Site icon Citizen News

QUANTITY OF HEAT SIMPLIFIED PHYSICS NOTES

Chapter Nine

QUANTITY OF HEAT

HEAT CAPACITY

It takes shorter time to prepare tea for fewer people as compared to preparing tea for many.

This is because less amount of energy is needed to prepare tea for fewer people. On a hot day, the land surface is much warmer compared to the sea. This is because of the nature of the surfaces. Different materials have different rates of heat absorption.

EXPERIMENT: To investigate the relationship between the mass of a body and the quantity of heat required to cause a unit temperature rise in it

Apparatus Water, beaker, Bunsen burner, thermometer, stopwatch, wire gauze.

Procedure

Observation

It takes a longer time for the larger volume of water to attain the same temperature rise than for the smaller volume of water.

Explanation

Conclusion

The SI unit of heat capacity is JK-1

Example 1

Calculate the quantity of heat required to raise the temperature of a metal block with a heat capacity of 460 IK-1 from 15 °C to 45 °C.

Solution

Heat capacity, C = 460 J K-1

Temperature change 9 = (45°C — 15°C) = 30 °C

 Q = CΔT

= 460 x 30

= 13 800 J

 

SPECIFIC HEAT CAPACITY

c =             quantity of heat                                 (H)

Mass (m) x change in temperature (ΔT)

H = mc ΔT

Note:   If two different substances of the same mass are subjected to the same amount of heat, they acquire different temperature changes.

For example, the specific heat capacity of copper is 390 Jkg-1K-1. This means that 1 kg of copper would take in or give out 390 J when its temperature changes by 1 K.

Example 2

  1. A block of metal of mass 1.5 kg which is suitably insulated is heated from 30 °C to 50 °C in

8 minutes and 20 seconds by an electric heater coil rated 54 watts. Find:

(a) the quantity of heat supplied by the heater.

(b) the heat capacity of the block.

(c) its specific heat capacity.

Solution

Q = 54  x  500

= 27000J

(b) Heat capacity, C = Q/ΔT

But Q = 27000 J and ΔT= 50 – 30 =20K

C  = 27000

     20

= 1350 JK-1

(c)           Specific heat capacity, c = C/m
But c = 1 350 and m = 1.5

C = 1350

1.5

 

=900Jkg-1K-1

  1. Find the temperature of water if a heater rated 42 W heat 50 g water from 20C in five minute. (Specific heat capacity of water is 4200Jkg-1K-1)

 

Assuming no heat losses;

Heat supplied by the heater = heat gained by the water.

42 x 5 x 60 = mcΔT

42 x 300 = 50 X 10-3 X 4 200 x ΔT

ΔT =       42 x 300

50 X 10-3 X 4 200

ΔT= 60

But ΔT= T – 20, where T is the final temperature.

60= T- 20
T = 60 + 20

T = 80°C

  1. A piece of copper of mass 60 g and specific heat capacity 390 Jkg-1K-1cools from 90°C to 40°C Find the quantity of heat given out.

Solution

Q = mcΔT= 60 x 103 x 390 x 50
= 1 170 J

 

DETERMINATION OF SPECIFIC HEAT CAPACITY

Method of Mixtures

Solids

EXPERIMENT: To determine the specific heat capacity of a solid by the method of mixtures

Apparatus

Metal block, thread, beaker, water, tripod stand, heat source, well-lagged calorimeter, stirrer, thermometer, cardboard.

 

 

Procedure

Heat lost by           heat gained by                          gained by

metal block     =    calorimeter with stirrer   +       water in calorimeter

mscs (T2 – T3) =  mccc (T3 – T1) + mwcw (T3 – T1)

 

cs = mccc(T3 – T1) + mwcw (T3 – T1)

                      (T2-T3)

 

Where CcCs and Cw are specific heat capacities of the calorimeter, the solid and water respectively.
The specific heat capacity of the material of the cube can therefore be calculated as;

  1. A lagged copper calorimeter of mass 0.75 kg contains 0.9 kg of water at 20⁰C. A bolt of mass 0.8 kg is transferred from an oven at 400⁰C to the calorimeter and a steady temperature of 50⁰C is reached by the water after stirring. Given Specific heat capacity of copper is 400 Jkg-1K-1and that of water 4200 Jkg-1K-1. Determine:
  2. Heat lost by the bolt

Let c be the specific heat capacity of the material of the bolt.

