Understanding Specific Latent Heat

Understanding Specific Latent Heat

Specific Latent Heat

  1. The amount of heat required to change the phase of a substance depends on the mass and the type of material of the substance.
  2. A small ice cube melts quickly but a large block of ice melts very slowly.
  3. 100 J of heat energy can melt a large amount of wax, but the same amount of heat can only melt a small amount of iron.
  4. The specific latent heat of a substance is the amount of heat required to change the phase of 1 kg of the substance at a constant temperature.
  5. The specific latent heat, l, of a substance can be calculated from the latent heat, Q, absorbed or released and the mass, m, of the substance that undergoes the change of phase.
    Understanding Specific Latent Heat 1where, Q = latent heat absorbed or released by the substance
    m = mass of the substance
  6. The SI unit for specific latent heat is J kg-1.
  7. The latent heat absorbed or released when a substance of mass m changes from one phase to another is given by:
    Understanding Specific Latent Heat 2
  8. The specific latent heat of fusion of a substance is defined as the amount of heat required to change 1 kg of the substance from the solid to the liquid phase without a change in temperature.
  9. The specific latent heat of vaporisation of a substance is defined as the amount of heat required to change 1 kg of the substance from the liquid to the gaseous phase without a change in temperature.
  10. The specific latent heat of vaporisation of a substance is usually larger than the specific latent heat of fusion. This is due to the extra work done against atmospheric pressure during the change of phase from liquid to gas.
  11. If heat is supplied electrically by a heater to change the phase of a substance, the formula Q = ml can be written as:
    Understanding Specific Latent Heat 3where, P = power of the heater, in watts (W)
    t = time the heater is switched on, in seconds (s)
  12. Figure shows the heat and latent heat required to change a block of ice into steam. Note that the largest amount of heat is required for the change of phase from water to steam. This is due to the additional work done against atmospheric pressure when the steam expands into the air.
    Understanding Specific Latent Heat 4
  13. Table shows the specific latent heats of some common substances.

Understanding Specific Latent Heat 5

People also ask

What is the Latent Heat of Fusion?

  1. The particles in a solid vibrate about their fixed mean positions. Figure shows the transition of a solid to a liquid.
    What is meant by Latent Heat 4
  2. During melting, the latent heat absorbed is used to weaken the bonds between the particles.
  3. The particles are freed from their fixed positions and are able to vibrate and move among each other as particles of a liquid.
  4. The heat absorbed by a melting solid is known as the latent heat of fusion.
  5. The heat required to convert a solid into the liquid state is called latent heat of fusion.
  6. In other words ‘The latent heat of fusion of a solid is the quantity of heat in joules required to convert 1 kilogram of the solid to liquid, with out any change in temperature.
    Ex: The latent heat of fusion of ice = 3.34 × 105 J/kg
  7. For a liquid to solidify at its freezing point, latent heat of fusion has to be removed from it.

What is the Latent Heat of Vaporisation?

  1. The particles in a gas are in continuous random motion and they are much further apart from each other than the particles of a liquid.
    What is meant by Latent Heat 5
  2. When a liquid boils, the latent heat absorbed is used to:
    (a) completely break the bonds between the particles,
    (b) do work against atmospheric pressure when the gaseous vapour expands into the atmosphere.
  3. The heat absorbed during boiling is known as the latent heat of vaporisation.
  4. The heat required to convert a liquid into the vapour state is called latent heat of vaporisation.
  5. The other words ‘The latent heat of vaporisation of a liquid is the quantity of heat in joules required to convert 1 kilogram of the liquid to vapour or gas, without any change in temperature.
    Ex: Latent heat of vaporisation of water = 22.5 × 105 J/kg
  6. When vapour condenses back into a liquid, latent heat of vaporisation is released.

Latent Heat of Fusion of Ice Experiment

Aim: To determine the specific latent heat of fusion of ice.
Material: Crushed ice
Apparatus: Two immersion heaters, two filter funnels, stopwatch
Method:
Specific Latent Heat Experiment

  1. Beaker A and beaker B are placed on their respective electronic balances. The readings of the electronic balances are reset to zero.
  2. The apparatus is set up as shown in Figure. Initially the filter funnels are not placed over the beakers.
  3. The heater for experimental set-up is switched on. When water is dripping out of the filter funnels at a constant rate, the filter funnels are placed over Beaker A and Beaker B respectively, and a stopwatch is started.
  4. After 10 minutes, the mass of water collected in beaker A and beaker B are measured and recorded as m1 and mrespectively.
  5. The power of the immersion heater, P, is recorded.

