Evaporation of Liquids  
Evaporation is a gradual change of state from liquid to gas that occurs at the surface of a  
liquid.  
Factors affecting the rate of Evaporation  
•
•
There are four primary factors that affect the rate of evaporation; temperature, surface area,  
wind, and humidity.  
Effect of Temperature on Evaporation  
•
•
Temperature is directly proportional to the rate of evaporation  
The wet clothes dry faster on a hot sunny day because of the high temperature. When the  
temperature is high, the kinetic energy of the water particles increases, leading to the  
quicker conversion of liquid water into its gaseous state. This explains why hot water  
evaporates more quickly than cold water.  
Effect of Surface Area on Evaporation  
•
Surface area is directly proportional to the evaporation increase in Surface area increases the  
rate of evaporation  
•
Suppose there is water in a wide shallow bowl, and the same amount of water is also kept in  
a narrow glass. In which one of the two situations will the water evaporate quickly? The water  
in the bowl will evaporate much faster than in the glass because the larger surface area gives  
more space for the water particles to evaporate.  
•
You might have also noticed that the wet clothes dry quickly when they are spread out  
properly on the clothesline compared to the clothes that are not spread out properly. This is  
again due to the larger surface area of the spread-out wet clothes, leading to faster water  
evaporation.  
Effect of humidity on Evaporation  
•
Evaporation is also affected by humidity, or the amount of water vapor in the air. The lower  
the relative humidity, the dryer the air is and the faster it evaporates. The higher the humidity,  
the closer the air is to saturation, and less evaporation is possible  
•
Thus; Humidity and rate of evaporation are in inverse relation to each other. ie,. As the  
humidity decreases, the rate of evaporation increases  
.
Effect of Wind (The rate of flow of air) on Evaporation  
•
•
Wind is directly proportional to the evaporation. With rising wind speeds, the rate of  
evaporation of liquid increases. When the wind speed increases, the water vapor particles  
travel away with the wind, reducing the amount of water vapor in the environment. This  
increases the rate of water evaporation.  
For example; when the clothes are spread out on a clothesline on a windy day, they will dry  
up quicker than on a still day. This is because the water vapor in the surrounding air will  
move away from the clothes by the wind, leading to faster water evaporation from wet  
clothes.  
Applications of evaporation  
(i) Evaporation has cooling effect  
Because it takes thermal energy away from the surface. This is because the faster particles  
escape from the liquids surface leaving behind the slower ones and the faster ones have  
more energy, hence the temperature of the liquid drops  
(ii) Sweating uses the cooling effect caused by evaporation  
As the sweat evaporates, It takes away thermal energy from the skin to the surrounding  
(iii) Evaporation coolers can significantly cool a building by simply blowing dry air over a filter  
saturated with water  
(iv) Cooling effect from evaporation used in Refrigerators  
(v) In a clothes drier, hot air is blown through the clothes allowing water to evaporate very rapidly  
Vapour Pressure (VP)  
•
When a liquid evaporates in a closed container, the vapor formed above the liquid exerts a  
pressure. According to kinetic molecular theory, the molecules of the vapor are in constant  
motion and will hence exert a pressure just like the molecules of a gas. This pressure is called  
the vapor pressure of the liquid  
.
Question: How can you differentiate between gas and vapor  
•
A gas refers to a substance that has a single defined thermodynamic state at room  
temperature whereas vapor refers to a substance that is a mixture of two phases (ie.  
gaseous and liquid phase) at room temperature  
Types of Vapour Pressure (VP)  
▪
Saturated vapour pressure  
▪
Unsaturated vapour pressure  
Saturated Vapour Pressure (SVP)  
•
•
A saturated vapor is a vapor that is in contact with its own liquid within a confined space.  
When a partially filled container of liquid is sealed with a stopper, some liquid molecules at  
the surface evaporate into the vapor phase. However, the vapor molecules cannot escape  
from the container and so after a certain amount of time, the space above the liquid reaches  
a point where it cannot hold any more vapor molecules.  