=0.8 x c x (400 – 50)

=280c

  1. Heat absorbed by the water

=0.9 x 4200 x (50 – 20)

=113,400J

  1. Heat absorbed by calorimeter

HC=mc∆T= 0.75 x 400 x (50 – 20)

=9000J

  1. Specific heat capacity of the bolt

 

Solution

Heat lost by bolt = heat gained by calorimeter + heat gained by water

280c = 9000+ 113 400

280c = 122 400

c = 437 Jkg-1K-1

Specific heat capacity of bolt is 437 Jkg-1K-1.

 

 

Liquids

Using a solid of known specific heat the capacity and replacing the water in the calorimeter with liquid whose specific heat capacity is to be determined, the same procedure as in experiment 9.2 is repeated.

Calculations

Replacing the mass of water and the specific heat capacity of water with those of the liquid, ml and cl and making the same assumptions on heat losses to the surroundings, the specific heat capacity of the liquid can be calculated as below;

Heat lost by       =     heat gained by            + heat gained by liquid

The hot solid             calorimeter and stirrer     in the calorimeter

mscs (T2 – T3) = mccc (T3 – T1) + mlcl (T3 – T1)

mscc (T2 – T3) – mccc (T3 – T1) = mlct (83 – 81)

Hence, the specific heat capacity of the liquid can be calculated as;

    cl =   mscs (T2 – T3) – mccc (T3 – T1)

               ml(T3-T1)

Note:

The following precautions need to be taken to minimize heat losses to the surroundings:
(i) Use of a highly polished calorimeter.

(ii) Heavy lagging of the calorimeter.

(iii) Use of a lid of poor conduction.

Example 6

A block of iron of mass 1.25 kg at 120°C was transferred to an aluminium calorimeter of mass 0.3 kg containing a liquid of mass 0.6 kg at 25°C. The block and the calorimeter with its contents eventually reached a common temperature of 50 °C. Given the specific heat capacity of iron as 450 Jkg-1K-1 and that of aluminium 900 Jkg-1K-1, calculate ;

  1. Heat lost by the iron

Hm=mc∆T =1.25 x 450 x 70

=39 375J

  1. Heat absorbed by the liquid

Let c be the specific heat capacity of the liquid.

Hl=mc∆T=0.6 x c x 25

=15c

  1. Heat absorbed by calorimeter

HC=mc∆T= 0.3 x 900 x 25

=6 750 J

  1. Specific heat capacity of the liquid

Heat lost by iron block = heat gained by calorimeter + heat gained by liquid
1.25 x 450 x 70 = 0.3 x 900 x 25 + 0.6 x c x 25

39 375 = 6 750 + 15c

39 375 – 6 750 = 15c

c = 2 175 J kg-1K-1

 

  1. A girl heats 5 kg of water to temperature of 800 When she adds m kg of water at 150 C the mixture attains temperature of 40⁰C. Determine the value of m. (ignore heat changes due to the container)           (3 marks)
  2. Equal masses of water and paraffin with specific heat capacities C­Wand CP respectively are heated using identical sources of heat, for the same length of time. The final temperature θP of paraffin was found to be greater than final temperature  of water, Show that CW is greater than CP. (2 marks)

 

Electrical Method

Solids

EXPERIMENT; To determine the specific heat capacity of a metal by electrical method

Apparatus

Metal block with two holes, heater, thermometer, ammeter, voltmeter, stopwatch, lagging material, power source, rheostat, connecting wires, weighing balance.

Procedure and sample results

Temperature change of the metal block = T2 – T1

Assuming no heat losses to the surroundings, the specific heat capacity of the material of the metal block is calculated as follows;

Electrical energy supplied by the electrical heater coil = heat gained by the metal block.
VIt = mc (T2 – T1)

where c is the specific heat capacity of the material of the block.
Therefore, specific heat capacity c is given by;

c =          VIt  

m(T2-T1)

Precautions

(i) The metal block must be highly polished and heavily lagged.

