Latent Heat and Evaporation, Physics tutorial

Concept of latent heat:

Latent heat is the name given to energy that is either lost or gained through a substance if it changes state, for illustration from gas to liquid. This is measured as an amount of energy, joules, instead of as a temperature.

Most of the substances can exist in three states: solid, liquid and gas; however there is an additional state named plasma. The major difference between substances in each state is how fast its molecules are moving. As a liquid, the molecules move at a speed where they can repeatedly join altogether, break apart and then join altogether again. If they move slowly, they stay joined altogether, forming a solid. If they move fast, they stay broken apart, making a gas.

For illustration, we generally think of water as a liquid. Though, it can as well be a solid (ice) or a gas (steam). However you can observe whenever you boil water in a kettle or when the surface of a pond freezes, not all of the molecules in a substance modify state at similar time.

Types of Latent Heat:

There are three stages of the substance: solid, liquid and gaseous. Accordingly there are three kinds of latent heats, which have been explained below:

1) Latent heat of fusion: Let us assume that we have one kg of ice and we have to transform it into water, the amount of heat needed to do this is termed as latent heat of fusion. In common amount of heat needed to melt the substance of unit mass from solid to liquid or the amount of heat needed to be eliminated from the unit mass of substance to freeze it from liquid to solid is termed as latent heat of fusion denoted by 'lf'.

2) Latent heat of vaporization: We have notice the illustration for boiling water. The amount of heat needed by the unit mass of substance to vaporize from liquid to the gaseous state or the amount of heat needed to be eliminated from the unit mass of substance to condense from the gaseous to liquid stage is termed as latent of vaporization denoted by 'lv'.

3) Latent heat of sublimation: A few substances such as naphthalene get directly transformed from solid to gaseous state whenever kept in the open atmosphere. The amount of heat needed by the unit mass of substance to change directly from solid to gaseous stage or the amount of heat needed to be eliminated from the gaseous substance to change it to the solid stage is termed as latent heat of sublimation represented by 'lsub'.

Applications of Latent Heat:

From the prior, we realized that if a liquid is changed into vapor, heat is absorbed from the liquid or any object in contact with it. The heat is the latent heat employed in causing the transformation from liquid to vapor.  The phenomenon is used in the working principles of some general household devices like: Pressure cooker, Refrigerator and the air conditioner.

Melting point:

The melting point (or, generally, liquefaction point) of a solid is the temperature at which it transforms state from solid to liquid at atmospheric pressure. At melting point, the solid and liquid stage exists in equilibrium. The melting point of a substance based on pressure and is generally specified at standard pressure. Whenever considered as the temperature of the reverse change from liquid to solid, it is termed to as the freezing point or crystallization point. Due to the capability of few substances to super cool, the freezing point is not considered as a quality property of a substance. When the 'characteristic freezing point' of a substance is determined; however the actual methodology is nearly always 'the principle of observing the disappearance instead of the formation of ice', that is, the melting point.

Boiling point:

The boiling point of a substance is the temperature at which the vapor-pressure of the liquid equivalents the pressure surrounding the liquid and the liquid converts into a vapor.

The boiling point of a liquid differs based on the surrounding atmospheric pressure. A liquid in a partial vacuum consists of a lower boiling point than when that liquid is at atmospheric pressure. A liquid at high pressure consists of higher boiling point than when that liquid is at atmospheric pressure. For a particular pressure, various liquids boil at dissimilar temperatures.

Effects of Impurities and Pressure on the Melting and Boiling Points of a Substance:

The melting and boiling points of a substance are influenced by pressure and impurities.

1) Effect of pressure on boiling point:

Rise in the pressure raises the boiling point of a liquid. The pressure cooker is a practical application of the effect of the pressure on boiling point. Increased pressure of the trapped air (or gas) above the liquid in a pressure cooker increases the boiling point of liquid. Therefore a high cooking temperature is reached extremely fast. Therefore, food is cooked quicker in the pressure cooker.

2) Effect of impurities on the boiling point of a substance:

Impurities increase the boiling point of a pure solvent. Therefore, the boiling point of a solution of salt in water is greater than that of pure water.

3) Effect of impurities on the melting point of a substance:

The melting point of a pure solid is lowered through the presence of impurities. For illustration, a freezing mixture of ice and salt consists of a lower temperature than 0oC, the freezing point of pure water (or melting point of ice)

Concept of evaporation:

Evaporation takes place if a liquid converts its state to form a gas or vapor. Most of them which take place on Earth are the change from the liquid state of water to a water vapor. However it looks extremely simple, water is not just the most plentiful compound on Earth and it is as well extremely complicated. The quantity of water on the Earth never varies; it simply transforms its form as it passes via what is termed as the water cycle. Each phase comprises various methods which are needed to change water to a new form.

Water consists of three different states: Ice, liquid and vapor. Evaporation of water is the procedure through which it converts from liquid to a vapor. To do this, water needs one of some conditions to be in place. For a body of water like a river, lake or ocean to let some of its water molecules to switch from a liquid and be discharged as a water vapor into the air, heat from the sun should be present to initiate the procedure. A change in energy or pressure is for all time needed from a few source for this procedure to take place.

Factors which affect the rate of evaporation:

1) Temperature: Evaporation is faster when the surrounding temperature is higher. This is due to the reason that the method of evaporation comprise absorbing of latent heat of vaporization from the surroundings. The warmer the surroundings, the more quantity of heat supplied by it to provide the latent heat of vaporization.

