WBCHSE Class 11 Physics Change Of State Of Matter Vaporisation Condensation Notes

Properties Of Bulk Matter – Change Of State Of Matter Vaporisation Condensation

Vaporization and Condensation Notes for Class 11

Condensation or liquefaction

The gaseous state ot a liquid is called the vapour of that liquid. The process by which a liquid changes into vapour is called vaporisation. The opposite process is called condensation, i.e. the conversion of vapour into liquid is called condensation or liquefaction.

Just as a solid substance absorbs latent heat when it converts into a liquid, a liquid also absorbs latent heat when it converts into vapour.

The amount of heat absorbed by unit mass of a liquid to convert into vapour is called latent heat of vaporisation. This value depends on the temperature at which the vaporisation occurs.

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Vaporisation of liquids may take place in two ways:

1. Evaporation and
2. Boiling.

 

1. The gradual change of a liquid to its vapour state, at any temperature, from the surface of the liquid is called evaporation.
2. The rapid change of a liquid to its vapour state, at a certain temperature, from the whole volume of the liquid, is called boiling. This particular temperature depends on the pressure on the liquid and remains constant until the whole liquid transforms into vapour.

Sublimation: It is another process of vaporisation where a solid changes directly to its vapour state, without passing through the liquid state. The vapour when condensed, regains the solid state directly. Camphor, iodine, napthalene etc. change into vapour under normal temperature.

Evaporation: If we store water in a wide-brimmed pot, it is noticed that the level of water decreases in a couple of days. This happens due to evaporation from the surface of the water. During summers, small ponds and canals dry up because of evaporation.

• This process is slow but continuous and can take place at all temperatures. The rate of evaporation increases with increase in temperature. Wet clothes dry up by the process of evaporation of water.
• All liquids evaporate somewhat under ordinary temperatures. Liquids, which evaporate at ordinary temperature at a very high rate, are classified as volatile liquids. Ether, alcohol, chloroform, carbon tetrachloride are examples of volatile liquids.
• Rate of evaporation for some liquids, at ordinary temperature, is very low. Such liquids are termed as nonvolatile liquids. Mercury, glycerine etc. belong to this group.

Factors affecting the rate of evaporation:

1. Nature of the liquid: Different liquids have different rates of evaporation. The liquids with low boiling points, also called volatile liquids, have high rates of evaporation. So, spirit, ether, petrol, etc. evaporate quickly.

2. Surface area of liquid: Rate of evaporation increases with the increase in surface area of the liquid. Due to larger surface area, a higher number of molecules of the liquid can leave the surface at a time. The surface area of wet clothes are increased by spreading them as much as possible; this increases the rate of evaporation and the clothes dry up faster.

3. Dryness of air: The drier the air (i.e., the lower the humidity), the faster the evaporation. The air is less humid in winter than during monsoon. So wet clothes dry faster in winter than during monsoon.

4. Atmospheric pressure: Higher the pressure on the liquid surface, lower the rate of evaporation. The converse is also true. For this reason, evaporation is very rapid in vacuum.

5. Temperature of liquid and the surrounding air: Higher the temperature of the liquid and the surrounding air, higher the rate of evaporation. Hence, ponds, tanks etc. dry up quickly in summer.

6. Flow of air: Flow of air over the liquid surface increases the rate of evaporation. Continuous flow of air shifts moist air from over the surface of the liquid replacing it with dry air that helps evaporation. So, perspiration dries up faster under a fan.

Cooling due to evaporation: A liquid needs some latent heat to change into its vapour state. If heat is not supplied from outside, liquid absorbs the latent heat from its own body and from the surroundings during evaporation. Hence, the liquid and the surroundings get cooled.

