Chapter 8 Change Of State Of Matter Long Answer Type Questions
Question 1. A solid is heated at a constant rate. Temperature of the specimen changes. Study the graph and answer the following questions.
- What do the horizontal parts, AB and CD In the graph, represent?
- If CD = 2AB, what inference can be drawn?
- What does the slope of DE indicate?
- The slope of OA is more than the slope of BC. What is the inference?
Answer:
Given
A solid is heated at a constant rate. Temperature of the specimen changes.
1. Horizontal parts AB and CD represent respective quantities of heat absorbed for the changes of state at respective constant temperatures.
AB represents melting and CD represents the vaporisation of the substance.
2. CD = 2AB indicates that, latent heat of vaporisation is twice the latent heat of fusion, for the material in question.
3. From the slope of DE, the reciprocal of thermal capac¬ity of the specimen in vapour state can be calculated.
4. The slope of OA is greater than that of BC. It shows that the specific heat in liquid state is more than that in solid state.
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Question 2. Why Is ice at 0°C more effective in cooling of a body than an equal amount of water at 0°C?
Answer:
Ice Is at 0°C more effective in cooling of a body than an equal amount of water at 0°C
When 80 cal of heat is extracted from 1 g of water at 0°C, it changes to 1 g of ice at 0°C. Hence, 1 g ice can absorb 80 cal more heat than 1 g water at 0°C. Thus, to cool a body, ice at 0°C is a more effective than an equal amount of water at 0°C.
Long Answer Questions on Change of State of Matter
Question 3. Water at 100 C and Meant of the same amount at 100 C— which one can release more heat? Which will cause more severe burns?
Answer:
Water at 100 C and Meant of the same amount at 100 C
An addition of 540 cal of heat to 1 g of water at J00°C produces steam at 100°C and therefore, the steam tan release more heat than water at 100°C. Thus, steam produces more severe burns than water of the same mass at 100°C.
Question 4. Some water is kept In a hole drilled into a melting Ice block. Will the water freeze to ice?
Answer:
Some water is kept In a hole drilled into a melting Ice block.
Water will not freeze to ice. The melting ice block is at 0°C. Water, if initially at a higher temperature, gives heat to the ice block at the hole, melting more ice.
Once water reaches 0°C, thermal equilibrium is established between the block and water and so heat exchange stops. The water can not transform into ice as it does not release any latent heat.
Question 5. State whether water in a beaker can be made to boil with a flow of steam at normal pressure.
Answer:
Water in the beaker will not boil. Steam is at 100°C under normal pressure. Steam condenses to water by delivering the latent heat of condensation and the water in the beaker absorbs it to rise up to 100°C.
Heat exchange stops at this stage. Hence, water in the beaker cannot take any latent heat of vaporisation from the steam. So, water will remain at 100°C but will not boil.
Latent Heat: Fusion and Vaporization Explained
Question 6. An astronaut carried some water at 20°C in a thermoflask and on the moon he poured it into a beaker. Comment on what happens to the water.
Answer:
An astronaut carried some water at 20°C in a thermoflask and on the moon he poured it into a beaker.
Moon has no atmosphere and hence no air pressure acts there. With the lowering of pressure on a liquid surface, its boiling point decreases and the rate of vaporisation becomes very high.
Thus, water from the flask at 20°C starts boiling and vaporises very fast collecting the necessary latent heat from the rest of the mass. Hence, the temperature of the rest of the water decreases and ultimately it changes to ice in the beaker.
Question 7. Why is a cooling effect produced due to evaporation?
Answer:
Cooling effect produced due to evaporation because
When a liquid changes to its vapour state it requires latent heat. If no heat is supplied from outside then the liquid takes the required latent heat from its own body and from its surroundings. As a result, the liquid and its surroundings become cooler. So, a cooling effect is produced due to evaporation.
Question 8. Melting point of ice decreases on increasing the pressure on ice, but melting point of wax increases with the Increase in pressure. Explain with reasons.
Answer:
Melting point of ice decreases on increasing the pressure on ice, but melting point of wax increases with the Increase in pressure.
For solids which contract on melting, like ice, the melting point decreases on increase of the pressure on it (as increased pressure helps contraction). But for solids which expand on melting, like wax, an increase in pressure opposes the expansion and the melting point increases.
Question 9. Why does it take comparatively more time to completely convert some water to its vapour state than to attain boiling point from 0°C, by supplying heat continuously at the same rate?
Answer:
To convert 1 g of water from 0°C to 100°C, 100 cal of heat is required. But 1 g of water at boiling point takes 537 cal to change to its vapour state. So, time taken to change the state of 1 g of water at 100°C to its vapour form is 5.37 times the time taken to change the temperature of lg of water from 0°C to 100°C.
Question 10. Water boils at a lower temperature in vacuum and its temperature decreases during boiling. Why?
Answer:
Water boils at a lower temperature in vacuum and its temperature decreases during boiling.
We know that a decrease in the pressure on a liquid results in a decrease in its boiling point. So, water boils at a lower temperature in vacuum. If no heat is supplied externally then water takes the latent heat from the remaining water. That is why the temperature of boiling water decreases.
