WBBSE Solutions For Class 9 Physical Science Chapter 6 Saturated Unsaturated Vapour And Anomalous Expansion Of Water

Chapter 6 Saturated Unsaturated Vapour And Anomalous Expansion Of Water Synopsis

If the vapour pressure in a closed space at a particular temperature is equal to the highest possible vapour pressure at that temperature, then that vapour is called
saturated vapour.

1. If the vapour pressure in a closed space at a particular temperature is less than the highest possible vapour pressure at that temperature, then that vapour is called
   unsaturated vapour.
2. Saturated vapour remains in equilibrium when in contact with the liquid whereas unsaturated vapour does not remain in equilibrium when in contact with the liquid.
3. Dew point is that temperature at which a certain volume of air is saturated with the water vapour present in it.
4. If the temperature of the atmosphere goes below the dew point, then some amount of water vapour of the atmosphere condenses as tiny water particles to accumulate on grass, leaves etc. These tiny particles are known as dew.
5. If the temperature of a large part of air drops below the dew point, water vapour present in the atmosphere condenses as deposits on dust, coal particles etc. floating in air. A thick layer thus formed is called fog.
6. Relative humidity is the ratio of the mass of water vapour present in a particular volume of air to the mass of water vapour required to saturate that volume at a particular temperature.
7. In other words, relative humidity is the ratio of partial pressure of water vapour to the saturated pressure of water vapour at a particular temperature.
8. ∴ relative humidity = \(\frac{m}{M} \times 100 \%\) = \(\frac{f}{F} \times 100 \%\)

where m is the mass of water vapour present in a particular volume of air at t°C, M is the mass of water vapour required to saturate that volume of air, f is the water vapour pressure in air at t°C and F is the saturated vapour pressure of water at that temperature.

Read and Learn More WBBSE Solutions for Class 9 Physical Science and Environment

Absolute humidity is defined as the amount of water vapour in grams present per cubic meter of air (g/m³).

Generally, if the temperature of any liquid is increased, its volume also increases. But there is an exception to this rule in case of water for a specific range of temperature,i.e., from 0°C to 4°C.

If the temperature of water is increased from 0°C, its volume decreases and density gradually increases. At a temperature of 4°C, density of water is maximum (1 g/cm³) and volume of a particular mass of water is minimum. This abnormal property of water from a temperature range of 0°C to 4°C is called the anomalous expansion of water.

Chapter 6 Saturated Unsaturated Vapour And Anomalous Expansion Of Water Short And Long Answer Type Questions

Question 1. Define saturated and unsaturated vapour.

Answer:

Saturated And Unsaturated Vapour:-

Saturated Vapour: At a particular temperature, if maximum amount of vapour is accommodated in a closed space, then that vapour is called saturated vapour.

Unsaturated Vapour: If the vapour in a closed space at a particular temperature is less than the highest possible vapour that can be accommodated at that temperature, then that vapour is called unsaturated vapour.

Question 2. What is vapour pressure?

Answer:

Vapour Pressure:-

A liquid can vaporise at any temperature. If a liquid is taken to fill a closed vessel partially, then the liquid vaporises and the empty portion of the vessel is filled up by that vapour. This vapour exerts pressure on the inner surface of the vessel. This pressure is called vapour pressure.

Question 3. What is saturated vapour pressure and unsaturated vapour pressure?

Answer:

Saturated Vapour Pressure: If vapour pressure in a closed space at a specific temperature is equal to the maximum vapour pressure at that temperature, then that vapour pressure is called saturated vapour pressure of that closed space.

Unsaturated Vapour Pressure: If vapour pressure in a closed space at a specific temperature is less than the maximum vapour pressure possible at that temperature, then that vapour pressure is called unsaturated vapour pressure of that closed space.

Question 4. Write down the differences between saturated and unsaturated vapour.

Answer:

Differences Between Saturated And Unsaturated Vapours Are:

Question 5. What do you mean by dew point?

Answer:

Dew Point:-

1. The atmosphere is filled with water vapours almost at all times. If temperature is more, capacity to hold water vapour becomes more. As temperature remains high during the day, capacity to hold water vapour also remains high.So generally during day time, air is not saturated by water vapour present in it.
2. At night, earth’s surface gets colder by radiation of heat, so atmosphere surrounding the surface gets cold. Then capacity of air to hold water vapour also decreases. Thus when temperature of the atmosphere decreases, air at a specific temperature gets saturated by the water vapour present it.
3. This temperature is called dew point. In short, the specific temperature at which a certain volume of air is saturated with water vapour present in it is known as dew point.