(ii)   The two holes should be filled with a light oil to improve thermal contact with the heater and thermometer.

Example 7

A metal cylinder of mass 0.5 kg is heated electrically. If the voltmeter reads 15 V, the ammeter 3.0 A and the temperature of the block rises from 20°C to 85 °C in 10 minutes, calculate the specific heat capacity of the metal cylinder.

Solution

Heat supplied by the heater = heat gained by the metal cylinder

VIt = mcΔT

15 x 3 x 10 x 60 = 0.5 x c x 65

c = 15 x 3 x 600

0.5 x 65

= 831 Jkg-1K-1

EXPERIMENT: To determine the specific heat capacity of a liquid by electrical method

Apparatus

Well-lagged calorimeter, stirrer, 12 V heating coil, liquid (water), ammeter, voltmeter, weighing
balance, thermometer, stopwatch, rheostat.

 

 

 

 

 

 

 

 

 

Fig. 9.4: Determination of specific heat capacity of a liquid by electrical method

Procedure

Mass of water = m2 –  m1 = mw

Assuming no heat is lost to the surroundings, the specific heat capacity of the water can be calculated as follows; ,

heat supplied by electric heater = heat gained by water + heat gained by calorimeter
Vlt = mwcw(T2 – T1) + m1cc(T2 – T1)

cw =  VIt – m(T2-T1)

            mw(T2-T1)

Where cw and cc are the specific heat capacities of water and material of the calorimeter respectively,

Example 8

In an experiment to determine the specific heat capacity of water, an electrical heater was used. If the voltmeter reading was 24 V and that of ammeter 2.0 A, calculate the specific heat capacity of water if the temperature of a mass of 1.5 kg of water in a 0.4 kg copper calorimeter rose by 6 ˚C after 13.5 minutes. (Specific heat capacity of copper is 400 Jkg+K:’)

Solution

Electrical energy supplied by heater = heat gained by calorimeter + heat gained by water 24 x 2 x 13.5 x 60 = 0.4 x 400 x 6 + 1.5 x c x 6, where c is specific heat capacity of water. 38 880 – 960 = 9c

c = 37920

9

= 4213 Jkg-1K-1

 

2002 Q19 P1

An immersion heater rated 90W is placed is a liquid of mass 2kg.  When the heater   is switched on for 15 minutes the temperature of the liquid rises from 20oC to 30oC Determine the specific heat capacity of the liquid. (Assume no heat losses)

 

Continuous Flow Method

EXPERIMENT 9.5: To determine the specific heat capacity of water by the continuous flow method

Apparatus

Constant head tank, electric heating coil, two thermometers, glass tube (surrounded by an evacuated glass jacket which prevents heat escape from the liquid by conduction or convection), stopwatch, rheostat.

 

 

Procedure

Results and Calculations

First rate of flow of water

Ammeter reading = II

Voltmeter reading = VI

Inflow temperature of water = T1
Outflow temperature of water = T2
Mass of the empty beaker = ml
Mass of the beaker with water = m2
Time of collecting the water = t
Mass of water collected = m2 – ml = mc
Temperature difference = T2 – T1

Second rate of flow of water
Ammeter reading = I2

Voltmeter reading = V2

Mass of the beaker with water = m3
Mass of water collected = m3-m1= mn

Under steady conditions, none of the electrical energy supplied is used in heating the apparatus
and, therefore;

Electrical energy = heat energy absorbed    +        heat energy lost to

Supplied                   by collected water                  the surrounding

VIt = mw c(T2 – T1) + H

After the rate of flow is altered, temperature difference is the same and the heat lost in time t is again H.
:. V2I2t = mn c(T2 – T1) + H

Hence;

(V2 I2 – V1I1) t = (mn -mw) c(T2 – T1)