Cooling due to evaporation Example:

1. Ether or spirit, dropped on our skin, produces a cooling effect. Such a volatile liquid, evaporates fast by absorbing latent heat from our body and this producing this cooling effect.
2. Earthen pitcher keeps water cooler than a metal pitcher. An earthen pitcher has a large number of pores on its surface. Water comes out through the pores and evaporates, collecting the necessary latent heat from the pitcher and also from the water in the pitcher.
   • This causes a fall in the temperature of the water in the pitcher. Pores of old earthen pitchers get blocked by dust particles and so water kept in them do not cool down as much.
   • Water in metal or glass vessels do not cool down much because of the lack of pores on the surface of the container. Evaporation can only take place from the top of the liquid surface. So the water inside does not cool down as much.
3. To keep a room cool in summer, vetiver grass (khas khas or khus khus) mattings with water sprayed on the surface, are hung on doors and windows. The sprayed water evaporates and collects the latent heat from the air inside, thereby cooling the room.
4. Cloth pieces soaked in water are spread on the forehead of a person with high fever. Water in the wet cloth piece evaporates, taking the necessary latent heat from the forehead. So it helps in lowering the body temperature and hence the fever.
5. Wet clothes, when allowed to dry on the body, often cause cough and cold. Water of the wet clothes evapo¬rates taking its latent heat from the body, thus lowering the body temperature; this is the reason for catching cold.
6. If we stand under a fan (or in a breeze) after being covered in perspiration we immediately experience a cooling sensation. This is because perspiration evaporates rapidly under a fan and as a result, it takes away latent heat from the skin as well as the body.
7. Water is poured on roads, room floors or rooftops to lower the temperature during summer. This water, while evaporating, absorbs latent heat from the roads, floors or rooftops, thereby creating a cooling effect.

Understanding Vaporization and Condensation in Physics

• Saturated Vapour and Saturated Vapour Pressure
• Unsaturated Vapour and Unsaturated Vapour Pressure

Vaporisation takes place at any temperature from the upper surface of a liquid. In a closed container when the liquid changes to vapour, the container gradually becomes filled with that vapour.

• The vapour thus formed, exerts pressure on the surface of the container like any other gas. This pressure is termed as vapour pressure.
• As the vapour formed due to evaporation in the closed container increases, vapour pressure also increases. There is a limit up to which a closed space of fixed volume can hold a certain amount of vapour at a fixed temperature. When these closed container holds maximum amount of vapour at a certain temperature that it can hold, the container is saturated with vapour.
• In this condition, the vapour container over the liquid surface in the container is called I saturated vapour and the pressure exerted by that vapour on the container is called saturated vapour pressure.
• On the other hand, if the vapour present is less than the amount that a closed space can hold maximum at that temperature, the space is called unsaturated with that vapour.
• In this state, the amount of vapour that is present in the container is called unsaturated vapour, and the pressure it exerts on the surface of the container is known as unsaturated vapour pressure.

Saturated vapour and saturated vapour pressure: The maximum possible amount of vapour that a closed container can hold at a specific temperature is called saturated vapour and the pressure exerted by this vapour is known as saturated vapour pressure.

Unsaturated vapour and unsaturated vapour pressure: If the amount of vapour present in a closed container is less than the maximum possible amount that the closed container can hold at a specific temperature, then the vapour is known as unsaturated vapour and the pressure it exerts is called unsaturated vapour pressure.

It is important to note that though it is possible to get a fixed value of saturated vapour pressure at any certain temperature, unsaturated vapour pressure is devoid of any fixed value. Also, the saturated vapour pressure, of different liquids at a fixed temperature are different.

Change in Volume of Vapour at Constant Temperature:

Unsaturated vapour: Unsaturated vapour follows Boyle’s law at a constant temperature. Hence, in a closed space

1. Unsaturated vapour pressure decreases with the increase in space (i.e., Volume)
2. When the volume of the space is decreased, initially the vapour pressure increases in inverse proportion. As the reduction in space continues, the capacity of holding vapour in the space also decreases and unsaturated vapour finally changes to saturated vapour. Now, even on further reduction of volume in space, the vapour pressure does not follow boyle’s law any more.

Saturated vapour: Saturated vapour pressure at a fixed temperature is independent of the volume of vapour i.e. saturated vapour does not obey Boyle’s law.

• The observations can be represented graphically as shown. Curved portion of the graph, DC shows that unsaturated vapour obeys Boyle’s law. At point C, the vapour becomes saturated.
• Its pressure remains constant even if its volume i$ decreased more and the saturated vapour gradually condenses into liquid under pressure. CB part of the graph represents this change.
• Obviously, CB is parallel to the volume axis. At point B, the entire vapour condenses into liquid. No appreciable change in the volume of the liquid is noticed with the increase in pressure.
• This is because liquid is almost incompressible. Hence, the BA pan of the graph is almost parallel to the pressure axis.