Question 6. How does dew originate? What are the essential conditions for formation of dew?

Answer:

Dew Originate:-

When temperature of the atmosphere comes down below the dew point, some amount of water vapour of the atmosphere is condensed to form small droplets of water and settle on grass, leaves etc. These droplets are called dew.

Essential Conditions For Formation Of Dew:

1. Cloud free clear sky;
2. Calm and still air;
3. Excess of water vapour in atmosphere;
4. Closeness to objects which are bad conductors of heat.

Question 7. Explain how fog is formed.

Answer:

Fog Forms As:-

If atmospheric temperature of a vast region falls below the dew point due to some reason, then some amount of water vapour present in the atmosphere condenses as deposits on coal particles, dust etc. and floats in the atmosphere.

This is called fog. When formed over a lake, it is called mist. As air in an industrial area contains considerable amount of dust, coal particles etc., dense fog is formed in those areas.

Question 8. Explain how cloud is formed.

Answer:

Cloud Forms As

Density of water vapour is less than density of air. So, water vapour present in air naturally goes up. As it goes up from the earth’s surface, air pressure decreases. So, volume of water increases due to reduction of air pressure and hence, its temperature decreases.

Now as we know that temperature of upper atmosphere remains low, when water vapours come in contact, the temperature of water vapour decreases further. Now if that temperature becomes less than the dew point, then water vapour condenses on dust particles present there and float around in the form of small water particles which is called cloud.

Question 9. ‘Cloudy sky is not favourable for formation of dews’—explain.

Answer:

Cloudy Sky Is Not Favourable For Formation Of Dews:-

Since radiated heat from the earth gets reflected back by cloud. For this the earth surface does not get cool sufficiently to help the formation of dew. That is why cloudy sky is not favourable for formation of dews.

Question 10. What do you mean by relative humidity?

Answer:

Relative Humidity Means:-

Relative humidity is the ratio of the mass of water vapour present in a particular volume of air to the mass of water vapour required to saturate that volume at a particular temperature. In other words, relative humidity is the ratio of the partial pressure of water vapour to the saturated pres- sure of water vapour at a particular temperature.

Now suppose, m is the mass of water vapour present in a specific volume of air at t°C and M is the mass of water vapour to be present at that temperature to make that volume of air saturated.

∴ relative humidity (RH) = \(\frac{m}{M}\) x 100%

Question 11. What do you mean by absolute humidity? What is its practical unit?

Answer:

Absolute Humidity:-

Absolute humidity (AH) is defined as the amount of water vapour in grams present per cubic meter of air.

Suppose, water vapour of mass m g is present in a specific amount of air of volume V m³ at temperature t°C.

∴ absolute humidity (AH) = \(\frac{m}{M}\)

Practical unit of absolute humidity is g/m³.

Question 12. On a summer day, suppose the temperatures of Delhi and Puri are the same. Which place feels more uncomfortable?

Answer:

As Puri is situated on the seashore, the value of the relative humidity of air in Puri is higher than that in Delhi. Perspiration takes more time to dry up in Puri, as a result, an uncomfortable feeling is there, though the temperature of these two places are the same.

Question 13. If the temperature of a closed room is increased, is there any change of relative humidity?

Answer:

We know that, relative humidity = \(\frac{m}{M}\) x 100%

If room temperature is increased, capacity of air to hold water vapour increases. Hence, water vapour of greater mass is required (say, M₁ where M₁ > M) to saturate the air of that room. Since amount of water vapour in the room remains unchanged, hence relative humidity decreases.

Question 14. Is there any change of relative humidity when water is sprinkled in a closed room?

Answer:

When water is sprinkled in a closed room, mass of water vapour present in the air of the room increases. Now as the temperature of the room remains unchanged, mass of water vapour necessary to saturate remains unchanged. So, relative humidity of air in the room increases.

Question 15. Is there any change of dew point when water is sprinkled in a dosed room?

Answer:

When water is sprinkled in a closed room, amount of water vapour in the room increases. Now as the temperature of the room remains unchanged, so in order to make the air of the room saturated, comparatively less reduction of temperature has to be brought about. Hence, dew point increases.

Question 16. What do you mean by anomalous expansion of water? Explain it with a graphical representation of volume and temperature.