Advantages of the method over the other methods

Exercise 9.1

  1. A lady wanted to have a warm bath at 40°C. She had 5.0 kg of water in a basin at 85˚C. What mass of cold water at 25°C must she have added to the hot water to obtain her choice of bath? Neglect heat losses and take specific heat capacity of water as 4 200 Jkg-1K-1.
  2. 2 kg of iron at 100°C is dropped into 0.09 kg of water at 26 °C inside a calorimeter of mass 0.15 kg and specific heat capacity 800 Jkg-1K-1. Find the final temperature of the water. (Specific heat capacity of iron is 460 J kg-1K-1and that of water 4200 Jkg-1K-1)
  3. A copper calorimeter of mass 0.12 kg contains 0.1 kg of paraffin at 15°C. If 0.048 kg of aluminium at 100°C is transferred into the liquid and the final temperature of the mixture is 27°C, calculate the specific heat capacity of paraffin, neglecting heat losses. (Specific heat capacity of aluminium is 900 Jkg-1K-1and that of copper 400 Jkg-1K-1)

CHANGE OF STATE

 

Observation

  1. The thermometer records a temperature rise of the ice until it reaches O°C.
  2. The temperature remains at 0 °C while ice changes to water at O°C.

 

Explanation

Conclusion

The heat supplied to ice at 0 °C does not change the temperature of the ice, but changes its state from solid to liquid (melting).

The heat absorbed as the ice melts is called latent heat. The term ‘latent’ means ‘hidden’. It is used thus because the ice at 0 °C is converted water at 0 °C without change in temperature.

Latent Heat of Fusion

Latent heat of fusion is defined as the heat required to change the state of a material from solid to liquid without temperature change. Conversely, as a liquid changes to solid state, latent heat of fusion is given out.

 

EXPERIMENT: To explore the change of state of naphthalene using the cooling curve
Apparatus

Naphthalene is heated to melt and then allowed to cool.

The temperature is recorded in intervals of time and a graph of temperature against time is plotted

 

                                      

Observation

During cooling, the temperature of liquid naphthalene falls from about 90°C to about 80 °C, where it remains constant for some time.

At this temperature, all the naphthalene gradually changes to solid, after which the temperature falls further to room temperature.

The graph of temperature against time is as shown in figure

Cooling curve for naphthalene

 

Explanation

Specific Latent Heat of Fusion

the specific latent heat of fusion of a substance is defined as the quantity of heat energy required to change a unit mass of the substance from solid to liquid without change in temperature.

Q = mLf, where Lf is the specific latent heat of fusion. The SI unit of specific latent heat is Jkg-1.

Lf = Q/m

Note: A unit mass of a material changing from liquid to solid would give out heat energy equal to its specific latent heat of fusion.

 

 

 

EXPERIMENT: To determine the quantity of heat required to change unit mass of ice to water
Apparatus

Water, ice pieces, thermometer, calorimeter, stirrer.

 

 

Determining latent heat of fusion of ice
Procedure

Note

  1. Warming the water so that its temperature rises by a given value above room temperature then cooling it to a temperature which is the same value below room temperature balances the heat exchange between the calorimeter with its contents and the surrounding.
  2. ‘Dry ice’ is one which has minimum water moisture on its surface. It is used so that any heat absorbed is utilized in changing of state from solid to liquid, but not in warming the water.

Results and Calculations

Mass of calorimeter and stirrer = m1
Mass of water and calorimeter =m2
Mass of calorimeter and mixture = m3
Temperature of water in calorimeter = T1
Final temperature of mixture = T2

Mass of water used = m2 – m1 = mw

Mass of ice melted = m3 – m2 = mm
Temperature change = T2 – T1

 

Heat lost   heat lost =     heat gained         +     heat gained      +              heat gained

(By warm water)          (By calorimeter)      (Ice at 0 °C to water at 0°C)    (Water at0 °C to final temp T2)

 

 

Let the quantity of heat required to melt a unit mass of ice at 0 °C to water at 0 °C be Lf
mw Cw (T1 – T2) + m2cc(T1 – T2) = mm Lf+ mw Cw (T2 -0)

where Cw and Cc are specific heat capacities of water and calorimeter material respectively.