2) Pressure: The bigger the pressure, the slower the evaporation and vice-versa. When an open vessel of water is placed within a bell-jar from which air has been pumped out (decreased pressure), evaporation occurs at faster rate.

3) Area of liquid surface exposed: The greater the surface area of liquid exposed, the faster will be the evaporation.

4) Dryness of the air: The drier the air (that is, the less water vapor it includes) the quicker the evaporation. Water droplets on a watch-glass evaporate faster when left in the open than when covered by a bell jar. As the water evaporates, the air in the bell jar gets filled by water vapor and this slows down the evaporation. In case of the watch glass in the open, the water vapor rapidly escapes into the open air and more evaporation occurs. This is the cause why wet clothes dry faster in dry weather than in the wet weather.

5) Motion of the Air: Wet clothes put in the draft from a fan will dry up sooner than when put it in still air. The air stream or wind eradicates the vapor and brings fresh and drier air into contact with the liquid.  Therefore, evaporation is more quick.

6) Nature of the Liquid: Various liquids evaporate at different rates. The lower the boiling point of a liquid, the more will be the rate of evaporation. For illustration: mercury by means of a boiling point of 357oC almost not evaporates while either (boiling point 35oC) evaporates very fast. Liquids similar to ether that evaporates rapidly are termed as volatile liquids.

Concept of boiling:

Boiling takes place if a substance in the liquid state is thrown into violent agitation as an outcome of continuous supply of heat to the liquid molecules.

Evaporation and boiling:

The major difference between evaporation and boiling are:

1) Evaporation occurs at all temperatures, whereas boiling takes place at specific temperature.

2) Evaporation occurs from the surface, while the complete liquid boils.

3) Evaporation can take place by employing the internal energy of the system, whereas boiling needs an external source of heat.

4) Evaporation generates cooling however boiling doesn't.

5) Evaporation is a slow procedure while boiling is a rapid procedure.

Vapor and Vapor Pressure:

Vapor: As you know a liquid converts into vapor when adequate heat energy is supplied to it.  The change from liquid to vapor occurs all through the whole liquid at a specific temperature.

Vapor pressure: Whenever a liquid is allowed to evaporate in a closed vessel, the vapor formed will apply a particular pressure. This is termed as vapor pressure.

Saturated and unsaturated vapor:

Saturated vapor: A saturated vapor is one that is in contact having its own liquid in a closed space. The pressure applied by such a vapor is the saturated vapor pressure.

Unsaturated vapor: An unsaturated vapor is one which is not in contact having its own liquid in a closed space. The gas laws are approximately followed in such a vapor.

Concept of Sublimation:

This is a procedure whereby some solid substances if heated, change directly into gaseous state devoid of the intermediate liquid state. For instance, solid iodine and solid carbon dioxide converts from solid state to gaseous state devoid of changing to the intermediate liquid state.

Humidity:

The atmosphere consists of varying amounts of water vapor. Humidity is the amount of water vapor in the atmosphere.

Relative humidity:

When the air is saturated or almost saturated by means of water vapor, it is explained as very humid. When the air is highly unsaturated it is stated to be dry. Humid weather makes one feel uncomfortable as perspiration doesn't evaporate simply from the skin to the already unsaturated air. As well, very dry air evaporates the moisture from the skin too fast. This is injurious to the skin. A comparison among the amount of moisture in reality present in the air with the quantity of moisture it can hold when saturated is known as relative humidity. This is deduced as a ratio which points out the extent to which air is saturated by water-vapor.

Relative humidity is generally represented as a percentage. This percentage states you how close the air is to being saturated. When the relative humidity is 100 percent, this signifies that the air is saturated.  When the relative humidity is 50 percent, the air consists of half the water vapor needed for it to be saturated. 

Dew point:

Dew point temperature is the absolute measure of how much water-vapor is in the air. The warmer the air, the more the amount of water vapor it can have.

Humidity and Weather:

The requirement for efficient weather forecasting is becoming much significant in all countries of the world. Though, to accomplish a good level of accuracy in such weather forecasting, it needs accurate determination of the humidity level of the air. This job in most of the situations is left for the meteorologist. Their reports regarding humidity and weather are found much helpful in setting up flights in airports, navigation, radio transmission, farming, factories, food storage and so on.

As well, the construction of particular household equipment like refrigerators and air conditioners need knowledge of how to keep the relative humidity of an enclosure at a particular level. The formation of clouds, fog, rain, hailstone and snow based among other things on the relative humidity of the atmosphere.

Formation of Dew, Mist and Fog:

a) Formation of dew:

It is a general experience that in the evening, the ground and the air in contact starts to cool as they lose heat via radiation.  A clear sky and still air might cause temperature air to fall even lower so that the air becomes almost saturated with water-vapor. Moreover, the surfaces of a few objects specifically grass and other plants, give off water-vapor, this adds to the amount of vapor in the air. Whenever compared by the surrounding air, such surfaces are good conductors and lose heat sooner than the air. As the temperature of ground and grass falls beneath the dew point, the moisture in the surrounding air will condense and deposited in the surfaces as dew.

When the dew point is below 0oC, the vapor transform into solid form of ice crystals termed as frost.

b) Formation of mist and fog:

Whenever the air close to the surfaces of the land cools until it reaches dew point, the water vapor starts to condense around small dust particles in the atmosphere. Such particles have surfaces on which the condensed vapor collects.  Such a collection of water particles is termed as mist. A thick mist is termed as a fog. The water particles of the mist or fog are small adequate to float in air. If they become bigger, they fall as rain.

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