QUANTITY OF HEAT PHYSICS NOTES, REVISION QUESTIONS & ANSWERS

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  HEAT CAPACITY

  1. Define specific latent heat of fusion of a substance        (1mk)

 

  1. Distinguish between heat capacity and specific heat capacity of a substance.

 

  1. Give the property of water which makes it suitable for use as a coolant in machines                                                                           (1mk)

 

  1. Equal masses of water and paraffin are heated for same length of time. The final temperature of paraffin was found to be greater than the final temperature of water. Explain the observation.                 (2mk)

 

  1. A student states that when two bodies at different temperatures are brought into contact the resulting rise of temperature in one will be equal to the fall in temperature in the other. Give two reasons why this statement is wrong
  2. (i) Describe how you would attempt to measure the specific heat capacity

of aluminium by method of mixtures.                                        (3mk)

(ii)    State one source of experimental error in this method.         (1mk)

(iii)    Give one way you could take to reduce the magnitude of the error you have mentioned.                                                                (1mk)

  1. Briefly describe an experiment to determine the specific heat capacity of a liquid substance using the electrical method.                         (6mk)

 

  1. A liquid at 80°C in a cup was allowed to cool for 20 minutes. State two factors that determine the final temperature.         (2mk)

 

  1. Equal masses of water and ice at 00C are added separately into two identical beakers containing equal amount of water. State the reason why ice may cause a greater change of temperature.                         (1mk)

 

  1. Figure below shows the temperature changes against quantity of heat supplied to one kilogram each of liquids A and B.

 

 

 

 

 

 

 

 

 

Which of the two liquids has a high specific heat capacity?  Give a reason.(2mk)

 

  1. 500g of water at 20oc is mixed with 200g of water at 55oc. Find the final temperature of the mixture.

 

  1. Calculate the heat evolved when 100g of copper are cooled from 900c to 100 (Specific Heat Capacity of Copper = 390J/Kgk).

 

  1. An iron block of mass 20g is left in water boiling at 950C for some minutes then transferred quickly to a well-lagged copper can of mass 40g containing 60g of water at 200C. The mixture is well stirred and the final temperature T is noted. Given that the specific heat capacity of copper = 400JKg-1K-1, Iron = 460JKg-1K-1 and water = 4200JKg-1K-1, find the value of T.

 

  1. 100g of boiling water are poured into a metal vessel weighing 800g at a temperature of 200C if the final temperature is 500 What is the specific heat capacity of the metal? (Specific Heat capacity of water 4.2 x 103J/kgk)

 

  1. You are provided with two beakers. The first beaker contains hot water at 700  The second beaker contains cold water at 200C.  The mass of hot water is thrice that of cold water.  The contents of both beakers are mixed.  What is the temperature of the mixture?
  2. Water at 240C fall through a height of 72m to the bottom of a dam. Calculate the temperature of the water at the bottom of the dam. (Take specific heat capacity of water as 4200JKg-1K-1)

 

  1. A lead weight is dropped from a helicopter hovering at 100m above the ground. Assuming that all the energy is converted into heat energy, determine the rise in temperature of lead. Take specific heat capacity of lead to be 130J/kgk.                                  (3mks)

 

  1. Water flows from a high dam down a vertical cliff 100m high. The temperature of the water reaching the ground is 230C Determine the temperature of water in the dam. Assume evaporation and sound produced due to the flow are negligible and the specific heat capacity of water is 4200J kg -1 K-1 (3mk)           

 

  1. An energy saving stove when burning steadily has an efficiency of 60%. The stove melts 03kg of ice at 00c in 180 seconds.

