Evaporation

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Q30. Define latent heat.

The heat that is gained or lost, when a substance changes from one physical state to another state without any change in temperature is called latent heat.

Q31. Why latent heat does not cause a rise in temperature of the substance?

The latent heat which is applied to a substance is used up in overcoming the forces of attraction between the particles of a substance during the change of state. It does not increase the kinetic energy of the particles of the substance. Hence, the temperature of the substance does not rise during the change of state.

Q32. What are the types of latent heat?

Latent heat is of two types.
a) Latent heat of fusion (melting)
b) Latent heat of vapourisation

Q33. Define latent heat of fusion.

The latent heat of fusion of a solid is the quantity of heat in joules required to convert one kilogram of the solid (at its melting point) to liquid without any change in temperature.
The latent heat of fusion of ice is 3.34×10⁵ joules per kilogram or 80 cal / g

Q34. Ice at 0°C is more effective in cooling a substance than water at 0°C. Explain?

Ice is more effective in cooling a substance than water at 0°C because each kilogram of ice, for melting and hence, cools it more effectively. On the other hand, water at 0°C cannot take any such latent heat from the substance.

Q35. Why we feel cold when we hold a piece of ice in our hand?

The piece of ice held in our hand starts melting slowly by taking the latent heat (required for melting) from our hand, our hand loses heat to ice and hence we feel cold.

Q36. Define latent heat of vapourisation.

The latent heat of vapourisation of a liquid is the quantity of heat in joules required to convert one kilogram of the liquid (at its boiling point) to vapour without any change in temperature.
The latent heat of vapourisation of water is 22.5×10⁵ joules per kilogram.

Q37. Why the spirit (or after shave lotion or nail enamel remover) gives a sensation of cold when rubbed on the skin?

When spirit is rubbed on the skin, it evaporates rapidly by taking the latent heat of vapourisation from our hand and so our hand loses heat and gets cooled.

Q38. Why burns caused by steams are more severe than those caused by boiling water though both of them are at the same temperature of 100°C?

Burns caused by steam are more severe than those caused by boiling water because steam contains more heat in the form of latent heat than boiling water. So when steam falls on our hand and condenses to form water, it gives out 22.5×10⁵ joules per kilogram more heat than boiling water at the same temperature.
This is the reason why steam is better than boiling water in heating purpose.

Q39. Define sublimation.

The change of solids directly into vapour on heating and of vapours into solid on cooling is known as sublimation.
  • The solid substances which undergo sublimation is said to be ‘sublime’.
  • The solid obtained by cooling the vapours of the solid is called ‘sublimate’.
  • The common substances which undergo sublimation are ammonium chloride, iodine, amphora, naphthalene and anthracene.

Q40. Why naphthalene balls disappear with time without leaving behind residue?

Naphthalene balls undergo sublimation and keep on forming naphthalene vapours slowly, which disappears into the air.

Q41. What is dry ice? Why is it called so?

  • Solid carbon dioxide (CO₂) is called dry ice.
  • It is called so because solid CO₂ directly changes in CO₂ gas and does not melt to produce a liquid on decreasing the pressure.
  • Dry ice is used as a refrigerant under the name ‘dricold’.

Q42. Define evaporation.

The phenomenon of change of a liquid into its vapour state at any temperature below its boiling point and without the external source of heat is called evaporation.

Q43. Explain how the evaporation takes place.

Some particles in a liquid always have more kinetic energy than others. So, even when a liquid is all below its boiling point, some of its particles have enough energy to break the force of attraction between the particles and escape from the surface of the liquid in the form of vapour are constantly escaping from the liquid.
• Evaporation of a liquid can take place even at room temperature.
• The latent heat of vaporization must be supplied whenever the liquid changes into vapour or gases.

Q44. Differentiate between evaporation and boiling.

insert table

Q45. What are the factors affecting evaporation?

The factors effecting evaporation are as follows:
a) Temperature: The rate of evaporation increases with the increase in the temperature of the surrounding.
The rate of evaporation of a liquid becomes maximum at its boiling point.
b) Surface area of liquid: The rate of evaporation increases with the increase in the surface area of liquid.
c) Humidity of air: The rate of evaporation decrease with increase in humidity of air and vice-versa.
d) Wind speed: The rate off evaporation increases with the increase in wind speed.

Q46. Why should we wear cotton clothes in summer?

During summer we perspire more because of the mechanism of our body which keep us cool. When evaporation takes place, the sweat particles gain energy from body surface and change into vapour. The heat energy equal to latent heat of vapourisation is absorbed from the body, living the body cool. Cotton, being a good absorber of water helps in absorbing the sweat and exposing it in the atmosphere for easy evaporation.
• Synthetic clothes made of nylon, polyester etc do not absorb much of sweat and therefore they cannot keep our body cool in summer.

Q47. Why do we see water droplets on the outer surface of the glass containing ice cold water?

The air contains water vapour, which on coming in contact with the cold surface of glass, losses energy and gets converted into liquid state. This can be seen in the form of water droplets. So, we see water droplets on the outer surface of the glass containing ice cold water.

Q48. Why a desert cooler does cools better on a hot dry day?