Change in Temperature of Vapour at Constant

Unsaturated vapour: Unsaturated vapour pressure is directly proportional to the temperature when volume remains constant, similar to the pressure law for gases.

Saturated vapour: Saturated vapour pressure also increases with the increase in temperature at a constant volume, but not as per pressure law. The nature of variation between saturated vapour pressure of water and temperature can be represented graphically as shown.

Initially, vapour pressure increases at a slower rate. Soon the rate of increase in pressure becomes very high. Two important points are to be noted here.

1. Saturated vapour pressure of water at 0°c is not zero, but about 0.4 cm of mercury pressure and
2. At 100°c, the saturated vapour pressure of water is 76 cm of mercury which is normal atmospheric pressure. For other pure liquids too, the graph is almost the same.

Differences between Saturated and Unsaturated Vapours

1. At a constant temperature, if the amount of any vapour and amount of the liquid in contact with that vapour do not change with time then it can be said that the vapour and the liquid are in equilibrium to each other.

2. Saturated vapour pressure of a liquid is equal to its superincumbent pressure during boiling. For example at 98°C the saturated vapour pressure of water is 707.3 mm, i.e., at the atmospheric pressure of 707.3 mm of mercury, water will boil at 98°C.

Saturated vapour pressure (SVP) of water at different temperatures (Regnaulfs list)

• Critical Temperature
• Gas and Vapour

Critical temperature: It has been observed that when a gas is cooled below a certain temperature, it can be converted to its liquid state by application of pressure alone. When the temperature of the gas is above that temperature, the gas cannot be compressed to liquid.

This fixed temperature for a gas is called the critical temperature. The pressure applied to change a gas to its liquid state at critical temperature is called its critical pressure. Every gas has its characteristic critical values of temperature and pressure.

Gas and vapour: A gas below its critical temperature is a vapour and vapours can be compressed to the liquid state. But when the temperature of a gaseous matter is above its critical temperature, it is a gas. Naturally, a gas cannot be liquefied only by application of pressure.

• Critical temperatures of oxygen and hydrogen are -119°C and -24 °C respectively. Hence, at room temperature, these are permanent gases and cannot be liquefied by applying pressure alone.
• But, the critical temperatures for carbon dioxide, ammonia, sulphur dioxide are 31°C, 132.2°C and 157.2°C respectively, and are above the normal room temperature.
• So, they can be compressed to liquid at room temperature and are therefore called vapours. It is to be noted that to liquefy a gas it is, at first, cooled below its critical temperature and then appropriate amount of pressure is applied to convert it to liquid.

Boiling: The temperature of a liquid increases on heating and vapour starts rising from the surface. When the liquid reaches a particular temperature, it gets vigorously agitated and the whole volume of the liquid starts to transform into vapour.

This state of the liquid is called boiling. This temperature remains constant until the entire liquid changes into vapour, and is called the boiling point of the liquid. Heat supplied during boiling is entirely used for transition of liquid into its vapour and here is no rise in the temperature.

Boiling Definition: The temperature at which the whole volume of a liquid starts transforming into its vapour state rapidly, under a fixed pressure is called the boiling point of that liquid at that pressure.

Different liquids have different boiling points. The boiling point of any liquid depends on the pressure on the liquid surface. At standard atmospheric pressure, the temperature at which a liquid boils is called its normal boiling point. Every liquid has a normal boiling point. For example, the normal boiling point of water is 100°C, i.e., at standard atmospheric pressure, water boils and transforms into its vapour at 100°C.

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Normal boiling points of a few liquids

Differences between Evaporation and Boiling

Examples of Vaporization and Condensation in Daily Life

Latent Heat of Vaporisation and Latent Heat of Condensation: We know that the heat applied during boiling does not increase the temperature of the liquid. This heat transforms liquid into its vapour. The heat required to transform unit mass of a liquid into its vapour is called latent heat of vaporisation.

Latent heat of vaporisation: The amount of heat required by unit mass of a liquid to change into its vapour at a constant temperature, is the latent heat of vaporisa¬tion of the liquid.