•
Now some of the vapor molecules condense back into a liquid. The system reaches the point  
where the rate of evaporation is equal to the rate of condensation (ie,.. the number of  
molecules escaping from the liquid per unit time is equal to the number of molecules  
returning to the liquid per unit time). See Figure below.  
•
This is called a dynamic equilibrium between the liquid and vapor phases. At this state of  
equilibrium, the vapor is said to be saturated. The pressure of the vapor when it is saturated  
above the liquid surface is called the saturated vapor pressure (S.V.P).  
Unsaturated Vapour Pressure (U.S.VP)  
•
On the other hand, the unsaturated vapor is the vapor which is not in contact with its own  
liquid in a confine space. It is not in dynamic equilibrium with its own liquid. The rate at which  
the liquid evaporates is greater than the rate at which the liquid condenses. Thus, the  
pressure exerted by a vapor which is not in contact with its own liquid in a confined space is  
called unsaturated vapor pressure. See the figure below  
NB:  
•
•
A substance of high vapour at room temperature is called volatile  
Increase in temperature of a solid or liquid ,the rate evaporation or condensation increases  
which results to an increase in the vapor pressure  
Measurement of SVP  
•
•
•
Saturated vapour pressure is measured by mercury barometer.  
It is given by, SVP = (760 - x) mmHg  
Where: 760mmHg=atmospheric pressure (atm), X mmHg=vapour pressure  
Humidity  
•
Humidity is the measure of wetness of the atmosphere. The exact amount of water vapor in  
the atmosphere at a given temperature is called Absolute humidity. At higher temperature,  
the atmosphere contains more water vapor compared to water vapor present at low  
temperature.  
•
•
OR – Humidity –  
Is the amount of water vapour present in the atmosphere  
Water vapor from atmosphere condenses to form clouds, fog, dew and frost  
Sources of Humidity  
o
o
Evaporation from rivers, lakes and oceans  
Transpiration (evaporation of plant leaves)  
Dew point and its relationship to weather  
•
Dew – Is water in the form of droplets that appears on exposed objects in the morning or  
evening due to condensation  
•
•
The dew point is the temperature to which the air must be cooled in order for it to become  
saturated. It provides a measure of the actual amount of water vapor in the air – so the higher  
the dew point, the more moisture in the air.  
Fog is a cloud floating just above the ground. It is formed when water vapor in the air is  
cooled down to its dew point. Fog is of more effect than the mist as it can reduce visibility to  
less than 200m.  
•
•
•
Mist is the condensation of vapour into water droplets occurring near the ground. Mist limits  
visibility to about 1000m or less.  
Hailstones are water droplets in clouds formed due to super cooling below 00 C without  
freezing  
Frost is a deposit of small white ice crystals formed on the ground or other surfaces when the  
temperature falls below freezing  
•
Snow is formed when the dew point is below the freezing point (00 C)  
NB:  
✓
Hot Air contains more moisture (humidity) than cold air,  
✓
Dew is formed at night because hot air comes into contact with a cold surface ,now water vapour  
present in it condenses on the cold surface in the form of droplets (dew drops)  
✓
✓
The formation of dew is more when the sky is clear and less when it is cloudy.  
DP is measured by Renault hygrometer  
✓
✓
DP occurs when RH of air is 100%  
Below DP clouds, dew or frost formed  
Factors influencing the formation of Dew  
(a) Temperature  
•
The warmer the air, the more humid is the air. For this case, at low temperature, air will take  
shorter time to be saturated. Since the air saturate quickly at low temperature than at high  
temperature, dew form quickly at low temperature than at high temperature. The temperature  
of the atmospheric air must fall below the dew point for dew to be formed  
(b) Wind  
As wind increases, the rate of evaporation also increases, thus it prevents the formation of  
water droplets (dew)  
(c) Water Vapour  
•
•
The atmospheric air must be saturated with water vapour for dew to be formed  
(d) Cloud cover  
•
When a surface loses heat to the atmosphere, it cools, and can form dew on that surface.  