Calculate; –

  1. The power rating of the stove.
  2. ii) The heat energy wasted by the stove.

 

  1. Water at 200c spills over a waterfall of height 10m. Calculate the rise in temperature of water at the bottom of the waterfall if 80% of potential energy at the top of waterfall is converted into heat at the bottom of the waterfall. Take specific heat capacity of water 4200j/Kg-1 K-1) (4mk)

 

  1. In a domestic oil-fired boiler, 5kg of water flows through the boiler every second. The water enters the boiler at a temperature of 300C and leaves at a temperature of 700C, re-entering the boilers after flowing around the radiators at 300C.  3.0x 107J of heat is given to the water by each kilogram of oil burnt.  The specific heat capacity of water is 4200Jkg -1K-1
  • Use the information above to calculate the energy absorbed by the water every second as it passes through the boiler
  • Use the same information above to calculate the mass of oil which would need to be burnt in order to provide this energy.

 

ELECTRICAL MTD

  1. A heating element rated 2.5 KW is used to raise the temperature of 3.0 kg of water through 500 calculate the time required to effect this. ( Specific heat capacity of water is 4200J/kgK).                                  (3mk)

 

  1. An electric heater rated 300 W heats some liquid of heat capacity 1680 J/K for 2 minutes. Find the rise in temperature.                (3mk)

 

  1. An immersion heater rated 1500W is used to heat a block of ice of mass 500g initially at -10°C for 2.5 minutes. If the final temperature is 20°C determine the specific latent heat of fusion of the ice (Take specific heat capacity of water as 4200J/KgK, specific heat capacity of ice = 2100J/KgK) (4mk)

 

  1. An immersion heater takes a current of 10A when connected to 240V supply. If it is used to heat 5Kg of water at 200C, find the temperature of water after 4 minutes of heating.                                  (3 mks)

 

  1. A hot-water tank for a house contains 150kg of water at 150 the tank itself has a heat capacity of 6000 JK-1. An immersion heater is used to heat the water to 500c. the tank is well insulated and the power of the heater is 2500W (specific heat capacity of water =4200 JKg-1K-1)
  • Find the amount of heat transferred to the water (2mk)
  • find how much heat is absorbed by the tank              (2mk)
  • Determine the time it will take the heater to raise temperature to 500

(3mk)

  1. A metal cylinder of mass 500g is heated electrically. If the voltmeter reads 15V, the ammeter 3.0A and the temperature of the block varies from 200 C to 850 C in 10 minutes. Calculate the specific heat capacity of the metal cylinder.(3mk)

 

  1. An-immersion heater rated 150W is placed in a liquid of mass 5 kg. When the heater is switched on for 25 minutes, the temperature of the liquid rises from 200C – 2700C.  Determine the specific heat capacity of the liquid.  (Assume no heat losses)

 

  1. An immersion heater which takes a current of 3A from 240V mains raised the temperature of 10kg of water 300c to 500 How long did it take?

 

  1. An immersion heater rated 90W is placed in a liquid of mass 2kg. When the heater is switched on for 15 minutes, the temperature of the liquid rises from 200C to 300C. Determine the specific heat of the liquid.

 

 

 

 

  1. Figure below shows a set up in an experiment to determine specific heat capacity of water.

 

 

 

 

 

 

 

 

 

 

 

The data below was obtained was obtained from the experiment

  • Voltage V across the heater = 12V
  • Current I in the circuit   = 1.4A
  • Time (t) heating                = 600s
  • Mass m of water = 0.4kg
  • Change in temperature DT = 6ºc
  1. Define specific heat capacity
  2. State two improvements that would be made in the set up to obtain accurate
  • Use the above results to determine the specific heat capacity of water.

 

GRAPH

  1. The figure below shows a graph of the variation of temperature with time for a pure substance heated at a constant rate

 

 

 

 

 

 

 

 

 

 

Assuming that heat transfer to the surrounds is negligible, state the changes observed on the substance in region.