Answer:

1. In general, if the temperature of a liquid increases, its volume also increases. But in case of water, there is an exception to this rule in a specific range of temperature, i.e., from 0°C to 4°C temperature.
2. If temperature of water is increased from 0°C, it volume decreases and density gradually increases. Density of water is maximum (1g • m³) at 4°C, so the volume of a specific mass is lowest. If temperature is increased after 4°C, the volume of water increases and density decreases.
3. Change of volume of 1 g of water, when temperature in increased from 0 C to 10°C is shown.

This behaviour of water from 0°C to 4°C temperature is known as anomalous expansion of water.

Question 17. Discuss the influence of anomalous expansion of water on aquatic animals.

Answer:

In cold countries, when atmospheric temperature decreases below 0°C, temperature of the upper surface of lakes, water bodies etc. also comes down gradually. During this time, cold and heavy water from the top of the lake goes down and comparatively warm and light water from the bottom of the lake comes upward.

This convection cycle goes on till the temperature of the water at the bottom reach 4°C. At this temperature 4°C, density of water is maximum, i.e., after this temperature, water does not rise any further. Now, temperature of the upper surface of the lakes, water bodies etc. keeps on decreasing.

At this temperature, density of water at the top starts decreasing causing the water not to go down further.

Hence, temperature of the upper surface comes down to 0°C and finally is converted to ice. As ice is lighter than an equal volume of water, it floats on water. Ice is a bad conductor of heat and so rate of transmission of heat from the lower layer is very low and as a result, the thickness of ice increases gradually.

Some water remains below the ice layer or crust. Temperature of water just below this ice crust is 0°C and temperature increases down the water layers to become 4°C at the bottom. For this reason, though the atmospheric temperature in cold countries comes below 0°C to make the upper surface of lakes frozen, aquatic animals survive easily.

Chapter 6 Saturated Unsaturated Vapour And Anomalous Expansion Of Water Very Short Answer Type Questions Choose The Correct Answer

Question 1. Hygrometer is used to measure

1. Amount of oxygen in air
2. Density of air
3. Increase of density with temperature
4. Amount of water vapour present in the atmosphere

Answer: 4. Amount of water vapour present in the atmosphere

Question 2. If room temperature is equal to the dew point, relative humidity becomes

1. 100%
2. 0%
3. 70%
4. 85%

Answer: 1. 100%

Question 3. Which of the following statements regarding saturated vapour is true?

1. Follows Boyle’s law
2. Remains in equilibrium when in contact with the liquid
3. Can be made unsaturated by reducing the temperature
4. All of the above

Answer: 2. Remains in equilibrium when in contact with the liquid

Question 4. Anomalous expansion is seen for

1. Mercury
2. Kerosene
3. Glycerine
4. Water

Answer: 4. Water

Question 5. In a cold country if the upper surface of a lake is converted into ice, temperature of water at the bottom of the lake becomes

1. 0°C
2. 1°C
3. 2°C
4. 4°C

Answer: 4. 4°C

Question 6. On a particular day, temperature of Kolkata and Delhi are the same. But the levels of relative humidity in air are 80% and 60%, respectively. Which of the following statements is correct?

1. Weather of both the places are equally comfortable
2. Given information regarding weather is incomplete
3. The weather of Kolkata is more comfortable
4. Weather of Delhi is more comfortable

Answer: 4. Weather of Delhi is more comfortable

Question 7. The physical quantity determining the amount of water vapour in a definite volume of air is

1. Humidity
2. Dew point
3. Temperature
4. Water equivalent

Answer: 1. Humidity

Question 8. If the amount of water vapour increases in air, then

1. Density of air increases
2. Density of air decreases
3. Relative humidity of air decreases
4. Dew point of air decreases

Answer: 2. Density of air decreases

Question 9. Formation of fog takes place generally in

1. Summer
2. Rainy season
3. Winter
4. Autumn

Answer: 3. Winter

Question 10. Formation of dew takes place generally in

1. Summer
2. Rainy season
3. winter
4. Autumn

Answer: 4. Autumn

Question 11. Anomalous expansion of water is observed in the temperature range of

1. 10°C-14°C
2. 4°C-14°C
3. 0°C-4°C
4. 0°C-10°C

Answer: 3. 0°C-10°C

Question 12. If relative humidity of air decreases from 80% to 60%, then

1. We feel comfortable
2. Uneasiness due to perspiration increases
3. We feel hotter
4. Temperature comes down from 80°C to 60°C