Lf = mw cw (T1 – T2) + mccc(T1 – T2) – mw cw T2

                  mm

This is the quantity of heat energy required to melt unit mass of ice at constant temperature and is referred to as the specific latent heat of fusion of ice.

 

EXPERIMENT: To determine specific latent heat of ice by electrical method

Apparatus

Crushed ice, two filter funnels, two beakers, voltmeter, ammeter, rheostat, heater, two
thermometers, and stopwatch.

 

Put equal quantities of crushed ice into two identical filter funnels P and Q,

Results and Calculations

Mass of beaker under P before experiment = m1
Mass of beaker under P after experiment =m2

Mass of ice melted in P during experiment = (m2 – m1) = mn
Mass of beaker under Q before experiment = m3

Mass of beaker under Q after experiment = m4

Mass of ice melted in Q during experiment = (m4 – m3) =mm
Reading of ammeter = I

Reading of voltmeter = V

Time during which heater is switched on = t seconds.

Lf = VIt

         m

where Lf is the specific latent heat of fusion of ice.

The SI unit for specific latent heat of fusion is Jkg-1

Table gives values of specific latent heat of fusion of some common materials.

Material Specific latent heat of Jus ion (x 105 Jkg-1}
Copper 4.0
Alwninium 3.9
Water (Ice) 3.34
Iron 2.7
Wax 1.8
Naphthalene 1.5
Solder 0.7
Lead 0.026
Mercury 0.013

Example 9

In an experiment to determine the specific latent heat of fusion of ice, 0.025 kg of dry ice at 0˚C is melted up in 0.20 kg of water at 21°C in a copper calorimeter of mass 0.25 kg. If the final temperature of the mixture falls to 11 ˚C, what is the specific latent heat of fusion of ice? (Take the specific heat capacity of water as 4200 Jkg-1K-1 and that of copper as 400 Jkg-1K-I).

Solution

Heat loss by +                                       heat lost by water   =    heat gained                           + heat gained

calorimeter                           by melting ice      by melted ice

 

0.25 x 400 x 10 + 0.2 x 4 200 x 10 = 0.025L + 0.025 x 4 200 x 11,

where L is the specific latent heat of fusion of ice.

1 000 + 8 400 = 0.025L + 1 155

0.025L == 9400 – 1 155

L == 8245

0.025

= 329 800 Jkg-1

= 3.298 x 105 Jkg-1

Hence the specific latent heat of fusion of ice is 3.298 x 1 ()5 Jkg-l.

Latent Heat of Vaporization

Q=mLv 

The SI unit of specific latent heat of vaporization is Jkg-1

 

EXPERIMENT: To determine the specific latent heat of vaporization of a liquid (water)
USING THE METHOD OF MIXTURES

Apparatus

Calorimeter, stirrer, water, thermometer, flask, delivery tube, heat source.

 

PROCEDURE

 

Results and Calculations

Mass of water = (m2 – mc) = mw

Mass of condensed steam =m3 – m2= ms
Temperature change = T2 – T1

 

 

 

Therefore

 

Heat lost by steam + heat lost by water = heat gained by water   + heat gained by calorimeter

(condensation)          (100˚C-T2)    

msLv + mscw(100-T2)  =  mwcw(T2-T1) +  mccc(T2-T1)

 

Lv  =  mwcw(T2-T1) +  mccc(T2-T1)- mscw(100-T2) 

                    ms

Note:

Errors due to heat loss to the surroundings can be minimized by first cooling the water in the calorimeter by a given value below room temperature and then passing the steam until the temperature rises above room temperature by the same value.