Cloud cover prevents the heat from escaping and thus, hinders dew formation. The absence  
of clouds will allow heat to escape to outer space hence promoting dew formation  
Experiment to show the Presence of water vapour in the Air  
Water vapor is contained in the atmosphere, which can be easily depicted using the following  
experiment:  
Apparatus – A glass beaker and ice cubes  
Procedure – The following procedure can be performed to show the presence of water vapour in  
air :  
•
•
Take the glass beaker and dry it from outside.  
Place the ice cubes in the beaker.  
•
Leave the ice along with the glass untouched for some minutes.  
Observations;  
Water droplets appear on the outer surface of the glass, and it can be proved that it is water  
vapor by using a dry cobalt chloride paper that turns pink in the presence of water.  
•
Conclusion;  
The water vapour present in the air condenses on the cold surface of the glass. This shows that  
air contains water vapour.  
•
Relative Humidity (RH)  
•
Relative Humididty –Is the ratio of the saturated vapour pressure at the dew point to the  
saturated vapour pressure at the current air temperature  
Mathematically:  
•
푺푽푷 풂풕 풅풆풘 풑풐풊풏풕  
푹. 푯 =  
풙 ퟏퟎퟎ%  
푺푽푷 풂풕 풕풉풆 풐풓풊품풊풏풂풍 풂풊풓 풕풆풎풑풆풓풂풕풖풓풆  
OR  
•
•
Relative humidity is the ratio of the Actual vapour density to the saturated vapour density  
Mathematically:  
풂풄풕풖풂풍 풗풂풑풐풖풓 풅풆풏풔풊풕풚 (풑풓풆풔풖풓풆)  
푺풂풕풖풓풂풕풆풅 풗풂풑풐풖풓 풅풆풏풔풊풕풚 (풑풓풆풔풖풓풆)  
•
Since absolute humidity is the density of water vapor in saturated air  
Thus  
풂풃풔풐풍풖풕풆풉풖풎풊풅풊풕풚  
•
Actual vapour density and Saturated vapour density has equal volume, therefore relative  
humidity can also be defined as  
Absolute Humidity  
•
Abosolute humidity – Is the mass of water vapor divided by the mass of dry air in a certain  
volume of air at a specific temperature, ie.,  
풎풂풔풔 풐풇 풘풂풕풆풓 풗풂풑풐풓  
풎풂풔풔 풐풇 풅풓풚 풂풊풓  
푴ퟏ  
푴ퟐ  
(
)
푨퐛퐬퐨퐥퐮퐭퐞 퐡퐮퐦퐢퐝퐢퐭퐲 퐀. 퐇 =  
=
Also can be defined as;  
Specific Humidity (Humidity Ratio)  
Is the ratio of water vapour to dry air in a particular mass  
풎풂풔풔 풐풇 풘풂풕풆풓 풗풂풑풐풓  
푴풗  
Thus, Specific humidity (S.H)  
=
=
풎풂풔풔 풐풇 풅풓풚 풂풊풓  
푴풂  
NB;  
The difference between air temperature and dew point temperature can indicates whether the  
Relative humidity is LOW or HIGH  
•
•
When the air and the dew point temperature are far apart, the Relative humidity is low  
When the air and the dew point temperature are close to the same value, the Relative humidity  
is high  
•
When the air and the dew point temperature are the same, the air is saturated and the Relative  
humidity is 100%  
Worked examples  
1. If the actual vapor density is 12 g/m3 at 200 compared to the saturation vapor density at that  
temperature of 20.5 g/m3, then what will be its relative humidity?  
Solution:  
AVD = 12g/m3, SVD = 20.5 g/m3, T = 200 C  
풂풄풕풖풂풍 풗풂풑풐풖풓 풅풆풏풔풊풕풚(풑풓풆풔풖풓풆)  
From: RH =  
x
100%  
푺풂풕풖풓풂풕풆풅풗풂풑풐풖풓 풅풆풏풔풊풕풚(풑풓풆풔풖풓풆)  
ퟑ
ퟏퟐ 품/풎  
∴ 푹. 푯 =  
x 100% = ퟓퟖ. ퟓ%  
ퟑ
ퟐퟎ.ퟓ 품/풎  
2. A mass of air at 200C has a relative humidity of 36%. If the air is cooled to 160C, what will its  
relative humidity be? Saturation vapor pressure at 200C and 160C are 17.5mmHg and 13.6mmHg  
respectively.  