Applications of Phase Changes in Real Life
Question 11. What will happen if we pour a few drops of ether on the bulb of a thermometer? Also, what will be the consequence if the bulb of a thermometer is kept completely immersed in a bottle of ether?
Answer:
As ether is volatile, the drops of ether will vaporise fast on contact with air. During vaporisation, it will take the necessary latent heat from the bulb of the thermometer.So, the reading of the thermometer will drop.
- However, when the bulb is kept completely immersed in a bottle of ether, it does not come in contact with air.
- So, the ether in contact with the bulb does not evaporate, nor is any latent heat absorbed from the thermometer so the reading does not drop. The reading of the thermometer, in this case, gives the temperature of the ether in the bottle.
Question 12. What is the function of the energy supplied by latent heat of fusion or of vaporisation?
Answer:
Change of a solid to its liquid state involves breaking of the crystal structure of the solid. Latent heat of fusion provides the energy for this. Similarly, to change a liquid to its vapour state, intermolecular separations need to be so increased that there remains practically no attraction between the molecules the latent heat of vaporisation supplies the required energy for this.
Question 13. Why is a fuse wire in electrical lines made up of an alloy and not of pure metals?
Answer:
Melting point of an alloy is always less than the melting point of each of the constituent pure metals. So, a fuse wire made of an alloy can melt before the current in the circuit reaches a danger level and hence can break the circuit. Thus the other parts of the circuit remains safe.
Question 14. More work has to be done to vaporise a substance than to melt it. Explain.
Answer:
More work has to be done to vaporise a substance than to melt it.
In melting, the crystal structure of the molecules of a solid breaks down. For this, in most cases, the intermolecular separation changes very little. So, the work done against the intermolecular force is comparatively low. During vaporisation, the intermolecular separation increases by a lot more. So, more work has to be done against the intermolecular force.
Question 15. Boiling point of water at Darjeeling is usually less than 100°C. Explain why?
Answer:
Boiling point of water at Darjeeling is usually less than 100°C.
Air pressure decreases as altitude increases. Lower pressure on a liquid lowers its boiling point. Darjeeling being at a higher altitude, has a low air pressure and therefore water has a lower boiling point; so water begins to boil at a temperature below 100°C.
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Question 16. Water boils at nearly 120°C in a pressure cooker. Why?
Answer:
Water boils at nearly 120°C in a pressure cooker.
Boiling point increases with the increase in pressure on a liquid. Pressure on a liquid in a pressure cooker rises to about 2 atm and so the boiling point of water at that pressure is nearly 120°C.
Question 17. At 100°C, during boiling, what is the specific heat of water?
Answer:
Heat gained during boiling (H) = latent heat of vaporisation
Rise in temperature t = 0
Now, H = mst
So, specific heat, s = \(\frac{H}{mt}\).
According to this equation, the value of specific heat is infinite.
Question 18. Why does a liquid evaporate even at a temperature below its boiling point?
Answer:
The air around any liquid has a tendency to become saturated with the vapour of the liquid. A certain mass of air has a capacity of holding a certain maximum amount of the vapour.
- When that maximum amount is present, air is said to be saturated with that vapour. But in most cases, the air surrounding a liquid is unsaturated. So, in order to saturate the surrounding air, molecules on the surface of a liquid continuously transform into vapour molecules that mix with air, whatever the temperature of the liquid be.
- This is the principle of evaporation of a liquid at all temperatures. However, when air attains saturation or near saturation, the rate of evaporation becomes negligibly small.
- That is why wet clothes do not dry up easily in rainy season. When the temperature of air becomes very low in winter, the atmosphere often becomes supersaturated with water vapour. Then the vapour turns back into water particles, thus forming dews and fogs.
Detailed Explanation of Phase Changes in Matter
Question 19. Sometimes why does a layer of ice stick to the heel of j a shoe during walking on ice?
Answer:
Under the pressure of the heel of a shoe on ice, its melting point decreases and ice melts. Now on raising the shoe above, the pressure on ice diminishes and its melting point increases. So water again freezes into ice which stick to ihe heel of the shoe.
Question 20. Liquid oxygen at 50 K is heated to 300 K at a constant pressure of 1 atm. The rate of heating is constant. Which of the following graphs represents the variation of temperature with time?
Answer:
Given
Liquid oxygen will undergo a change of state when heated from 50K to 300K. During the change of state, the temperature will not change until all the liquid aw gen has changed to gaseous state.
After that, the temperature will increase because the rate of heating is constant. Hence the correct graph is (c).
Examples of Phase Changes with Explanations
Question 21. The melting point of ice in vacuum is 0.0073°C What do you mean by the statement?
Answer:
Given
The melting point of ice in vacuum is 0.0073°C
Melting point of ice reduces due to application of pressure on it. It has been found that the melting point of ice reduces by 0.0073°C for one-atmosphere increase of pres¬sure. Conversely, if the pressure is decreased by one atmosphere, i.e., if the pressure is nil. then ice will melt at 0.0073°C instead of at 0°C. So it can be said that the melting point of ice in vacuum will be 0.0073°C.