Similarly, if we cool a gas, it will start condensing on reaching the boiling point. Until the whole gas condenses, the temperature remains constant. Clearly, a gas loses heat during condensation. The amount of heat lost by unit mass of a gas during condensation is called latent heat of condensation.

Latent heat of condensation: The amount of heat extracted from unit mass of a vapour to change it into its liquid state at a constant temperature, is the latent heat of condensation of the vapour.

• For any substance, the latent heat of vaporisation is equal to the latent heat of condensation and is denoted by L. Thus if a quantity H of heat is given or extracted to change the state of mass m, for vaporisation or condensation respectively then H = mL. Unit of L in CGS system is cal · g^-1, and in SI it is J · kg^-1
• The latent heat of vaporisation of water or latent heat of steam is 537 cal · g^-1; it means that 537 cal of heat is required by 1 g of water at 100°C to change into 1 g of steam at 100°C. Again, 537 cal heat is to be extracted from 1 g of steam at 100°C to change it into 1 g of water at 100°C.
• So, it is clear that, 1 g of steam at 100°C can transfer 537 cal more heat than 1 g of water at 100° C. Hence, burns caused by steam at 100°C are more severe than those caused by boiling water at 100°C.
• Besides, the latent heat of vaporisation of water is more than that of any other liquid. So steam at 100°C is a very good warming agent, in cold countries steam is used, instead of hot water to keep the houses warm.

Values of latent heat of vaporisation of water:

In CGS system, L = 537cal · g^-1

In SI, L = 537 x 4.2 x 10³ = 2.26 x 10⁶ J. kg^-1

Latent heat of vaporisation of a few liquids

Latent heat of evaporation and latent heat of boiling are same for a particular liquid at a particular temperature. This is why both these quantities are referred to as latent heat of vaporisation. But in general, evaporation and boiling do not take place at the same temperature.

In case of water, the latent heat of vaporisation increases with decrease in temperature. For example, at 100°C, during boiling of water latent heat is 537cal-g^-1. But when water is evaporated from ponds, rivers and other water bodies at 40°C or below, the latent heat of vaporisation is higher than 537 cal • g^-1.

Properties Of Bulk Matter – Change Of State Of Matter Vaporisation Condensation Numerical Examples

Practice Questions on Vaporization and Condensation for Class 11

Example 1. How much heat is required to convert 1 g of ice at -10°C to steam at 100°C? Specific heat of ice = 0. 5 cal · g^-1 · °C^-1; latent heat of fusion of ice = 80 cal · g^-1; latent heat of vaporisation of water = 540 cal · g^-1
Solution:

Given

Specific heat of ice = 0. 5 cal · g^-1 · °C^-1; latent heat of fusion of ice = 80 cal · g^-1; latent heat of vaporisation of water = 540 cal · g^-1

Heat required to bring 1 g of ice from -10°C to 0°C

= 1 x 0.5 x {0 – (-10)} = 5 cal

Heat required to convert 1 g ice at 0°C to water at 0°C = 1 x 80 = 80 cal

Heat required to bring that water to 100°C 1 x 1 x (100-0) = 100 cal

To convert water at 100°C to steam at 100°C, heat required = 1 x 540 = 540 cal

∴ The total heat required to convert the ice to steam = 5 + 80 + 100 + 540 = 725 cal.

Example 2. What will be the consequence of extracting 64800 cai of heat from 100 g of steam at 100°C? Latent heat of condensation of steam = 540 cal · g^-1; latent heat of fusion of ice = 80 cal · g^-1.
Solution:

Given

Latent heat of condensation of steam = 540 cal · g^-1; latent heat of fusion of ice = 80 cal · g^-1.

The heat extracted from 100 g of steam at 100°C to form water at 100°C = 100 x 540 = 54000 cal.

Then, heat extracted to bring that water to 0°C = 100 x 100 = 10000 cal

The total heat extracted in these two stages = 54000 + 10000 = 64000 cal

The remaining amount of extracted heat = 64800 – 64000 = 800 cal. It freezes some amount of water into ice.

∴ The amount of ice formed = 800/80 = 10 g.

∴ On extracting 64800 cal of heat, the specimen will consist of 10 g of ice at 0°C and (100 – 10) g i.e., 90 g of water at 0°C.