Solution;  
풂풄풕풖풂풍 풗풂풑풐풓 풑풓풆풔풔풖풓풆  
From; Relative humidity  
=
× ퟏퟎퟎ%  
풔풂풕풖풓풂풕풊풐풏 풗풂풑풐풓 풑풓풆풔풔풖풓풆  
ퟑퟔ% = 풂풄풕풖풂풍 풗풂풑풐풓 풑풓풆풔풔풖풓풆  
× 100 → 푉푎푝표푟 푝푟푒푠푠푢푟푒 = ퟔ. ퟑmmHg  
ퟏퟕ.ퟓ  
풗풂풑풐풓 풑풓풆풔풔풖풓풆  
ퟔ.ퟑ  
Relative humidity  
=
× ퟏퟎퟎ% =  
× ퟏퟎퟎ%  
풔풂풕풖풓풂풕풊풐풏 풗풂풑풐풓 풑풓풆풔풔풖풓풆  
ퟏퟑ.ퟔ  
ퟔ.ퟑ  
Therefore; Relative humidity =  
× ퟏퟎퟎ% = ퟒퟔ. ퟑ%  
ퟏퟑ.ퟔ  
3. Late on an autumn day, the relative humidity is 45.0% and the temperature is 20.0ºC. What will  
the relative humidity be that evening when the temperature has dropped to 10.0ºC, assuming  
constant water vapor density?  
Given that the saturation vapor density at 200C and 100C are 17.2g/m3 and 9.40g/m3 respectively  
풗풂풑풐풓 풅풆풏풔풊풕풚  
From; Relative humidity  
=
× ퟏퟎퟎ%  
풔풂풕풖풓풂풕풊풐풏 풗풂풑풐풓 풅풆풏풔풊풕풚  
ퟒퟓ% = 푽풂풑풐풓 풅풆풏풔풊풕풚 × ퟏퟎퟎ → 푽풂풑풐풓 풅풆풏풔풊풕풚 = ퟕ. ퟕퟒg/m3  
ퟏퟕ.ퟐ  
풗풂풑풐풓 풅풆풏풔풊풕풚  
풔풂풕풖풓풂풕풊풐풏 풗풂풑풐풓 풅풆풏풔풊풕풚  
ퟕ.ퟕퟒ  
ퟕ.ퟕퟒ  
Relative humidity  
=
× ퟏퟎퟎ% =  
× ퟏퟎퟎ%  
ퟗ.ퟒ  
Therefore; Relative humidity =  
× ퟏퟎퟎ% = ퟖퟐ. ퟑ%  
ퟗ.ퟒ  
4.  
A
closed vessel contains moist air at 200C, the relative humidity being 30%. What should be the  
relative humidity if the vessel were cooled to 100C? saturation vapor pressure of water at 200C is  
17.5mmHg and at 100C is 9.2mmHg.  
Solution;  
풂풄풕풖풂풍 풗풂풑풐풓 풑풓풆풔풔풖풓풆  
From; Relative humidity  
=
× ퟏퟎퟎ%  
풔풂풕풖풓풂풕풊풐풏 풗풂풑풐풓 풑풓풆풔풔풖풓풆  
ퟑퟎ% = 풂풄풕풖풂풍 풗풂풑풐풓 풑풓풆풔풔풖풓풆× 100 → 푉푎푝표푟 푝푟푒푠푠푢푟푒 = ퟓ. ퟐퟓmmHg  
ퟏퟕ.ퟓ  
풗풂풑풐풓 풑풓풆풔풔풖풓풆  
ퟓ.ퟐퟓ  
Relative humidity  
=
× ퟏퟎퟎ% =  
× ퟏퟎퟎ%  
풔풂풕풖풓풂풕풊풐풏 풗풂풑풐풓 풑풓풆풔풔풖풓풆  
ퟗ.ퟐ  
ퟓ.ퟐퟓ  
Therefore; Relative humidity =  
× ퟏퟎퟎ% = ퟓퟕ%  
ퟗ.ퟐ  
5. The temperature and relative humidity of air on a certain day are 200C and 80%, respectively.  
Find the fraction of mass of water vapor that will condense if the temperature falls to 50C.  