Example 10

Dry steam is passed into a well-lagged copper calorimeter of mass 0.25 kg containing 0.50 kg of water and 0.02 kg of ice at 0 ⁰C. The mixture is well stirred and the steam supply cut off when the temperature of the calorimeter and its contents reaches 25°C. Neglecting heat losses, if 25g of steam is found to have condensed to water and Specific heat capacity of copper is 400 Jkg-1K-1and latent heat of fusion of water is 3.36 x105 Jkg-1) find:

  1. Heat lost by the steam

H=m Lv

=0.025Lv

  1. Heat lost by hot water

H=mc∆T

=0.025 x 4 200 (100 ⁰C – 25°C)

=7 875J

  1. Heat gained by calorimeter

H=mc∆T

=0.25 x 400 x 25

=2500J

  1. Heat gained by ice

H=m Lf

=0.02 x 336000

=6 720J

  1. Heat gained by cold water

H=mc∆T

=(0.5 + 0.02) 4 200 x 25

=54600J

  1. the specific latent heat of vaporization of water

 

Solution

heat lost by steam+ heat lost by water =  heat gained    +  heat gained by      + heat gained by

(condensation)        (100 ⁰C – 25 ⁰C)      by ice (fusion)    water (0 ⁰C – 25 ⁰C)     calorimeter

 

Let L be the specific latent heat of vaporization of water + heat gained by

0.025L + 7 875 = 6 720 + 54 600 + 2 500 0.025L = 55 945
L= 55945
     0.025

= 2237800 Jkg-1.

Specific latent heat of vaporization of water is 2.238 x 106 Jkg-1.

 

Determining specific latent of vaporization of water using ELECTRICAL METHOD

Beaker, flask, heater coil, condenser, water, stop watch, voltmeter, rheostat, ammeter power source.

 

                                       

Procedure

Results and Calculations

Ammeter reading = I

Voltmeter reading = V

Mass of empty beaker = mB

Mass of beaker with condensed water = m2
Time taken to collect the condensed water = t
Mass of condensed water = m2 – mB= ms

Assuming that all the heat given by the heater coil is used in vaporizing the water and all the steam is condensed, the specific latent heat of vaporization of water is calculated as follows;
heat supplied by the heater coil = heat used to vaporize the water.

VIt = mcLv 

Lv = ( VIt ) ,

            mc   where Lv is the specific latent of vaporization.

 

 

Table 9.3 gives values of specific latent heat of vaporization for some common materials.

Material Specific latent of vaporization
  ( x 105 Jkg-1)
   
Water 22.6
Alcohol 8.6
Ethanol 8.5
Petrol 6.3
Benzene 4.0
Ether 3.5
Turpentine 2.7

 

Example 11

Calculate the quantity of heat required to change 0.50 kg of ice at -10°C completely into steam at 100 °C Take:

specific heat capacity of ice = 2 100 Jkg-1K-1

specific heat capacity of water = 4 200 Jkg-1K

specific latent heat of fusion of ice = 3.36 x 1()5 Jkg-1
specific latent heat of vaporisation of steam 2.26 x 106 Jkg-1

Solution

Quantity of heat gained by ice to raise its temperature to 0˚C
mcΔT = 0.50 x 2100 x 10

= 10500J

Quantity of heat required to change the ice into water;
mL= 0.50 x 336 000

= 168000 J

. Quantity of heat required to raise temperature of water to 100 DC;
mcΔT = 0.50 x 4 200 x 100

= 210 OOOJ

Quantity of heat required to vaporize water;
mLv = 0.5 x 2 260 000

= 1130000J

Total heat energy = 10 500 + 168000+ 210 000 + 1 130000
= 1518500 J

 

Factors Affecting Melting and Boiling Points

  1. pressure on melting point

Apparatus

Block of ice, thin copper wire, two heavy weights, and wooden support.

 

Procedure

Observation

The wire gradually cuts its way through the ice block, but leaves it as one piece.

Explanation

Conclusion

Application of pressure on ice lowers the melting point.
Note:

The process of refreezing is known as regelation.

Applications of the Effects of Pressure on Melting Point of Ice
Ice Skating

Joining Ice Cubes under Pressure

 

  1. Impurities

 

 

BOILING

  1. a)Increasing the pressure

 

 

 

 

 

 

 

 

 

 

 

 

 

Procedure

Explanation

Conclusion

 

b)Reducing the pressure

Procedure

 Observation

Explanation

Conclusion

  1. the effect of impurities on boiling point

 

 

 

 

 

Procedure

Observation

Conclusion

 

Evaporation

 

  1. Effects of Evaporation

(ii)           In a fume chamber, pour some ether into a test tube. Bubble air through the ether using a long rubber tubing.