Answer: 1. We feel comfortable

Question 13. If water is sprinkled in a closed room, then the room’s

1. Relative humidity decreases
2. Relative humidity increases
3. Dew point decreases
4. Dew point remains unchanged

Answer: 2. Relative humidity increases

Question 14. If temperature of water is decreased from 4°C to 0°C, then

1. Density increases
2. Density decreases
3. Volume decreases
4. Both density and volume decrease

Answer: 2. Density decreases

Question 15. If the temperature of water is increased from 4°C to 10°C, then

1. Volume of water decreases
2. Mass of water decreases
3. Volume of water increases
4. Mass of water increases

Answer: 3. Volume of water increases

Question 16. Volume expands due to application of heat. On the basis of this information, water behaves normally

1. In the range of 0°C-4°C
2. In the range of 0°C-10°C
3. In the range of 4°C-10°C
4. In the range of 3°C-4°C

Answer: 3. In the range of 4°C-10°C

Question 17. Temperature of air and its relative humidity are 30°C and 80%, respectively. If 32 mm Hg is the pressure of saturated water vapour at 30°C, then the pressure of water vapour present in air becomes

1. 23.2 mm Hg
2. 24.2 mm Hg
3. 25.6 mm Hg
4. 27.2 mm Hg

Answer: 2. 24.2 mm Hg

Question 18. What is the relative humidity if pressure of water vapour is 27 mm Hg and pressure of saturated water vapour at that temperature is 30 mm hg?

1. 60%
2. 70%
3. 80%
4. 90%

Answer: 4. 90%

Chapter 6 Saturated Unsaturated Vapour And Anomalous Expansion Of Water Answer In Brief

Question 1. What is the value of relative humidity, when temperature of the atmosphere, is equal to the dew point?

Answer: When temperature of the atmosphere is equal to the dew point, value of relative humidity is 100%.

Question 2. Which liquid shows anomalous expansion?

Answer: Water shows anomalous expansion.

Question 3. At what temperature, density of water is maximum?

Answer: At 4°C or 277 K, density of water is maximum.

Question 4. If the upper surface of a lake in a cold country freezes to ice, what is the temperature of water at the bottom surface?

Answer: If the upper surface of a lake in a cold country freezes to ice, the temperature of water at the bottom surface must be 4°C.

Question 5. If the upper surface of a lake in a cold country freezes to ice, what is the temperature of water just below the ice crust?

Answer: If the upper surface of a lake in a cold country freezes to ice, the temperature of water just below the ice crust must be 0°C.

Question 6. Does saturated vapour follow Charles’ law and Boyle’s law?

Answer: No, saturated vapour does not follow Charles’ law and Boyle’s law.

Question 7. What is the range of temperature in which anomalous expansion of water is seen?

Answer: Anomalous expansion of water is seen in the temperature range of 0°C to4°C.

Question 8. The volume of a definite mass of water is minimum at what temperature?

Answer: The volume of a definite mass of water is minimal at 4°C.

Question 9. Due to which property of water, aquatic animals survive in spite of atmosphere temperature going down below the freezing point in cold countries?

Answer: This happens due to anomalous expansion of water.

Question 10. A bowl made up of glass is filled with water up to the brim. What is the result if the temperature of the water is raised and reduced?

Answer: Water spills out for both the rise and fall of temperature.

Question 11. How is unsaturated vapour converted to saturated vapour?

Answer: By increasing the pressure or by decreasing the temperature, unsaturated vapour can be converted to saturated vapour.

Question 12. How is saturated vapour converted to unsaturated vapour?

Answer: By decreasing the pressure or by increasing the temperature, saturated vapour can be converted to unsaturated vapour.

Question 13. Which of the two remains in equilibrium when in contact with the liquid, saturated vapour or unsaturated vapour?

Answer: Saturated vapour remains in equilibrium when in contact with the liquid.

Question 14. What is the required temperature of the atmosphere for the formation of fog?

Answer: If the temperature of the atmosphere comes down below the dew point, the formation of fog takes place.

Chapter 6 Saturated Unsaturated Vapour And Anomalous Expansion Of Water Fill In The Blanks

Question 1. The capacity of a closed space to contain maximum vapour ________ with increasing temperature.

Answer: Increase

Question 2. _______ vapour does not remain in equilibrium when in contact with a liquid.

