Vapour And Humidity

Vapour

The Process of Evaporation of Liquid

Explain the process of evaporation of liquid

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 temperatur e.

Alcohol and ether evaporate rapidly but lubricating oil and mer cury hardly evaporate.

Evaporation of a liquids result in the formation of vapour.

Factors Affecting Evaporation of a Liquid

Identify factors affecting evaporation of a 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

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0

C while that of water is 100

o

C.

b.

Pressure above the  liquid (atmospheric  pressure):

When  the  atmospheric  pressure is

high, the rate of evaporation may be reduced.

c.

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

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the atmosphere.  Some liquids  such as alcohol have low surface energy ,  hence they evaporate

rapidly.

Question Time 1

Why do molecules escapes when the liquid is heated?

When  the  liquid  is heated,  the  molecules  tend  to gain  (absorb) kinetic energy  hence  the random  speed of the

moleculesincreases.

The  process of evaporation of liquid can be  explained  using Kinetic Theory.When a liquid is left to evaporate

in a closed container, the pressure of the 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 from 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, a vapour being a gas below its critical temperature.

This  phenomenon is known as evaporation. The number of molecules leaving  the  surface, and

hence the rate of evaporation, will increase with temperature as the liquid contains more energy

at a  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 powerful jet.

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You can show that the rate of evaporation may be increased by:

a.

Warming the flask gently.

b.

Increasing the area of the liquid surface.

c.

Blowing a stream of air across the surface.

d.

Reducing the pressure above the liquid surface.

Saturated vapours

When a liquid is in 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 molecules continually escaping and reentering the liquid. At any

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moment the number of molecules leaving the surface will be equal to the number returning to it

and so a 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  returning to  it)  and Figure 2(b), which shows the saturated

state. A dynamic equilibrium exits here.

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.

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Notice that since the velocity of the molecules increases 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 (s.v.p.)

The Effect of Temperature on Saturated Vapour Pressure (S.V.P) of a Liquid

Explain the effect of temperature on saturated vapour pressure (S.V.P) of a liquid

Saturated vapour  pressure (S.V.P):

Is 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  mercur y  represents  the  saturated  vapour  pressure  of  the  liquid  in  the  flask.

Saturated vapour pressure increases with the increase in Temperature (Ti) and the incr eased with

decrease in Temperatur e (Td).

The graph of saturated vapour pressure (svp) against Temperature

The graph shows as saturated vapour pressure (s.v.p) increases then the temperature will increase

and vice versa for decrease of temperature.

Boiling  point  (B.P):Is  the  temperatur e reached where the  saturated  vapour  pressure (S.V.P)  is

equal to external atmospheric pr essure.

The Boiling point of alcohol is 78

C water is 100

C and pressure of the atmosphere as 76cm of

O

O

mercury.The intersection of the normal atmosphere pressure line with the liquids S.V.P curve.

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Humidity

The Concept of Humidity

Explain the 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.

0

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  (g/m

). The absolute  humidity is  not  very frequently  used

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since in practice we're more often concerned with the degree of wetness of the air.

Relative Humidity (R.H) is the ratio of the mass of water vapour actually in a unit volume of air

to that is required to saturate it at the same temperature.

R.H = Mass of water vapour/Volume of air required to saturate the air at the same temperature

It is common practice to quote relative Humidity as a fraction or a percentage.

Thus

R.H = m/M X 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.

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Relative Humidity

is also  defined as the saturation vapour pressure  of water at  the  dew point

divided by the saturation vapour pressure of water at the original air temper ature.

R.H = s.v.p at dew point/s.v.p at air 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 from the surface of water.

Humid atmosphere

is saturated with water vapour.

Perspiration: 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

(R.H) Is High.

Cotton fibres must not become too dr y, otherwise they become  Brittle and hence cause

difficulties in spinning.

In contrast, a dr y 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 ver y common type of Hygr ometer (often known as Mason s hydrometer). A piece of muslin

wick  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.

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

the temperature of the wet bulb falls.

The reading of the wet bulb thermometer is normally found to be several degrees below

that of the dry bulb Thermometer is known as the wet bulb depression.

Example 1

If  the  dry  the  temperature is  30

C  when  the wet  bulb temperature is  20

C  then the  wet  bulb

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depression is 10

C the rate Evaporation depends on the amount of water Vapour present in the

o

air.

The less moisture the air has the greater the difference between the two thermometer reading.The

difference is therefore greatest for dry air and zero for saturated air.

REGNAULT S  HYGROMETER:  Is  used  to  determine  dew  point  and  relative

Humidity.Simplified  form  of  regnault s hygrometer  consist  of two  test  –  tubes A  and  B  with

silvered ends C and D Respectively.

Test  tube  A  contains  ether  and  it  is  fitted  with  rubber  stopper  which  car ries  a

thermometer T and two narrow tubes E and F.

Test tube B. is empty and serves as comparison.

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Air is bubbled through ether via the narrow tube E 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 f orm on C while D appears unchanged.

Above show Regnault (dew point ) hygrometer.

The reading of the thermometer T is 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 if the tables the value of

S.V.P for water at Q1 and Q2 are X and Y millimeters of mercury Respectively.

R.H = S.V. Pat Q1 x 100%/S.V.P at Q2

Example 2

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The dew point  in a room  at  a  temperature of 10

0

c is  12,55C if  the saturated vapor pressure if

saturated  vapor  pressures  at  these  two  temperatures  are  15.5mm  and  10.9mm  of  mer cury

respectively calculate the Relative Humidity.

Solution

Required: To find relative humidity, RH.

RH = S.V.P at dew pointx 100%/S.V.P at air temperature

RH= 10.9/15.5× 100%

Relative Humidity, RH = 71%

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 of the tiny droplets of water or Ice floating in the 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 f all 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  (F.P)  0

C.Under  these

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conditions, the atmospheric vapour condenses directly into ice cr ystal.

Hail stones

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

below 0

o

C without freezing.

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

coming into contact with ice crystals in the upper atmosphere.

Hail stones are dangerous to air craft and human beings.

Mist:

Is the condensation of vapour Iito water dr oplets occurring near the ground.

Fog:

Is a mist In which Visibility does not extend beyond 1km.

Smog:

This is dense fog, where visibility is reducing to a few metres.

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