Tag: heat, internal energy and work

Questions Related to heat, internal energy and work

Choose the correct statements from the following

physics the kinetic model of matter evaporation heat, internal energy and work phase change

  1. Evaporation takes place from the surface of a liquid at all temperatures.

  2. The rate of evaporation depends upon the area of the exposed surface of the liquid, nature of the liquid and its temperature.

  3. The rate of evaporation depends on the pressure to which the liquid is subjected.

  4. The cooling produced in evaporation is a consequence of the fact that a liquid has latent heat.

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Correct Option: A,B,C,D
Explanation:

Evaporation is a type of vaporization of a liquid that occurs from the surface of a liquid into a gaseous phase. So there is loss of energy due to latent heat of the liquid.

For molecules of a liquid to evaporate, they must be located near the surface, they have to be moving in the proper direction, and have sufficient kinetic energy to overcome liquid-phase inter-molecular forces. When only a small proportion of the molecules meet these criteria, the rate of evaporation is low. Since the kinetic energy of a molecule is proportional to its temperature, evaporation proceeds more quickly at higher temperatures and high surface area.
Evaporation happens faster if there is less exertion on the surface keeping the molecules from launching themselves.
So all the options are correct.

Water evaporates under atmospheric pressure. Without changing the temperature, the same water is placed in a partial vacuum. The rate of evaporation will

physics the kinetic model of matter evaporation heat, internal energy and work phase change

  1. increase

  2. drop to zero

  3. decrease

  4. remain unaffected

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Correct Option: A
Explanation:
In partial vacuum, the pressure is very less.
According to Le-chatelier's principle on decreasing pressure, volume increases so more water converts into gas (vapour).

Soda bottles are made of thick glass so that they can withstand an increase in :

physics the kinetic model of matter evaporation heat, internal energy and work phase change

  1. pressure during summer

  2. temperature during summer

  3. viscosity

  4. potential energy

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Correct Option: A
Explanation:
Gas, liquid and solid expand on heating.
Gas expands the max. due to free of gas molecules or almost zero force of attraction between them. On heating they gain K.E. and move up to any place. This gives expansion of gases.
The pressure of gas inside closed container increases upon heating. This is seen that soda bottle explodes in summer. So the air made up of thick glass so that they can withstand pressure.

A fan produces a feeling of comfort because:

physics the kinetic model of matter evaporation heat, internal energy and work phase change

  1. A fan supplies cool air

  2. Of the evaporation of sweat

  3. Fan increase the humidity in air

  4. Fan decrease the humidity in air

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Correct Option: B
Explanation:

A fan produces a feeling of comfort because of evaporation of sweat because on hot day, our body sweats to loose heat through the evaporation of moisture.In still air evaporation is slow. A fan or breeze helps by replacing hot humid air with cooler dries air that allow for more evaporation


When the temperature of water rises, the rate of evaporation decreases. State whether true or false.

physics the kinetic model of matter evaporation heat, internal energy and work phase change

  1. True

  2. False

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Correct Option: B
Explanation:
The statement if False.
Actually, rate of evaporation of a liquid decreases as the temperature decreases. As intermolecular force increase rate of evaporation decreases. This happens because intermolecular force make it less likely for the molecules on the surface to escape from liquid and become vapour gas. As temperature increases rate of evaporation increases.

Why mercury do not evaporate at room temperature?

physics the kinetic model of matter evaporation heat, internal energy and work phase change

  1. Because melting point of mercury is higher

  2. Because boiling point of mercury is higher

  3. Because boiling point of mercury is lower

  4. Because boiling point of mercury is equal to room temperature

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Correct Option: B
Explanation:

Evaporation is a process in which some molecules of a liquid, which have greater kinetic energy, escape from the liquid and enter the atmosphere.

When the temperature of the liquid is closer to its boiling point, greater is the evaporation. 
Besides the boiling point being higher, the cohesive force of attraction among the molecules of mercury is greater than other liquids like water. This further reduces the evaporation rate.

Evaporation is the process of changing liquid into vapour below its boiling point. State whether true or false.

physics the kinetic model of matter evaporation heat, internal energy and work phase change

  1. True

  2. False

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Correct Option: A
Explanation:
Evaporation is the process of changing liquid into vapour below B.P. (boiling point)
It is the type of vaporisation that occurs on the surface of liquid due to increase in temperature or pressure it changes liquid into gaseous phase before reaching to B.P. The surrounding gas must not be saturated by evaporating substance.

A dog pants with its tongue sticking out on a hot day. Which of the following is the correct reason?

physics the kinetic model of matter evaporation heat, internal energy and work phase change

  1. It is its habit

  2. Producing more saliva cools down the body temperature.

  3. Saliva vapourises, cooling the tongue

  4. It is a genetic disease found in few animals like dogs.

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Correct Option: C
Explanation:

Evaporation causes cooling.
A dog pants with its tongue sticking out on a hot day because its saliva vapourises, cooling the tongue.

Relative humidity is the percentage of the

physics the kinetic model of matter evaporation heat, internal energy and work phase change

  1. absolute humidity value to the amount of humidity actually present

  2. increase of humidity/absolute humidity

  3. amount of humidity actually present to the absolute humidity

  4. none of these

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Correct Option: C
Explanation:

The ratio of actual amount of water vapour present in volume of air at a given temperature to maximum amount that air could hold at that temperature, expressed as percentage is relative humidity.

The molecules of liquid change into vapour when the pressure of molecules of liquid

physics the kinetic model of matter evaporation heat, internal energy and work phase change

  1. exceeds the atmospheric pressure

  2. becomes less than the atmospheric pressure

  3. becomes equal to the atmospheric pressure

  4. becomes zero

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Correct Option: C
Explanation:

When the pressure of molecules of liquid become equal to the atmospheric pressure, the molecules of liquid change into vapours.