Results:
Specific Latent Heat Experiment 1
Analysis of Data:
Mass of ice melted by the heat supplied from the heater,
m = (m1 – m2) g
= (97.0 – 12.0) g = 85.0 g
= 0.085 kg
Heat supplied, Q = Pt
= 48 x 10 x 60 J = 28 800 J
Specific Latent Heat Experiment 2
Discussion:

  1. The temperature of ice is lower than the room temperature. The ice will absorb heat from the surroundings and melt by itself. The control set of apparatus was set up to determine the mass of ice that was melted by the heat from the surroundings. Only then, the correct mass of ice melted by the heater can be determined.
  2. Crushed ice was used because the large surface area of the crushed ice allows for a more efficient absorption of heat.
  3. The value of the specific latent heat of fusion of ice obtained from this activity is larger than the standard value. Some of the heat supplied by the heater is lost to the surroundings. In the calculation, it was assumed that no heat was lost to the surroundings and all the heat supplied was absorbed by the ice only.

Conclusion:
The specific latent heat of fusion of ice determined by the activity is 3.39 x 105 J kg-1.

Latent Heat of Vaporisation of Water Experiment

Aim: To determine the specific latent heat of vaporisation of water. Material: Tap water
Apparatus: Immersion heater, beaker, electronic balance, stopwatch
Method:
Specific Latent Heat Experiment 3

  1. The apparatus is set up as shown in Figure.
  2. The heater is turned on to boil the water.
  3. When the water is boiling steadily, the stopwatch is started. At the same time, the reading on the electronic balance is recorded as m1
  4. After a time, t = 10 minutes, the reading on the electronic balance is recorded as m2.
  5. The heater is switched off. The power of the heater, P is recorded.

Results:
Specific Latent Heat Experiment 4
Analysis of Data:
Specific Latent Heat Experiment 5
Discussion:

  1. A high powered heater is used so that the boiling time is short. The amount of water that evaporates away during the short time of the experiment is very small and can be neglected. Therefore, a control set of apparatus is not necessary-for this activity.
  2. The value of the specific latent heat of vaporisation of water obtained from this activity is larger than the standard value. Some of the heat supplied by the heater is lost to the surroundings. In the calculation, it was assumed that no heat was lost to the surroundings and all the heat supplied was absorbed by the water only.

Conclusion:
The specific latent heat of vaporisation of water determined by the activity is 2.40 x 106 J kg-1.

Specific Latent Heat Example Problems With Solutions

  1. A plastic bag containing 0.80 kg of soup at 38°C is put into the freezer compartment of a refrigerator. Calculate the amount of heat that must be removed to completely solidify the soup.
    Assume the soup solidifies at -2°C.
    [Specific heat capacity of the soup c = 4250 J kg-1 °C-1; specific latent heat of fusion of the soup, l = 3.38 x 105 J kg-1]
    Solution:
    The changes from liquid soup to solid can illustrated in the following diagram.
    Specific Latent Heat Example
  2. In a boiler, water boils at 120°C. How much heat must be supplied to 4.0 kg of water at 30°C to change it to steam at the same temperature?
    [Specific heat capacity, c = 4200 J kg-1 °C-1; specific latent heat of vaporisation, l = 2.26 x 106 J kg-1]
    Solution:
    Specific Latent Heat Example 1
    Specific Latent Heat Example 2

What is meant by Latent Heat

What is meant by Latent Heat

Latent Heat

  1. Figure shows some ice cubes in a persons hand.
    What is meant by Latent HeatThe person makes the following observations and inferences, as shown in Table.