Desert cooler works on the principle, more is the evaporation, more will be the cooling. When hot and dry air passes through wet pads then water removes heat from the hot air and evaporates. This causes cooling effect in the pads as well as circulating water. Now, the incoming air coming in contact with the cooler water and pads gets cooled easily.
• The cooling of desert cooler is better on hot dry day because rate of evaporation is more in dry air rather than in humid air.

Q49. How does water kept in an earthern pot (matka) becomes cool during summer?

An earthern pot has large number of tiny pores in its wall from where water continuously keeps seeping to the outer surface. This water evaporates continuously and takes the latent heat required for evaporation from the earthern pot and the remaining water. Thus, the remaining water losses heat and gets cooled.

Q50. How are we able to sip hot tea or milk faster from a saucer rather than a cup?

Saucer has a large surface area and therefore the evaporation of hot tea or milk is faster in it. Thus, faster is the evaporation rate, quicker will be the cooling rate. This rapid cooling of hot tea or milk in saucer makes more convenient for us to cool tea or milk in it.

Q51. Why is ice at 273K more effective in cooling than water at the same temperature?

One Kg of ice at 273K needs 3, 36,000 joules of heat energy to get converted to water at the same temperature. Hence, ice at 273 K is more effective in cooling than water at the same temperature.

Q52. Why vegetables are cooked better and faster in pressure cooker?

In a pressure cooker, pressure is raised by not allowing the vapours formed to escape from the closed cooker. These causes water to boil at much higher temperature then its normal boiling point. Due to this reason, vegetables can be cooked better and faster in pressure cooker.

Q53. Why people sprinkle water on the roofs or open ground on a hot sunny day?

People sprinkle water on the roof or open ground on a hot sunny day because water evaporates by taking a large amount of latent heat of vapourisation from the ground and surrounding air. By losing heat the place becomes cool and comfortable.
• Water vapourizing from the leaves of trees also cools the surrounding air.

Q54. Some examples of evaporation.

a) In our daily life, we spread out the washed wet clothes which while drying to increase their surface area for the rapid evaporation of water present in them, while leads to quicker drying of wet clothes.
b) After rain, the wet roads dry quickly because the rain water is spread over a large area of road and these gives the particles of water a greater chance of escaping from the liquids.
c) In the later part of summer, the humidity of air increases and people sweat a lot in such weather. But the sweat from the body does not evaporate readily due to high humidity of air. As a result such weather becomes muddy (damp) and we feel hot and uncomfortable.
• This type of weather is experienced in cloudy days, in the rainy season and the area close to sea.
d) The washed wet clothes dry more quickly on a windy day because evaporation is faster due to high speed of the wind.
e) A wet handkerchief is placed on the forehead of a person suffering from high fever because as the water from the wet clothes evaporates it takes heat from the skull and the brain within it these, in turn lowers the temperature from the brain and protects it from any damage due to high temperature.
f) One often catches cold by wearing wet clothes because as the water evaporates from the clothes it takes lot of heat from the body. The heat lost by the body is sometimes so large that one gets cold.

Q55. Define kinetic energy.

The energy passes by a body by virtue of its motion is called kinetic energy.

Q56. Suggest a method to liquefy atmospheric gases.

Atmospheric gases can be changed into liquids by increasing the pressure accompanied by lowering of temperature.

Q57. Give two reasons to justify.

a) Water at room temperature is a liquid
  • Intermolecular forces are less
  • Intermolecular space and kinetic energy is more.
Thus, the molecules of water can inter change their spaces and hence water is in liquid state at room temperature.
b) An iron almirah is a solid at room temperature or a wooden table should be called a solid.
  • Intermolecular forces are very large.
  • Intermolecular spaces and kinetic energy is very small
Thus, the molecules are held very, very tightly and as a result, the iron almirah has a definite shape and volume and hence is a solid.

Q58. Why gases are poor conductors of heat and electricity?

Gases have very low densities and due to this reason they are poor conductors of heat and electricity.

Q59. Why a gas exerts pressure on the walls of the container?

In a gas, the particles move about randomly at high speed as they passes high kinetic energy due to this random movement, the particles hit each other and also the walls of the container. This leads to the development of force which is exerted by the gas particles on the walls of the container in the form of pressure.

Q60. What is the temperature called at which gas liquefies? What is the lowest possible temperature for liquefying gas?

The temperature at which a gas liquefies is called its critical temperature. It is different for different gases.

The lowest possible temperature for liquefying any gas is called absolute zero. The temperature of -273.15°C is given the value of absolute zero.

Q61. Liquids generally have lower density as compared to solids but you must have observed that ice-floats on water. Find out why?

The density of solids is generally more as compared to liquid but in case of water due to its anomalous expansion its density decreases since water expands just before its freezing point is reached. And hence, ice floats on water.

Q62. Why boiling point of a liquid is important?

The boiling point of any pure liquid at a pressure of 76 cm of a mercury is fixed and can be used to find the purity of a liquid. However, the pressure of impurities tend to increase the boiling point of liquid.

Physical nature of matter | Characteristics of matter particles | States of matter | Change of matter state | Evaporation

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