Example 3. 100 g of steam is passed through a mixture of 1 kg ice and 1 kg water. The entire amount of steam is converted into water. What is the final temperature of the mixture? What amount of ice will melt? The latent heat of fusion of ice is 80 cal • g^-1 and that of condensation of steam is 540 cal • g^-1.
Solution:

Given

100 g of steam is passed through a mixture of 1 kg ice and 1 kg water. The entire amount of steam is converted into water.

The initial temperature of the mixture of ice and water = 0°C

The heat released by 100 g steam at 100°C to be converted to water at 100°C = 100×540 = 54000 cal

Heat released by that water from 100°C to 0°C = 100 x 100 = 10000 cal

∴ Total heat released by steam = 54000 + 10000 = 64000 cal

The heat required to melt 1 kg ice = 1000 x 80 = 80000 cal

So, all the ice will not melt as the total heat released is less.

So, the amount of ice that melts = 64000/80 = 800 g

and the final temperature of the mixture will be 0°C.

Example 4. Divide 1 kg of water at 5°C in two parts such that the heat released in freezing one part into ice at 0 C can be used to convert the other part into steam. Latent heat of the solidification of water and that of the vaporisation of water are 80 cal · g^-1 and 540 cal · g^-1 respectively.
Solution:

Given

Latent heat of the solidification of water and that of the vaporisation of water are 80 cal · g^-1 and 540 cal · g^-1 respectively

Solution: Let the mass of the first part of water = x g.

∴ Mass of the remaining part of water = (1000 – x) g

The heat released by xg water for converting into ice = x x (5 – 0) + x x 80 = 85x cal

The heat required to convert (1000 – x) g of water to steam = (1000 -x)x (100 – 5) + (1000 – x) x 540 = (1000-x) x 635 cal

∴ 85x = (1000-x) x 635

or, 720x = 635000

or, \(=\frac{63500}{72}=881.9 \mathrm{~g}\)

∴ Mass of the first part = 881.9 g and that of the other part = 1000-881.9 = 118.1 g.

Example 5. A certain amount of water is heated from CTC to 100°C in an electric kettle. It takes 15 min to raise the temperature and 80 min to completely convert the.water to steam. What is the latent heat of the vaporisation of water?
Solution:

Given

A certain amount of water is heated from CTC to 100°C in an electric kettle. It takes 15 min to raise the temperature and 80 min to completely convert the.water to steam.

Let amount of heat generated in the kettle every minute = H cal.

Amount of water contained in the kettle = x g

∴ According to the given data,

15H = \(x(100-0) or, \text { or, } \frac{H}{x}=\frac{100}{15}=\frac{20}{3} \mathrm{cal} \cdot \mathrm{g}^{-1}\)

Again, in case of vaporisation, 80 H – x x L [where L = latent heat of steam)

or, 80\(\frac{H}{x}\)= L

or, L = 80 \(\times \frac{20}{3}=533.3 \mathrm{cal} \cdot \mathrm{g}^{-1}\)

∴ Latent heat of steam = 533.3 cal · g^-1.

Factors Influencing Boiling Point

1. Nature of the liquid: The boiling point of a liquid depends on the nature of the liquid. Different liquids have different boiling points. More volatile a liquid, the less its boiling point is.

2. Pressure on the liquid: The boiling point of a liquid depends on the pressure on the liquid. If the pressure decreases, boiling point decreases and vice versa.

3. Presence of impurities in the liquid: Boiling point of a pure liquid increases in presence of dissolved impurities. Saline water has a boiling point about 9 °C higher than pure water at the same pressure. Suppose, there is a possibility’ of presence of dissolved impurities in a liquid.

In this case, to determine the boiling point of the pure liquid, a thermometer is held in the vapour right above the liquid (instead of immersing it in the liquid). This works because if pressure on the liquid remains unchanged, the temperature of the vapour and the boiling point of the pure liquid (at the same pressure) are the same.

Effect of Pressure on Boiling Point: Boiling point of a liquid depends on the pressure on it. When pressure is reduced, the boiling point decreases. If the pressure on a liquid surface is increased, the boiling point of the liquid increases.