  1. a) BC                                                                                                                                                           (1mk)
  2. b) DE                                                                                                                                                           (1mk)
  3. The figure below shows a graph of temperature against time for a given substance (water)

 

 

 

 

 

 

 

 

 

 

 

State what happens in the section labelled

  • BC                     (1mk
  • CD            (1mk)
  • DE            (1mk)

 

  1. A 250 g sample of a solid was heated steadily in a lagged colorimeter of negligible mass. The observations were represented in a graph as shown below.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(i)     Explain the shape of the graph       (3mks)

(ii)     The solid requires 12.5J of energy to change its state. Calculate its specific latent heat of fusion.                                                                       (2mks)

  1. The cooling curve below is for a pure substance

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

(a) What is the melting point of the substance                   ( 1 marks )

(b) What point of the curve is the substance?

(i)     solid                                                                        (1mk)

(ii)     Liquid                                                                             (1mk)

(iii)    Solid and liquid                                                               (1mk)

 

  1. Figure below shows the variation of temperature ‘q’ with time t, when an immersion heater is used to heat a certain liquid. Study the figure and answer questions (i) and (ii).

 

 

 

 

 

 

 

 

 

 

 

(i)         State the reason for the shape of the graph in the section labelled BC.                                                    (1mk)

(ii) Sketch on the same axes the graph for another liquid of the same mass but higher specific heat capacity when heated from the same temperature.(1mk)

 

  1. In an experiment to determine the specific heat capacity of liquid, a student used 0kg of each of the liquids, water, glycerin and paraffin. Each of the liquids was supplied with 21600J of heat energy under the same conditions. The table below shows temperature rise for the liquids.

 

Liquid Water Glycerine Paraffin
Temp’ 0c 2.6 4.4 4.9

 

(i) Suggest a reason for the difference in the difference in the rise of

temperature.                                                                      (1mk)

(ii) Calculate the specific heat capacity of paraffin                       (3mk)

 

  1. Njoroge wanted to determine the heat energy, W, required to change one kilogram of water into vapour at a constant temperature. He set up the arrangement as shown below;

 

 

 

 

 

 

 

 

 

 

 

 

 

 

He took the reading M, of the balance at a given time interval. He obtained the following data

 

Mass (g) 100 97 95 93 91 81
Time (s) 0 135 210 330 405 495

(a)    Plot a graph of mass against time                             (5mk)

  • Determine the gradient of the graph                    (3mk)
  • What does the gradient represent? (1mk)
  • A steady current of 4A and potential difference of 12V were recorded during the experiment. Determine the value of W. (4mks)

 

  1. A 125W heater and a thermometer were immersed in 0.6kg of oil in a vessel of negligible heat capacity. The following observations were noted.

 

Temperature (K) 294 302 313 324 334
Time (minutes) 2 4 6 8 10
  1. i) Plot a suitable graph and use it to find: (4mks)
  2. ii) the average rise in temperature per minute.          (2mks)

iii)  the temperature at which the heating started.              (1mk)

  1. iv) Hence, calculate the specific heat capacity of the oil.(2mks)

 

 

  1. The table below shows values of temperature against time for a pure substance subjected to heat
Time (min) 0 0.5 1.0 4.0 7.0 8.0 11 13 14 16
Temperature 0C -10 5 10 10 10 15 53 78 80 80
  1. a) Draw a graph of temperature against time showing the effect of heating the solid substance          (5mks)
  2. b) From the graph, state the melting point and boiling point of the substance.

(2mk)

  1. c) If the loss in mass of the substance during vaporization is 70g and the heat supplied at a rate of 720J/min, calculate the specific latent heat of vaporization of the liquid substance                   (3mk)

 

  1. A 4g mass is receiving heat at the rate 100kJ per minutes and its temperature at various times recorded as follows
Time t (min) 0 1 2 6 13 18 23 28 29 30
Temperature T (K) 230 250 270 270 270 310 350 390 390 390

 

Plot a graph of temperature against time on the grid provided                  (4mks)

  • Use your graph to find

(i)    The specific heat capacity of the substance in its liquid state        (3mk)

(ii)    Its boiling point                                                               (1mk)

(iii)   Its melting point                                                                  (1mk)

(iv)    The specific latent heat of fusion of the substances       (3mk

 

 

  1. In an experiment to determine the specific latent heat of vaporization of a liquid using an electrical method, the amount of heat, Q, required to vaporize a given mass, m, of a liquid were recorded as shown.