Saturation vapor at 200C and 50 C are 17.5mmHg and 6.5mmHg, respectively.  
Solution;  
From; Relative humidity  
풂풄풕풖풂풍 풗풂풑풐풓 풑풓풆풔풔풖풓풆  
=
× ퟏퟎퟎ%  
풔풂풕풖풓풂풕풊풐풏 풗풂풑풐풓 풑풓풆풔풔풖풓풆  
ퟖퟎ% = 풂풄풕풖풂풍 풗풂풑풐풓 풑풓풆풔풔풖풓풆× 100 → 푉푎푝표푟 푝푟푒푠푠푢푟푒 = ퟏퟒmmHg  
ퟏퟕ.ퟓ  
NB;  
•
In order to solve this question, advanced formula of ideal gas equation may be applied.  
Ideal gas equation  
풎
푷 푽푴  
ퟏ
푷 푽푴  
ퟐ
푷푽 = 풏푹푻 = 푹푻  
→ 풎 =  
, 풎 =  
ퟏ
ퟐ
푹푻  
ퟏ
푴
푹푻  
ퟐ
푻ퟏ = ퟐퟕퟑ + ퟐퟎ = ퟐퟗퟑ푲, 푻ퟐ = ퟐퟕퟑ + ퟓ = ퟐퟕퟖ푲  
푷ퟏ = ퟏퟒ풎풎푯품, 푷ퟐ = ퟔ. ퟓ풎풎푯품  
When substituting for 풎ퟏ and 풎ퟐ  
In the fraction of water vapor condensed =  
;
풎
ퟐ
풎ퟏ−풎ퟐ  
= ퟏ −  
,
we will get  
풎
ퟏ
풎
ퟏ
푹푻ퟏ  
풎ퟐ  
풎ퟏ  
푷ퟐ푽푴  
푹푻ퟐ  
푷ퟏ푽푴  
푹푻ퟏ  
푷ퟐ푽푴  
)
ퟏ −  
= ퟏ − (  
) ÷ (  
) = ퟏ − (  
) × (  
푹푻ퟐ  
푷ퟏ푽푴  
ퟏ − (푷ퟐ) × (푻  
ퟐퟗퟑ  
푻
푷
ퟔ.ퟓ  
×
ퟏ) = ퟏ − (푻ퟏ) × (푷ퟐ) = ퟏ − ퟐퟕퟖ  
ퟏퟒ  
푻
ퟐ
푷
ퟏ
ퟐ
ퟏ
Therefore; the fraction of water vapor condensed = ퟏ − ퟎ. ퟒퟗ = ퟎ. ퟓퟏ  
6. A mass of air has a relative humidity of 57% and a dew point of 20˚C. What is the temperature of  
the air mass?  
Solution;  
푫풆풘 풑풐풊풏풕풕풆풎풑풆풓풂풕풖풓풆  
ퟐퟎퟎퟎ  
ퟓퟕ  
From; RH =  
x 100% ∴ RH = ퟐퟎ x 100% → 풙 =  
= ퟑퟓ. ퟎퟗ0C  
푶풓풊품풊풏풂풍 풂풊풓 풕풆풎풑풆풓풂풕풖풓풆  
풙
Difference between Absolute humidity and Relative humidity  
Absolute humidity  
Relative humidity  
Is the actual amount of water vapor present in  
the air  
Is a percentage of the amount of moisture the  
air could possibly hold  
It is expressed in moisture per cubic meter of air  
It is expressed in percentage as the ratio of  
(g/m3)  
vapor pressure to saturated vapor pressure  
If the temperature goes up, relative humidity  
goes down and vice – versa  
It is totally independent of the temperature  
Measurement of Relative Humidity  
•
•
It is measured by dry and wet bulb hygrometer and Renault hygrometer  
Hygrometer is an instrument used to measure the amount of humidity and water vapor in the  
atmosphere ,in soil or in confined spaces  
Dry and Wet Bulb Hygrometer  
•
•
One type of instrument used to find Relative humidity is a Psychrometer.  
