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VAPOUR AND HUMIDITY

EcoleBooks | PHYSICS O LEVEL(FORM THREE) NOTES - VAPOUR AND HUMIDITYContent
  • Evaporation
  • Factors affecting evaporation
  • Saturated vapor
  • Humidity
  • Rotation humidity (R. H)
  • Measuring Rotation humidity (R. H)
  • Dew point
  • Factors influence dew formulation
  • Application of evaporation
EVAPORATION
This is a gradual change of state from liquid to gas that occurring on surface of liquid.
According to kinetic theory of matter, when molecules of liquid gain heat they consequently gain kinetic energy .
As result of the random speed of the molecules increases. When the molecules at the surface gain large amount of energy they escape to the surrounding environment.This process is known as evaporation.
The molecules that have strong bonding force evaporate hardly. eg, evaporation of water , diesel etc. The molecules that have weak bonding force evaporate easily, eg , Methylated spirit , petrol.
Factor affecting evaporation
  • Temperature
  • Surface area
  • Concentration
  • Rate of air flow
1.TEMPERATURE
An increase in temperature increase the rate of evaporation . At high temperature the molecule are able to break out of the surface of the liquid and evaporate because they posses high kinetic energy (K.E). E.g. wet clothes hung on a line dry quickly on warm day

2. SURFACE AREA
We said that evaporation occur on a surface of a liquid. So when a surface area of a liquid is large also the rate of evaporation will be high because more molecule are escaping the surface of the liquid

3. CONCENTRATION
The concentration of the evaporating liquid in the surrounding air will determine the rate of its evaporation. When the concentration of the evaporating of liquid in air is high, the evaporation will be low and vice versa.

4. Rate of flow of air
Rate of flow of air determine the evaporation of liquid. In a wind environment the molecule will be carried away when arriving soon on the surface of liquid hence there is high rate of evaporation.
Saturated vapour pressure
Vapour pressure is the pressure created by the vapour of a substance that forms above of a liquid of the same substance.

EcoleBooks | PHYSICS O LEVEL(FORM THREE) NOTES - VAPOUR AND HUMIDITY
When the molecule gain K.E are able to escape from liquid. This is due to that temperature favour in raising the speed of molecules so when temperature increases also the rate of escaping molecule increase. These create more vapour pressure.
We said that as temperature increase also the rate of escaping molecule increase but other molecule return on it. So at any instant molecule are lowering the liquid and thus are returning to it. When the unsaturated more molecule leave the liquid than return to it per second and so the liquid evaporate until it all change into vapour.
Also when the spaces above the liquid become saturated i.e. some liquids present the number of molecule leaving the liquid per second equal to the number returning to it. The space above the liquid now has the maximum possible number of molecule at particular temperature
    • Saturated vapour pressure (SVP) if is in equilibrium
    • Unsaturated vapour pressure does not in equilibrium because if pressure is less than saturated vapour pressure S. V. P raising with him
EcoleBooks | PHYSICS O LEVEL(FORM THREE) NOTES - VAPOUR AND HUMIDITY
AMBIENT PRESSURE
Other gases that may be present beside the evaporating liquid also introduce pressure these are called ambient pressure e.g. atmospheric pressure.
Measuring saturated vapour pressure
Saturated vapour pressure is measured when vapour pressure is equal to atmospheric air by using Mercury Barometer.
    • A standard mercury barometer has tube with height about 76 cm closed at one and with an open mercury filled receiver at the base.
EcoleBooks | PHYSICS O LEVEL(FORM THREE) NOTES - VAPOUR AND HUMIDITY
The mercury in the tube adjusts until the weight of mercury column balance at atmospheric pressure (A.P) exerted on reservoirs.
High A.P exerts more force on the reservoir forcing the mercury in higher column. Lower pressures allow the mercury to drop to lower level in the column by lowering the reservoir.
At a pressure of 1atm = 760mm height of mercury. When you squirt a few drop of a liquid into mercury will form layers. Some of these liquid will evaporate at some time will condense and form saturated vapor pressure which force the mercury level down. This gives S.V.P the level of mercury has been forced down.
EFFECT OF TEMPERATURE ON S. V. P
If the temperature of the liquid/ solid increase the rate of evaporation and condensation increase and this leads to increase in vapor pressure.
NOTE: Liquid boils when vapor pressure is equal ambient pressure

HUMIDITY
This is the water vapor in the atmosphere. Water vapor enters in the atmosphere by evaporation from different source of water like river, ocean etc. Also transpiration form humidity.
Water vapor condenses to form dew, frost and clouds. The amounts of water vapor depend on the temperature of place so cause the water vapor varies from location and location. Also the level of saturation depends on temperature and availability of water.
RELATIVE HUMIDITY
We have seen that saturated vapor pressure increase as the temperature increases. This means that the high temperature there is some more vapor in situated air than at lower temperature.
Also the density of water vapor in saturated air is called Absolute humidity (AH)
EcoleBooks | PHYSICS O LEVEL(FORM THREE) NOTES - VAPOUR AND HUMIDITY
EcoleBooks | PHYSICS O LEVEL(FORM THREE) NOTES - VAPOUR AND HUMIDITY
The passage of the air through the ether causes it to evaporate and the temperature of the liquid to fall in consequence, while the bubbling ensures the mixing of the different layers of liquid, and therefore very approximately, at any rate, a uniform temperature of silver, ether, and thermometer. The passage of air is continued until a deposit of dew is seen on the silver, which shows that the temperature of the silver is below the dew-point. The thermometer is then read, and the temperature of the apparatus allowed to rise until the deposit of moisture has completely disappeared, when the thermometer is again read. The temperature is now above that of the dew-point, and the mean of the two readings so obtained may be taken as the temperature of the dew-point, provided that there is no more difference than two or three tenths of a degree centigrade between them.

1. Evaporation of perspiration cools our bodies.
2. Evaporation of water from the earth keeps the air moist and provides the moisture for clouds. Thus evaporation has an important influence on weather and climate.
3. It is important also in air conditioning, refrigeration, and distilling. Substances are evaporated in a partial vacuum to speed evaporation or to prevent deterioration by heat. Instant coffee and frozen orange juice is evaporated in this way, and so are blood plasma, vaccines, and antibiotics such as penicillin. Camphor is purified by sublimation.
4. In industry, the principles of evaporation and refrigeration are used to great extent in the treatment, separation, handling and storage of materials in any of the three states of matter – Solid, Liquid or Gas. Distillation concerns the separation of liquid mixtures by evaporation processes.
5. Evaporation of sea water to produce salt
6. Evaporation ponds are used to prevent pesticides, fertilizers and salts from agricultural waste water from contaminating the water bodies they would flow into.

BONUS –> Some of our power plants work by evaporation. The evaporation is the final step of water cycling driving small turbines before it is fed back into the primary pool.


EcoleBooks | PHYSICS O LEVEL(FORM THREE) NOTES - VAPOUR AND HUMIDITY

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1 Comment

  • EcoleBooks | PHYSICS O LEVEL(FORM THREE) NOTES - VAPOUR AND HUMIDITY

    Harry Justin, December 17, 2023 @ 5:39 pm Reply

    I have enjoyed so much thanks for
    provide us the privacy studying

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