Vapour The Process of Evaporation of Liquid Explain the process of evaporation of iguid Vapours

• These are molecules which escape into the atmosphere after liquids are heated.
• When a liquid is heated strongly then molecules tends to escape ( those molecules are

called vapour)

• Most liquids evaporates at any temperature however liquids may vary in the rate at which

they evaporate at ordinary temperature.

• Alcohol and ether evaporate rapidly but lubricating oil and mercury hardly evaporate
• Evaporation of a liquids result in the formation of vapour

Factors Affecting Evaporation of a Liquid

Identify factors affecting evaporation ofa liquid There are several factors which affect evaporation of liquids when heated which include the following a Nature of the liquid: Normally liquids evaporation differs depending on the nature of liquid. Example; Volatile liquids evaporate faster than non-volatile liquids, which evaporate slowly Alcohol evaporates faster than other liquids like water. The boiling point of alcohol is

78°C while that of water is 100°C > Pressure above the liquid (atmospheric pressure): When the atmospheric pressure is high, the rate of evaporation may be reduced. © Surface energy of the liquid: This forms a boundary or skin between the liquid and the atmosphere, The surface energy prevents molecules with lower kinetic energy from escaping into the atmosphere. Some liquids such as alcohol have low surface energy , hence they evaporate

rapidly Why do molecules escapes when the liquid is heated? Wren the liquid is heated, the molecules tend to gain (absorb) kinetic energy hence the random speed of the moleeulesinereases.

The process ofevaporation oF liquid can be explained using Kinetic Theory When a iid is lef o evaporate ina closed container, the pressure ofthe vapour in the container gradually increases,

Difference between Saturated and Evaporation of a Liquid

Distinguish between saturated and evaporation of a liquid Vapours The molecules in a liquid are in a state of continuous motion and some of those at the liquid surface will gain sufficient energy to escape fiom the surface altogether. The molecules that have left the surface are said to be in the vapour state. The difference between a vapour and a gas is purely one of temperature, 2 vapour being a gas below its critical temperature

This phenomenon 1s known as evaporation. ‘The number of molecules leaving the surface, and henee the rate of evaporation, will increase with temperature as the liquid contains more eneray ata higher temperature. The effect of the evaporation of a liquid can be shown clearly by the following experiment.

Some ether is run into the flask, as shown in the figure below It will evaporate in the enclosed space and the pressure that it exerts on the water will force a jet of water out of the tube.

Warming the liquid will increase this evaporation and give a more powerfil jt >\e Figure # You can show that the rate of evaporation may be inereased by. a. Warming the flask gently.

b. _Imereasing the area of the liquid surface. © Blowing a stream of air across the surface 4 Reducing the pressure above the liquid surface Saturated vapours When a liquid isin a closed container the space above the liquid is full of vapour, and the vapour is then described as a saturated vapour this means that the density of the liquid molecules in the air is a maximum, This is due to molceules continually escaping and reentering the liquid. At any

rai moment the number of molecules Teaving the surface will be equal to the number returning to it and s0 2 dynamic equilibrium is set up The properties of saturated vapours were first investigated by Dalton around 1800. This is shown in Figure 2(a), which shows a state before saturation has been reached (when there will be more molecules leaving the surface than retuming to it) and Figure 2(b), which shows the saturated

state. A dynamic equilibrium exits here. Figure 2(@) Y 6 Figure 26) This vapour will exert a pressure and if there is sufficient liquid the air above the liquid surface will be saturated with vapour; the pressure that this saturated vapour exerts is known as the saturated vapour pressure (s.v-p.) of the liquid at that temperature.

Notice that since the velocity of the molecules inereases with temperature the saturated vapour pressure also increases with temperature, and therefore the temperature of the vapour must be specified when quoting its saturated vapour pressure (sv p.) The Effect of Temperature on Saturated Vapour Pressure (S.V-P) of a Liquid Explain she effect of temperature on somrated vapour pressure (S..P) ofa liquid Saturated vapour pressure (S.V.P):ls the pressure exerted by vapour when a liquid is heated

and reaches a state of Equilibrium where eventually the rate at which the molecules leave the liquid is equal to the rate at which others return to it The Height, of mercury represents the saturated vapour pressure of the liguid in the Mask Saturated vapour pressure increases with the increase in Temperature (Ti) and the inereased with decrease in Temperature (Ta) The graph of sarurared vapour pressure (syp) against Temperature

‘The graph shows as saturated vapour pressure (.v.p) increases then the temperature will increase and vice versa for decrease of temperature. Boiling point (B.P) Is the temperature reached where the saturated vapour pressure (SVP) is equal to external atmospheric pressure The Boiling point of alcohol is 78°C water is 100°C and pressure of the atmosphere as 76em oF mercury. The intersection ofthe normal atmosphere pressure line with the liquids S.V.P eurve.

Humidity

The Concept of Humidity

Explain he concept of humidity HUMIDITY is the measure of the extent to which the atmosphere contains water vapour (moisture)

The Formation of Dew

Explain the formation of dew DEW These are deposits formed when the temperature fall slowly in the drops of water vapour. Dew point (D.P): Is the temperature to which air must be cooled to become saturated. For

example in an air container, if water vapour at pressure oF 8mm of mercury were to be cooled

dew would form at 7.9°C

Measurement of Relative Humidity

Measure relative humidity Absolute humidity is the mass of water vapour present in a unit volume of it and is usually expressed in grams per cubic metre (gim*). The absolute humidity is not very frequently used since in practice we're more often concerned with the degree of wetness ofthe air.