 

Q (J) X 103 3.0 4.0 5.0 6.0 7.0 8.0
M (kg) X10-3 4.0 6.4 8.8 11.2 13.6 16.0

 

(i) On the grid provided plot a graph of Q (y-axis) against m.          (5mks)

 

(ii) From the graph, determine the specific latent heat of vaporization of the

liquid.                                                                    (3mks)

(iii) Suggest a reason why the graph does not pass through the origin.                                                                                                               (1mk)

(iv) Write a possible equation of this graph.                                (1mk)

 

LATENT HEAT OF FUSION AND VAPORIZATION

 

  1. Define latent heat of fusion of a substance        (1mk

 

  1. Define specific latent heat of fusion of a substance        (1mk)

 

  1. Define specific latent heat of vaporization of a substance        (1mk)

 

  1. State two physical quantities that remain constant while pure ice is being converted to water.

 

  1. State two factors that affect the melting point of ice. (2mk)

 

  1. Explain why a burn from the steam of boiling water is more severe than that of water itself?

 

  1. A burn from steam is more severe than one from water boiling at the same temperature .Give reasons.

 

  1. The setup shown below was used to determine the specific latent heat of vapourisation of water.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Describe how you would use the apparatus to determine the specific latent heat of vapourisation of water stating any assumption made and the measurements one would take.                                    (6mk

 

  1. You are provided with the following apparatus:A filter funnel, a thermometer, a stop watch, ice at 0°C, an immersion heater rated P watts, a beaker, a stand, boss and clamp and a weighing machine. Describe an experiment to determine the specific latent heat of fusion of ice. Clearly state the measurements to be made. (4mk)

 

  1. In an experiment to determine the power of an electric heater, melting ice was placed in a container with an outlet and the heater placed in the ice as shown below. The heater was connected to a power supply and switched on for some time. The melted ice was collected.

 

 

 

 

 

 

 

 

 

 

 

 

  1. a) Other than the current and voltage, state the measurement that would be taken to determine the quantity of heat absorbed by the melted ice in unit time. (2mk)
  2. b) If the latent heat of fusion of ice is L, show how measurements in (i) above would be used in determining the power P, of the heater, (2mk)
  3. c) It is found that the power determined in this experiment is lower than the manufacturer’s value indicated on the heater.       (1mk)

 

 

 

 

CALCULATIONS

  1. 50g of steam at 100oC was passed into cold water at 20oC. The temperature on the water rose to 60oC.  Determine the mass of cold water used (specific heat capacity of water = 4200J/KgK and specific latent heat of vapourisation of water = 2.26 x 106 JKg-1)              (4mk)

 

  1. Dry steam is passed into a well-lagged copper can of mass 250g containing 400g of water and 50g of ice at 00. The mixture is well stirred and the steam supply cut off when the temperature of the can and the contents reach 200C.
  • Draw an experiment set-up that can be used to find the mass of steam

condensed                                                                       (4mk)

 (b) Neglecting heat losses, find the mass of steam condensed     (4mk)

 

  1. In an experiment to determine the specific latent heat of vaporization of water steam at 1000Cwas passed into water contained in a well lagged Calorimeter, the following measurements were made.

Mass of calorimeter                                              =60g

Initial mass of water                                   =80g

Final mass of calorimeter + condensed stem.        =143g.