During vaporisation of any liquid, its volume increases. The pressure on the liquid opposes this expansion in volume. So to boil the liquid, it needs to be brought to a higher temperature.

Effect of Pressure on Boiling Point Application:

1. Decrease in the boiling point with the decrease in pressure—this property of a liquid is utilised in different cases. For example,

1. In preparing a concentrated solution of hydrogen peroxide,
2. Making of condensed milk,
3. Forming sugar crystals from a sugar solution etc.

2. Increase in the boiling point with the increase in pres¬sure—this property is also utilised in different cases. For example,

1. Cooking in a pressure cooker,
2. Making paper pulp from sawdust and caustic soda,
3. Making artificial silk,
4. Sterilization of surgical instruments, bandages etc.,
5. Preservation of food materials in tin containers,
6. Extraction of pure alumina from bauxite, etc.,
7. Modem electric power plants utilise water at high temperatures by exerting high pressure.

Effect of altitude on boiling point: The atmospheric pressure decreases with increase ip altitiicje from the earth’s surface. If the altitude from the earth’s stirface is not very high, the decrease in pressure is about 85 mm of mercury for every kilometer rise in altitude.

• We know that the boiling point of a liquid depends on the pressure on its surface. So, the boiling point decreases with the rise in altitude. As the atmospheric pressure is low on mountains, water boils at a temperature lower than 100°C.
• So it is harder to boil anything on mountains. It has been calculated that on the peak of Mt. Everest (about 9 km high) water boils at only 70°C. At Darjeeling (about 2.2 km high) the boiling point of water is 93.6°C.

Determination of altitude from boiling point: The altitude of a place or the difference in altitudes of two places can be determined by measuring the boiling point of water.

Let the atmospheric pressures at the base (A) and at the top (B) of a hill be p_A and p_B respectively. The boiling points of water at A and at B are determined using a pressure hypsometer. Let these be T_A and T_B respectively.

The normal boiling point of water is 100°C. Although the vapour pressure variation with temperature is non-linear in nature, the boiling point variation can be approximated near 100°C by an empirical result.

For a 2.7 cm Hg rise or fall of pressure, the normal boiling point of water rises or falls by 1°C approximately.

Let pressure difference between A and B be h cm Hg.

∴ h = p_A -p_B=2.7(T_A-T_B)…(1)

Now, Hρg = h x 13.6 x g

[where p is the average density of air]

H = \(\frac{h \times 13.6}{\rho}\)

We get from (1) and (2)

H = \(\frac{2.7\left(T_A-T_B\right) \times 13.6}{\rho} \mathrm{cm}\)

∴ H = \(\frac{2.7\left(T_A-T_B\right) \times 13.6}{\rho} \times 10^{-2} \mathrm{~m}\)

Laws of Boiling: The facts related to boiling can be expressed in the form of the following laws. These law’s are known as laws of boiling.

1. Every liquid has a natural boiling point. The tempera¬ture at which a liquid starts boiling at normal atmospheric pressure is its normal boiling point. During boiling, the temperature remains constant until all the liquid changes to vapour.
2. Boiling point increases with the increase in pressure on die liquid and decreases when the pressure is lowered.
3. Boiling point of a liquid increases due to the presence of dissolved substances. But the temperature of the vapour above the liquid remains equal to the boiling point of the pure liquid.

Comparison Between Melting And Boiling Similarity:

1. Both melting and boiling produce change of state at respective fixed temperatures, called melting point and boiling point, and these temperatures remain constant as long as the change continues.
2. In both processes heat is absorbed.
3. Both processes are affected by any change in pressure on the substance. In other words, both melting point and boiling point of a substance depend on pressure on the substance.
4. In both processes there occur changes in volume.
5. Unstirred pure liquid can be gradually cooled below its melting point and it can still retain its liquid state. This is called supercooling. Similarly, unstirred pure liquid can be gradually heated above its boiling point and it can still retain its liquid state. This is called superheating. However both the states are very unstable. Even the slightest change in environment can change these states.

Comparison Between Melting And Boiling Dissimilarity:

1. Freezing point or melting point of a solution is always less than that of the pure solvent, but boiling point of a solution is always more than that of the pure solvent.
2. The volume of a solid may increase or decrease on melting, depending on the material. However, the volume of all liquids increase on boiling.