A Psychrometer consists of two thermometers. The bulb of one thermometer is covered with  
a wet cloth and is called the wet-bulb, whereas the bulb of the other thermometer is left bare  
and is referred to as the dry-bulb  
.
•
Relative humidity can be found by subtracting the temperature on the wet-bulb thermometer  
from the temperature on the dry-bulb thermometer and using a relative humidity chart.  
Worked Examples  
1. The dry bulb temperature reading of a hygrometer is 400 C and the wet bulb temperature reading  
is 300 C. What is the RH? (A: 48%)  
Solution;  
Dry bulb reading = 400C  
Wet bulb reading = 300C  
Difference in temperature = 400 – 300 = 100 C  
Therefore; from the Psychrometric table above, Relative Humidity = 48%  
2. A dry bulb thermometer reads 300 C and a wet bulb thermometer reads 240 C .What is the  
Relative Humidity of the air  
Solution  
:
Dry bulb reading = 300C  
Wet bulb reading = 240C  
Difference in temperature = 300 – 240 = 60 C  
Therefore; from the Psychrometric table above, Relative Humidity = 61%  
Renault Hygrometer  
•
•
It consists of an enclosed thin silver tube containing ether and a thermometer.  
There is also a tube through which air can be pumped into the ether.  
Diagram:  
Mechanism of Renault Hygrometer  
•
The heat transfer from atmosphere to ether by convection in a tube, Ether evaporates result  
cooling of the silver tube surface. Cooling continues until air adjacent to the outside surface of  
the tube becomes saturated with water vapour. Some water vapour condense outside the tube  
to form dew  
Applications of Humidity  
✓
✓
✓
✓
✓
It is used by meteorological departments to forecast the weather  
It is used to determine the appropriate site to locate cotton  
Electrical and electronic components are usually transported and stored in a dry air  
Used in hospitals in an operating room. RH at operating room is at least 50%  
It is used in storage and transportation of food items  
Class Activity – 9:1  
1. The dry bulb temperature reading of a hygrometer is 220C and the wet bulb temperature  
reading is 180C. What is the RH? (ANS: 68%)  
2. The dry bulb temperature reading of a hygrometer is 400 C and the wet bulb temperature  
reading is 250 C. What is the RH? (ANS: 29%)  
3. The relative density of a place was measured at 250 C and found to be 53.6%. if the absolute  
humidity is 23.05g/m3, determine the actual water vapour density at this experiment  
(
ANS: AVD = 12.35 g/m3)  
4. At a certain temperature and pressure, air can hold a maximum of 120g of water vapor. If this  
temperature and pressure the air is holding only 40g of water vapor, what is the relative  
humidity of the air? [ANS; R.H = 33.3%]  
5. The air in a room has 15g of water vapor per cubic meter. However, for saturation, one cubic  
meter of volume requires 20g of water vapor. What is the relative humidity. [ANS R.H = 75%]  
6. If the actual vapor density is 10 g/m3 at 20°C compared to the saturation vapor density at that  
temperature of 17.3 g/m3 , what is the relative humidity [ANS: 57.8%]  
7.  
A room with dimensions of 7m x 10m x 2m, holds air that is saturated with water vapor. The  
saturation vapor pressure of the water vapor is 7.37kPa. If all of the water vapor in the room  
was condensed, what volume would the water occupy? Give your answer in m3  
[ANS; V = 7x10x2 = 140m3]  
8. On a particular day, the relative humidity is 100% and the room temperature is 300C. What is  
the dew point? The saturated vapor pressure of water at 300C is 42400Pa [ ANS; 300C]  
Definition of Some terms used in this topic  
1. Dry air. The pure dry air is a mixture of a number of gases such as nitrogen, oxygen, carbon  
dioxide, hydrogen, argon, neon, helium etc. But the nitrogen and oxygen have the major portion  
of the combination.  