Relative Humidity (RH) is the ratio of the mass of water vapour actually in a unit volume of air to that is required to saturate itat the same temperature RLH = Mass of water vapour/ Volume of air required to saturate the air atthe same temperature [tis common practice to quote relative Humidity as a fraction ora percentage Thus

RH=m/MX 100%

Where M = mass of water vapour actually present in a unit of given volume of air. M= mass of water vapour required to saturate the air at the same temperature. Relative Humidity is also defined as the saturation vapour pressure of water at the dew point divided by the ssturation vapour pressure of water atthe original air temperature, RH =s.vp at dew points.v.p at ar Temperature Note:The low value of relative humidity of air means that evaporation takes place readily from

the surface of water.The high value of relative Humidity then evaporation does not take place readily fiom the surface of water Humid atmosphereis saturated with water vapour. Perspitation: Is the evaporation of sweat from the skin, is not so effective at cooling the body in humid atmosphere Note:

• Cotton manufacturing industries are constructed on sites where the relative Humidity

(RH) Is High.

• Cotton fibres must not become too dry, otherwise they become Brittle and henee cause

difficulties in spinning,

• Imcontrast, = dry atmosphere is required by ware House for the storage of food, Tobacco

and assembling of certain electrical components

• Air — conditioning plants are installed in ships and buildings for the purpose of

moderating Humidity

MEASURING RELATIVE HUMIDITY

Hygrometers are instruments used for finding relative humidity at a given place. With most hygrometers, the relative humidity is determined by first finding the dew point and then using vapour pressure tables A very common type of Hygrometer (often known as Mason's hydrometer) A piece of muslin wiek is wrapped round the bulb of one the thermometer and its lower and dipped into water capillary action keeps the Muslin wrapped around the bulb wet

2s

• Evaporation of water surrounding the wet bulb absorbs heat from the bulb, consequently

‘stomp often als x: Rysnlng Fin wet bl omens il nd Ba see lage Eeanaple) If the dry the temperature is 30°C when the wet bull temperature is 20°C then the wet bulb depression is, 10°C the rate Evaporation depends on the amount of water Vapour present in the The ks mish aias te greet french wo tment eating The i REGNAULF'S HYGROMETER: le ud Go doumine div pnt and le Humidity Simplified form of regnault’s hygrometer consist of two test — tubes A and B with

sivabeais nl Reply famotte elm nen betas

• Airis bubbled through ether via the narrow tube F by applying a fitted pump at the end of

the Narrow tube F. This causes the rapid evaporation which results in the absorption of latent heat from ether and the container:

• Air surrounding the tube cools to temperature at which the water vapour present is

sufficient to saturate it

• Consequently is seen to form on C while D appears unchanged

ae]

F Nd

i Above show Regnault (dew point ) kygrometer

• The reading of the thermometer Tis then noted.
• The flow of air through the ether is then stopped and the apparatus allowed warming up.
• The temperature at which the dew on C disappears is noted,
• ‘The dew point is then taken as the mean of the two temperatures
• Suppose the dew pint is Q1 and the actual air temperature is Q2 ifthe tables the value of

S.V.P for water at QU and Q2 are X and Y millimeters of mercury Respectively.

• RH=S.V. Pat QI x 100%/S.V.P at 2

Example2

a7 The dew point in a room at a temperature of 10°e is 12,$5C if the saturated vapor pressure i? saturated vapor pressures at these two temperatures are 15.Smm and 109mm of mercury respectively calculate the Relative Humidity

Solution

Required: To find relative humidity, REL.

The Knowledge of Humidity in Daily Life Apply the knowledge of humidity in daily life Water in the atmosphere exists in different forms example: Clouds, Rain, Snow, Hail stones,

Mist, Fog and Smog

Clouds,

• Consist ofthe tiny droplets of water or Ice floating inthe sky formed by condensation of

water vapor in the upper atmosphere.

• Clouds formation occurs when temperature falls below the dew point and small particles

‘of dust or salt crystals are present to act as nuclei on which condensation can begin.

• Cooling is due to upward movement of air accompanied by example
• Clouds may also formed when a warm moist air current meets a cold one if the drops

become big enough by joining together they may fall as rain Rain: These are drops of water that fall on the grounds when cooling occurs in the clouds Snow: Formed when the dew points is below the freezing point (FP) 0°C-Under these conditions, the atmospherie vapour condenses directly into ice erystal Hail stones

• Formed due to super cooling of water droplets in such a way that the droplets are cooling

below 0°C without freezing

• When these droplets are carried upwards by ascending air currents, they solidify upon

coming into coniact with ice crystals in the upper atmosphere

• Hail stones are dangerous to aircraft and human beings.

Mist: Is the condensation of vapour Tito water droplets occurring near the ground Fog: Is a mist In which Visibility does not extend beyond Lk. Smog:This is dense fog, where visibility is reducing to a few metres