Frost forms on the outside surface of the tube.

 

 

(iii) Place a beaker on film of water on a wooden block. Pour some ether into the beaker and blow air through the ether using a foot pump.

The ether quickly evaporates and after sometime, it is found that the beaker is stuck to the wooden block, a thin layer of ice having formed between them. This shows that evaporation causes cooling.

 

Factors affecting Rate of Evaporation

Temperature

Increasing the temperature of a liquid makes its molecules on its surface move faster. This
makes it easier for more of them to escape, enhancing evaporation. It takes shorter time for
cloths to dry on a hot day.

Surface Area

Increasing the area of the liquid surface gives the faster molecules greater chance of escaping.
A wet bed-sheet dries faster when spread out than when folded.

Draught

Passing air over the liquid surface sweeps away the escaping vapour molecules. This clears the way for more escaping molecules to enter the space. This is why wet clothes dry faster on a windy day.

Humidity

Humidity is the concentration of water vapour in the atmosphere. When humidity is high, there are more vapour molecules in the space above the liquid surface. This makes it more difficult for the water molecules to leave the surface. Wet clothes take longer time to dry up on a humid day.

Comparison between boiling and evaporation

 

Evaporation Boiling
Takes place at all temperatures Takes place at a fixed temperature.
Takes place on the surface of the liquid. Takes place throughout the liquid, with
No bubbles are formed. bubbles of steam forming all over.
Decreasing the atmospheric pressure Decreasing atmospheric pressure
increases the rate of evaporation. lowers the boiling point.

Application of Cooling by Evaporation
Sweating

When sweat evaporates, it draws latent heat from the skin, producing a cooling effect. Animals have different mechanisms of cooling their bodies. A dog exposes its tongue when it is hot, while the muzzle of a cow gets more wet when it is hot.

Cooling of Water in a Porous Pot

A porous pot has tiny pores, through which water slowly seeps out. When this water evaporates, it cools the pot and its contents.

The Refrigerator

The effect of cooling caused by evaporation is made use of in the refrigerator.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 11 shows the features of a domestic refrigerator. A volatile liquid circulates through the capillary tubes under the action of the compression pump.

 

 

 

 

 

 

 

 

 

(i) State the reason for using a volatile liquid                                                            (1 mark)

So that it vaporizes readily/ easily                                              

(ii) Explain how the volatile liquid is made to vaporize in the cooling compartment and to condense in the cooling fins                                 (2 marks)

In the freezing compartment the pressure in the volatile liquid is lowered suddenly by increasing the diameter of the tube causing vaporization in the cooling fins, the  pressure  is increased by the compression pump and heat lost to the outside causing condensation.

Acquires heat of the surrounding causing the liquid to vaporize

     (iii) Explain how cooling takes place in the refrigerator                                             (3 marks)

When the volatile liquid evaporates, it takes away heat of vaporization to form the freezing compartment, reducing the temperature of the latter. This heat is carried away and disputed at the cooling fins where the vapour is compressed to condensation giving up heat of vaporization

     (iv)    What is the purpose of the double wall?                                              (1 mark)

Reduces rate of heat transfer to or from outside ( insulates)

                           Reduces / minimizes, rate of conduction/ conversion of heat transfer                        

 

 

 

 

 

 

Revision Exercise 9

(Take specific heat capacity of water as 4 200 Jkg+K:’)

 

  1. Calculate the specific heat capacity of paraffin if 22 000 J of heat is required to raise the
    temperature of 1.5 kg of paraffin from 23°C to 30 °C.

 

  1. A block of metal of mass 5 kg is heated to 110°C and then dropped into 1.5 kg of water.

 

The final temperature is found to be 50°C. What was the initial temperature of the water?
(The specific heat capacity of the metal is 460 Jkg-1K-I)

 

  1. Water drops from a waterfall 84 m high. The temperature of the water at the bottom is
    found to be 26.2 0c. Calculate its temperature at the top.
Exit mobile version