Initial temperature of mixture                              =170C

Final temperature of mixture                               =320C

Specific heat capacity of copper                   =390J/KgK

  1. Determine the mass of condensed steam.     1mk
  2. Determine the heat gained by calorimeter and water. 3mk
  • Determine the latent heat of vaporization of steam. 3mk

 

  1. In an experiment to determine the specific latent heat of vaporization of water, steam at 1000c was passed into water contained in a well-lagged copper calorimeter. The following measurements were made:
        • Mass of calorimeter      = 50g
        • Initial mass of water     = 70g
        • Final mass of calorimeter + water + condensed steam = 123g
        • Initial temperature of water = 5oc
        • Final temperature of mixture = 300C

(Specific heat capacity of water = 4200 J kg –1K and specific heat capacity for copper = 390 J kg -1 K-1)

Determine the

  1. a) Mass of condensed steam
  2. b) Heat gained by the calorimeter and water
  3. c) Given that L is the specific latent heat of evaporation of steam

(i) Write an expression for the heat given out by steam

                 (ii)   Determine the value of L.

  1. Steam of mass 3.0g at 1000c is passes into water of mass 400g at 100 The final temperature of the mixture is T. The container absorbs negligible heat. (Specific latent heat of vaporization of steam= 2260 kJ/kg, specific heat capacity of water= 4200Jk-1)
  2. i) Derive an expression for the heat lost by the steam as it condenses to water at temperature T.          (2mk)
  3. ii) Derive an expression for the heat gained by the water. (2mk)

    iii)        Determine the value of T.                                   (2mk)

 

  1. What mass of steam initially at 1300C is needed to warm 200g of water in a glass container of mass 100g from 200C to 500?

Specific heat capacity of steam                = 200JKg-1K-1

                        Specific heat capacity of water                = 4200 JKg-1K-1

Specific latent heat of vaporization of water = 2.26 x 106JKg-1

Specific heat capacity of glass                 = 840JK-1K-1    4mks

 

  1. A can together with stirrer of total head capacity 60J/k contains 200g of water at 100 dry steam at1000c is passed in while the water is stirred until the whole reaches a temperature of 300c Calculate the mass of steam condensed.

 

  1. 02kg of ice and 0.01kg of water 00c are in a container. Steam at 1000c is passed in until all the ice is just melted.  How much water is now in the container?

 

  1. 200 g of ice at 0°C is added to 400g water in a well lagged calorimeter of mass The initial temperature of the water was 40°C. If the final temperature of the mixture is X°C, (Specific latent of fusion of ice L = 3.36 x 105 Jkg-1, specific heat capacity of water, C = 4200Jkg-1K-1, specific heat capacity of copper = 400 Jkg-1K-1.)

(i)     Derive an expression for the amount of heat gained by ice to melt it and raise its temperature to X°C                            (2mk)

(ii)     Derive an expression for the amount of heat lost by the calorimeter and its content when their temperature falls to X°C.          (2mks)

(iii)    Determine the value of X.                                        (3mks)

 

  1. In an experiment to determine the specific latent heat of vaporization L of water, steam at 100°C was passed into water contained in a well legged copper calorimeter. The following measurements were made.

Mass of calorimeter = 80g

Initial mass of water = 70g

Initial temperature of water = 5°C

Final mass of calorimeter + water +condensed steam =156g

Final temperature of mixture = 30°C

Specific heat capacity of water = 4200JKg-1K-1and specific heat capacity for copper =     390J/Kg-1K-1)

(a) Determine the:

(i) Mass of condensed steam                                    (2mks)

(ii) Heat gained by the calorimeter and water                     (1mk)

(b)    Given that L. is the specific latent heat of vaporization of steam

(I) Write an expression for the heat given out by steam.             (1mk)

(II) Determine the value of L                           (3mks)

 

  1. In an experiment to determine specific latent heat of water, steam at 1000C was passed into the water container, the following measurements were made.
    • Initial temperature of water = 150C
    • Mass of Calorimeter = 60g
    • Initial mass of water = 80g
    • Final mass of water + calorimeter + condensed steam = 160g
    • Final temperature of mixture = 400C
    • Specific heat capacity of water = 4200 J/Kg
    • Specific heat capacity of copper = 390J/Kgk

         (a) Calculate

         (i)    Mass of condensed steam.                                                (1mk)

                 (ii)   Heat gained by calorimeter and water.                              (5mk)

         (b) Given that Lv is the specific latent heat of vaporization of steam.