    ObservationInference
    The ice is colder than the surroundings.Heat energy is continuously being absorbed by the ice from the surroundings.
    The ice is at a constant temperatureThe heat energy absorbed by the ice does not cause an increase in temperature. The heat absorbed is not transferred to the molecules of ice as kinetic energy.
    The ice is meltingThere is change of phase from solid to liquid
  2. Therefore, when ice melts, it absorbs heat without a change in its temperature.
  3. A similar situation is observed when heat is supplied continuously to boiling water. Placing a thermometer in the boiling water will show that its temperature remains constant.
  4. The heat energy which has to be supplied to change the state of a substance is called its latent heat. (or)
    When a substance undergoes a change of phase such as melting or boiling, it absorbs heat without an increase in its temperature. The heat absorbed is known as latent heat.
  5. From the principle of conservation of energy, we can infer that latent heat is released
    (a) when a gas condenses at a constant temperature to become a liquid.
    (b) when a liquid solidifies or freezes at a constant temperature to become a solid.
  6. Table shows a summary of the processes and the latent heat involved.
    ProcessChange of phaseOccurs atLatent heat
    MeltingSolid to liquidMelting pointAbsorbed
    BoilingLiquid to gasBoiling pointAbsorbed
    CondensationGas to liquidBoiling pointReleased
    Solidification or freezingLiquid to solidMelting point or freezing pointReleased
  7. The four main changes of phase are melting, boiling, condensation and solidification.
  8. Figure shows the four main changes and the latent heat involved.
    What is meant by Latent Heat 1
  9. The heating curve for a substance in the solid state when it is heated uniformly and undergoes a change of phase from solid to liquid to gas is as shown in Figure.What is meant by Latent Heat 2
  10. The cooling curve for a substance in the gaseous state when it cools down and undergoes a change of phase from gas to liquid to solid is as shown in Figure.What is meant by Latent Heat 3
  11. There are three common characteristics when a substance undergoes a change of phase:
    (a) The change of phase occurs at a particular temperature.
    (b) Heat energy is transferred into or out of the substance during the change of phase.
    (c) During the change of phase, the temperature remains constant even though there is transfer of heat.
  12. The heat absorbed or the heat released at constant temperature during a change of phase is known as latent heat.
  13. The transfer of latent heat does not involve a change in the kinetic energy of the molecules.
    Latent heat does not raise the temperature but latent heat has always to be supplied to change the state of a substance. The word ‘latent’ means ‘hidden’.

People also ask

Latent Heat Experiment

1. Aim: To study the change in temperature when heat is supplied to a solid at its melting point.
Materials: Napthalene balls (mothballs) or octadecanol (stearyl alcohol), tap water
Apparatus: Boiling tube, 600 ml beaker, thermometer, stopwatch, Bunsen burner, tripod stand with wire gauze, retort stand with two clamps
Method:

Latent Heat Experiment

  1. Some mothballs are placed in the boiling tube until it is about one third full.
  2. The apparatus is set up as shown in Figure.
  3. The initial temperature of the mothballs is recorded.
  4. The mothballs are heated slowly in the water bath.
  5. The temperature of the mothballs is recorded every 30 s.
  6. The temperature is recorded for a few more minutes after the mothballs have started melting.
  7. The results are tabulated and a graph of temperature against time is plotted.

Results:

1. Tabulation of results.

Time / s0306090120150180210240270300330360390420
Temperature / °C254358708084878989898989899298

2. Graph of temperature against time.
Latent Heat Experiment 1
Discussion:

  1. Before melting, the temperature of the mothballs increases with time.
  2. At a temperature of about 89°C, the mothballs start to melt.
  3. During the melting process, the temperature remains constant although heat continues to be absorbed by the mothballs.
  4. After all the mothballs have melted, the temperature begins to increase again.

Conclusion:
When the mothballs are melting, the temperature remains constant.

2. Aim: To study the change in temperature when heat is supplied to a liquid at its boiling point.
Material: Tap water
Apparatus: 250 ml beaker, thermometer, stopwatch, Bunsen burner, tripod stand with wire gauze, retort stand with clamp
Method:
Latent Heat Experiment 2
Caution: Make sure you stop the heating before the water dries up.

  1. A beaker is filled with tap water.
  2. The apparatus is set up as shown in Figure.
  3. The initial temperature of the water in the beaker is recorded.
  4. The water in the beaker is heated.
  5. The temperature of the water is recorded every 30 s.
  6. The temperature of the boiling water is recorded for a few more minutes.
  7. The results are tabulated and a graph of temperature against time is plotted.

Results:

1. Tabulation of results.

Time / s0306090120150180210240270300
Temperature / °C204565809096100100100100100

2. Graph of temperature against time.
Latent Heat Experiment 3
Discussion:

  1. Before boiling, the temperature of the water increases with time.
  2. At a temperature of about 100°C, the water starts to boil. Bubbles of steam are formed in the water.
  3. The temperature remains constant while the water is boiling although heat continues to be absorbed by the water.

Conclusion:
When the water is boiling, the temperature remains constant.