2. Moist air. It is a mixture of dry air and water vapour. The amount of water vapour present in the  
air depends upon the absolute pressure and temperature of the mixture.  
3. Saturated air. It is mixture of dry air and water vapor, when the air has diffused the maximum  
amount of water vapor into it. The water vapours, usually, occur in the form of superheated  
steam as an invisible gas. However, when the saturated air is cooled, the water vapour in the  
air starts condensing, and the same may be visible in the form of moist, fog or condensation  
on cold surfaces  
.
4. Dry bulb temperature. It is the temperature of air recorded by a thermometer, when it is not  
affected by the moisture present in the air.  
5. Wet bulb temperature. It is the temperature of air recorded by a thermometer, when its bulb is  
surrounded by a wet cloth exposed to the air. Such a thermometer is called Wet bulb  
thermometer  
.
6. Wet bulb depression. It is the difference between dry bulb temperature and wet bulb  
temperature at any point. The wet bulb depression indicates relative humidity of the air  
7. Dew point depression. It is the difference between the dry bulb temperature and dew point  
temperature of air.  
Class Activity – 9  
1. Define evaporation and state the factors which affect the rate of evaporation of a liquid .How  
does the kinetic theory account for the cooling produced in a liquid which is evaporating  
2. Distinguish between :  
(a) snow  
(b) hailstones  
(c) mist  
3. Define the following terms (a) Dew  
(b) Dew point (c) Specific humidity  
4. What factors determine saturation of vapour pressure?  
5. Explain the principles used to measure relative humidity?  
6. Explain the factors that affecting evaporation  
7. Explain the difference between a vapour and a gas  
8. At a given pressure the thermometer of a wet bulb reads 210 C. If the Relative Humidity is 30 %,  
what is the temperature of the air?  
9. A mass of air has a relative humidity of 57% and a dew point of 20˚C. What is the temperature  
of the air mass?  
10. Why does air cool when it rises through the atmosphere? (ANS: As air rises, it expands  
because air pressure decreases with an increase in altitude. When expands, it cools  
adiabatically)  
11. The table gives the temperature and dew point in four towns at 12 noon.  
City  
Temperature (0C)  
Dew Point  
Arusha  
16  
24  
3
1
12  
5
Morogoro  
Zanzibar  
Dar es salaam  
28  
4
(a) In which town is the relative humidity the highest?  
(b) In which town is the relative humidity the slowest?  
12. Briefly explain the reasons for the following:  
(a) When a cold bottle is brought into a warm room, it becomes misted over  
(b) Frost is more likely to occur on a clear night than on a cloudy night.  
13. (a) What does the term saturation of water vapor mean?  
(b)What is the difference between vapour pressure and saturated vapour pressure  
14. Differentiate between  
(a) Absolute humidity and Relative humidity  
(b) Saturated and unsaturated vapour  
(c) Evaporation and Boiling  
15. Explain why dew is formed at night.  
16. What are the two processes that change a liquid into a gas. The state their difference  
17. A student is investigating the evaporation of water in the Laboratory. He pours 200 cm3 of water  
in a 250 cm3 beaker. He pours another 200 cm3 of water in a 1000 cm3 .The water in both  
beakers is initially at 500 C  
(a) Use the kinetic theory of matter to explain which beaker evaporates faster  
(b) List three other factors that affect evaporation of a liquid  
(c) Explain why the liquid that remains during evaporation cools  
18. The actual vapor density of a region at 230 C temperature is 15g/m3 , if the saturation vapour  
density at that temperature of 21.3 g/cm3 determine the region ‘s relative humidity  
19. At a temperature of 300 C the mass of water vapour in town K is 22 g while dry air has a mass of  
15 g. Determine the specific humidity of town K at the stated temperature  
20. A mass of air at 300 holds 15 g/cm3of water vapor .If the saturation point of the air is 30  
g/cm3 ,calculate the relative humidity of the air  
21. If the dry bulb temperature is 320 C and the wet bulb temperature is 240 C, what is the relative  
humidity of the air? [ANS; R.H = 51%]