  1. i)      Write an expression for the heat given out by steam.         (1mk)
  2.          ii)     Determine the value of Lv.                                      (3mk)

 

  1. In an experiment to determine the specific latent heat of vaporization of water. Steam at 1000C was passed into water container in a well lagged calorimeter. The following results were obtained Mass of calorimeter =50g
  • Initial temperature of water =200C
  • Initial mass of water =60g
  • Final mass of calorimeter +water +condensed steam =115g
  • Final temperature of mixture =300C
  • Specific heat capacity of water =4200J/kgk
  • Specific heat capacity of copper =400J/kgk

(a) Draw a possible set up that can be used in the above experiment   (3mks

(b)    Determine.

(i)The mass of condensed steam                                       (1mk)

(ii)The heat gained by the calorimeter and water                  (3mks)

(c)      Given that L is the specific latent heat of vaporization of steam,

(i)Write down an expression for the heat given out by steam   (1mk)

(ii)Determine the value of L                                              (2mks)

 

  1. Water of mass 200g at a temperature of 60oC is put in a well lagged copper calorimeter of mass 80g. A piece of ice at 0oC and mass 20g is placed in the calorimeter and the mixture stirred gently until all the ice melts. The final temperature of the mixture is then measured

(Latent heat of fusion of ice = 33400Jkg-1, specific heat capacity of water = 4200Jkg-1K-1) Determine:

(i)    The heat absorbed by the melting ice at 0oC     (2mks)

(ii)   The heat absorbed by the melted ice (water) to rise to temperature T                                                (2mks)

 (iii) The heat lost by the warm water and the calorimeter (Specific heat capacity of the calorimeter = 900Jkg‑1K-1)              (2mks)

(iv)   The final temperature T of the mixture             (3mks)

 

  1. A copper block of mass 800g is suspended in a freezing mixture -600C for some time and then transferred to a large volume of water at 00C. A layer of ice is formed on the block. Take specific heat capacity of copper =360JKg-1K-1, latent heat of fusion 336,000JKg-1
  2. i) Give a reason for the formation of ice                           (1mk)
  3. ii) State the temperature of the copper block after this change is complete

(2mk)

    iii)    Determine the mass of ice formed                              (3mk)

 

  1. In a cooling system 100g of steam at 1000C was passed into cold water at 400C. The temperature rose to 700C (specific heat capacity of water = 4200J kg-1K-1, latent heat of vaporization of steam = 2.2 x 106Jkg-1)

Calculate

  1. i) Heat lost by steam                                                   (2mk)
  2. ii) Heat lost by condensed steam                                  (3mk)

iii)     The mass of cold water used                                     (3mk)

 

  1. Water of mass 400g at a temperature of 60oC is put in a well lagged copper calorimeter of mass 160g. A piece of ice at 0oC and mass 40g is placed in the calorimeter and the mixture stirred gently until all the ice melts. The final temperature, T, of the mixture is then measured. Determine;

(i)     The heat absorbed by the melting ice at 0oC             (2mk)

(ii)    The heat absorbed by the melted ice (water) to rise the temperature T (Answer may be given in terms of T)  (2mk)

(iii)    The heat lost by the warm water and the calorimeter (The answer may be given in terms of T)                          (2mk)

(iv)   The final temperature T of the mixture (Specific latent heat of fusion of ice = 334000J/kg, specific heat capacity of water = 4200J/kgK, specific heat capacity of copper = 900J/kgK)                 (4mk)

 