Answer: Unsaturated

Question 3. _______ vapour may be converted into ________ vapour by increasing temperature or by reducing pressure.

Answer: saturated, unsaturated

Question 4. In general, an increase in temperature of any liquid _______  its volume but in the case of water, there is an __________ to this rule.

Answer: Increase, exception

Question 5. Though Delhi and Puri may have the same temperature on a particular day of summer, we feel warmer in __________

Answer: Puri

Question 6. When the temperature starts _____, the volume of water normally reduces to 4°C. When temperature starts anomalous expansion of water is observed up to 4°C.

Answer: Decreasing

Question 7. When temperature starts _______ anomalous expansion of water is observed up to 4°C.

Answer: Increasing

Question 8. _______ of water at 4°C is higher than its density at 3°C.

Answer: Density

Question 9. The volume of 1 g of water at 4°C is ________ than the volume of 1 g of water at 2°C.

Answer: Less

Question 10. ________ vapour exists on the top surface of a liquid in a closed vessel.

Answer: Saturated

Question 11. ________ indicates the amount of water vapour in air.

Answer: Humidity

Question 12. Air is _______ at the dew point.

Answer: Saturated

Question 13. The weather in Delhi is more dry compared to that of Kolkata. This means the amount of relative humidity in Delhi is comparatively ________

Answer: Lower

Question 14. _______ is defined as the amount of water vapour in grams present in every cubic metre of air.

Answer: Absolute humidity

Chapter 6 Saturated Unsaturated Vapour And Anomalous Expansion Of Water State Whether True Or False

Question 1. The saturated vapour is the maximum amount of vapour that can be accommodated in a closed space at a particular temperature.

Answer: True

Question 2. If the temperature of the atmosphere goes above the dew point, dew is formed.

Answer: False

Question 3. If the temperature of water is increased from 0°C to 4°C, then its volume increases.

Answer: True

Question 4. Clear sky is a favourable condition for the formation of dew.

Answer: True

Question 5. Dew forms only at night.

Answer: True

Question 6. Relative humidity has no unit.

Answer: True

Chapter 6 Saturated Unsaturated Vapour And Anomalous Expansion Of Water Numerical Examples

Useful Information

1. Relative humidity = \(\frac{M_P}{M_S}\)x 100%
2. M_P = mass of water vapour present in a certain volume of air
3. M_S = mass of water vapour required to saturate the same volume of air.
4. Relative humidity = \(\frac{f}{F}\) x 100%
5. Where, f = actual pressure of water vapour present in air.
6. F = Saturated vapour pressure of water at that temperature of air.

Question 1. At a particular temperature actual pressure of water vapour present in the air is 34 mm of Hg and the saturated vapour pressure of water at that temperature is 40 mm of Hg. Find relative humidity.

Answer:

At a particular temperature, the actual pressure of water vapour present in air (f) = 34 mmHg and saturated vapour pressure at that temperature (F) = 40 mmHg.

∴ Relative humidity = \(\frac{f}{F}\) x 100%

= \(\frac{30}{34}\) x 100% = 85%

Question 2. On a certain day, the room temperature is 20°C and the dew point is 15°C. Saturated vapour pressure at 20°C and 15°C are 20mmHg and 13mmHg respectively. Find the relative humidity of the aforesaid day.

Answer:

Saturated vapour pressure at dew point (f) = 13 mmHg and saturated vapour pressure at room temperature (F) = 20 mmHg.

∴ Relative humidity (RH) = \(\frac{13}{20}\) x 100% = 65%

Question 3. At a particular temperature actual pressure of water vapour present in air is 30 mmHg and relative humidity is 60%. Find saturated water vapour pressure at that temperature.

Answer:

At a particular temperature actual pressure of water vapour present in air (f) = 30mmHg.

Relative humidity (RH) = 60%

Let, saturated water vapour pressure be F at that temperature.

∴ Relative humidity (RH) = \(\frac{f}{F}\) x 100%

or, 60% = \(\frac{30}{F}\) x 100%

or, F = \(\frac{30 \times 100}{60}\) = 50

∴ Saturated water vapour pressure is 50 mmHg.

Chapter 6 Saturated Unsaturated Vapour And Anomalous Expansion Of Water Miscellaneous type Questions

Match the columns

1.

Answer: 1. C, 2. B, 3. D, 4. A

2.

Answer: 1. D, 2. A, 3. B, 4. C