  1. The melting point of a certain bullet is 3000c. If the initial temperature of the bullet is 200c , determine the least speed at which the bullet would be moving so that it just melt when suddenly stopped. Specific heat capacity of bullet = 840 J/kgK; specific heat of fusion of material making bullet = 6.3 x 104 J/kgK                                                                          (3mk)

 

  1. 32g of dry ice was added to 200g of water at 250C in a beaker of negligible heat capacity. When all ice had melted the temperature of water was found to be 100C. (Take specific heat capacity of water to be 4200JKg-1K-1)

(i)     Calculate the heat lost by water                                     (2mks)

(ii)     Write an expression for the heat gained by ice to melt and for temperature to rise to 100C.                                                              (2mks)

(iii)    Calculate the specific latent heat of fusion of ice.                       (2mks)

 

  1. A jet of dry steam at 1000C is sprayed on to the surface of 100g of dried ice at 00C contained in a well-lagged calorimeter of negligible heat capacity, until all the ice has melted and the temperature begin to rise. The mass in the calorimeter when the temperature reaches 400C is found to be 120g.Assuming that the specific latent heat of fusion of ice is 336000JKg-1, specific heat capacity of water is 4200JKg-1K-1, Determine the specific latent heat vaporization of water.                                           (4mks)
  2. 5 grammes of water at 200C is heated until it boils at 950 On further heating the temperature of Water does not change until it has all evaporated.

(i) State what happens to the energy supplied to the water after attaining a temperature of 950C.                                                                 (1 mk)

  1. ii) Calculate the amount of heat required to convert all the 5g of water to steam.

Latent heat of vaporization of water = 2260000 J/Kg)              (6mk)

 

  1. What mass of steam initially at 1300C will raise the temperature of 0 Kg of water contained in a 1.0 Kg glass container, from 200C to 500C? (4mks)
    • Specific heat capacity of steam =      200J Kg-1 K-1
    • Specific heat capacity of water =      4200 J Kg-1 K-1
    • Specific latent heat steam =      26 x 106 J Kg-1
    • Specific heat capacity of glass =      840 J Kg-1 K-1

 

 

ELECTRICAL

  1. An immersion heater rated 1000W is used to heat a block of ice of mass 500g initially at -10°C until the all the water evaporates at 95°C. Assuming that all the heat supplied is used to heat the ice, calculate the time in minutes for the whole process to take place. (Take specific heat capacity of water as 4200J/KgK, specific heat capacity of ice = 2100J/KgK specific latent heat of fusion = 3.35 x 105 J/Kg and specific latent heat of vapourization = 2.26 x 105J/Kg) (5mk)

 

  1. An electric kettle rated at 2.5kW contains 1.6kg of water. It is left switched on after boiling.

    (i)         How much heat energy will be used in turning all the water to steam?(2mk)

    (ii) Determine how long will it take for the 2.5kw kettle to boil dry (2mk)

 

  1. An electric heater rated 5 kW is used to melt 1.5 kg of ice at 0°C. Calculate the specific latent heat of fusion of ice, if it takes 5 minutes for the heater to melt all the ice. (3mk)

 

  1. A 180W heater is immersed in a copper calorimeter of mass 100g containing 200g of alcohol. When the heater is switched on after 36sec the temperature of the calorimeter and its contents raises by 120 what is the specific heat capacity of alcohol. (SHC of Cu = 400J kg-1 K-1) (4mks)

 

  1. An electric heater rated 6000W is used to heat 1kg of ice initially at -100c until all the mass turns to steam. Given that
      • Latent heat of fusion = 334kJ kg -1
      • Specific heat capacity of ice = 2,260J kg -1 K -1
      • Specific heat capacity of water  = 4, 200J kg-1 K-1
      • Latent heat of vaporization = 2, 260KJ kg -1 K -1

Calculate the minimum time required for this activity.

 

EVAPORATION AND BOILING

  1. Distinguish between evaporation and boiling (2 mks)

 

  1. State the two factors that affect the boiling point of water and in each case, explain how the boiling is affected. (4 mk)

 

  1. State two factors that would raise the boiling point of water to above 1000c

 

  1. When two pieces of ice blocks are squeezed together once they form one block. Explain

 

  1. Figure below shows a block of ice with two heavy weights hanging such that the copper wire / string connecting them passes over the block of ice.

 

 

 

 

 

 

 

 

 

  • It is observed that the wire gradually cuts its way through the ice block, but leaves it as one piece. Explain

(ii)     What change would be observed if the  copper wire used in the experiment was      replaced by a cotton thread.  Explain your answer.

 

  1. A student put some small pieces of ice in a beaker and sprinkled salt on the ice. He stirred until the ice melted and took the temperature of the content in the beaker- as shown.

 

 

 

 

 

 

 

(i)     State the observation made.                                      (1mk)

(ii)     Explain the observation.                                  (1mk)

 

  1. The figure below shows two identical containers A and B containing hot water and ice block.

 

 

 

 

 

 

 

 

State with reason which water cools faster assuming that the wire gauge absorbs negligible heat                                                                                      (2mk)

 

  1. Figure below shows two identical beakers A and B. Beaker A contains water at 0°C while B contains water and pieces of ice at 0°C. Both contents have the same mass.

 

 

 

 

 

 

 

Two identical metal blocks are removed from the same hot furnace and dropped into each of the beakers. Identify which of the two beakers would experience more evaporation and give a reason for your answer.(2mk)

 

  1. Figure below shows two sufurias with equal amount of water R heated at the same rate.

 

 

 

 

 

 

 

 

 

  1. i) State which water boiled first.          (1mk)
  2. ii) Explain your answer                  (1mk)

 

  1. The diagram below shows two cups of tea containing equal volumes of hot tea. The cups have different diameters.

 

 

 

 

 

 

State giving reasons, the cup whose tea will cool faster than the other.   (2mk)

 

  1. Ether is put into a beaker which is placed on a thin film of water. A student blows the ether through a pipe continuously. State and explain the observation made after some time         (2mk)

 

  1. The rate of evaporation can be increased in a number of ways. State two ways in which this could be achieved                                        (2mk)

 

  1. Some water is stored in a bag made of a porous material e.g. canvas which is hung where it is exposed to a draught of air (wind). Explain why the temperature of the water is lower than that of the air. (2 mk)

 

  1. Explain why food cooks faster in a pressure cooker than in an open sufurias

 

  1. Explain why the cooling unit (freezer) inside a refrigerator is placed near the top but an elastic immersion heater in water tank should be near the bottom.(2mk)

 

  1. A thin layer of water underneath a beaker containing ether freezes when air is blown through ether. Explain why the water freezes. (1mk)

 

  1. Hot milk in a bottle cools faster when wrapped in a wet cloth than when the bottle is immersed in cold water in a bucket. Explain why.          (3 mk)

 

  1. State the property of Freon that makes it useful as a refrigerant liquid (1mk)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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

 

 

 

 

                           

 

 

 

 

 

 

 

 

 

 

 

 

 

  • State the reason for using a volatile liquid. (1mk)

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

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

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

 

  1. Figure below shows a domestic electric refrigerator

 

 

 

 

 

 

 

 

 

 

 

(i)     Label the parts

X____________                                            (1mk)

         Y____________                                            (1mk)

(ii)     What property is considered when selecting the liquid used as refrigerant?                                           (1mk)

(iii)    If you stand near the back of a refrigerator you feel warm. Explain.                                                                                            (1mk)

(iv)    Explain how the refrigerator works                           (4mks)

SCHEME

  1. Give the property of water which makes it suitable for use as a coolant in machines                                                                                           (1mk)

Water has a high heat capacity hence can absorb a lot of heat Pj

  1. A burn from steam is more severe than one from water boiling at the same temperature .Give reasons.
    1. Steam already contains latent heat of ; P evaporation which water doesn’t have ;P

 

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