Heat Archives

Understanding Specific Latent Heat

December 18, 2020December 18, 2020 by Veerendra

Understanding Specific Latent Heat

Specific Latent Heat

The amount of heat required to change the phase of a substance depends on the mass and the type of material of the substance.
A small ice cube melts quickly but a large block of ice melts very slowly.
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.
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.
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.
where, Q = latent heat absorbed or released by the substance
m = mass of the substance
The SI unit for specific latent heat is J kg^-1.
The latent heat absorbed or released when a substance of mass m changes from one phase to another is given by:
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.
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.
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.
If heat is supplied electrically by a heater to change the phase of a substance, the formula Q = ml can be written as:
where, P = power of the heater, in watts (W)
t = time the heater is switched on, in seconds (s)
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.
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?

The particles in a solid vibrate about their fixed mean positions. Figure shows the transition of a solid to a liquid.
During melting, the latent heat absorbed is used to weaken the bonds between the particles.
The particles are freed from their fixed positions and are able to vibrate and move among each other as particles of a liquid.
The heat absorbed by a melting solid is known as the latent heat of fusion.
The heat required to convert a solid into the liquid state is called latent heat of fusion.
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
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?

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.
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.
The heat absorbed during boiling is known as the latent heat of vaporisation.
The heat required to convert a liquid into the vapour state is called latent heat of vaporisation.
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
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

Beaker A and beaker B are placed on their respective electronic balances. The readings of the electronic balances are reset to zero.
The apparatus is set up as shown in Figure. Initially the filter funnels are not placed over the beakers.
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.
After 10 minutes, the mass of water collected in beaker A and beaker B are measured and recorded as m₁ and m₂respectively.
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

Discussion:

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.
Crushed ice was used because the large surface area of the crushed ice allows for a more efficient absorption of heat.
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

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

Results:
Specific Latent Heat Experiment 4
Analysis of Data:

Discussion:

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.
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

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.
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:

What is meant by Latent Heat

December 3, 2020December 3, 2020 by Raju

What is meant by Latent Heat

Latent Heat

Figure shows some ice cubes in a persons hand.
What is meant by Latent Heat

The person makes the following observations and inferences, as shown in Table.

Observation	Inference
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 temperature	The 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 melting	There is change of phase from solid to liquid

Therefore, when ice melts, it absorbs heat without a change in its temperature.
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.
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.
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.

Table shows a summary of the processes and the latent heat involved.

Process	Change of phase	Occurs at	Latent heat
Melting	Solid to liquid	Melting point	Absorbed
Boiling	Liquid to gas	Boiling point	Absorbed
Condensation	Gas to liquid	Boiling point	Released
Solidification or freezing	Liquid to solid	Melting point or freezing point	Released

The four main changes of phase are melting, boiling, condensation and solidification.
Figure shows the four main changes and the latent heat involved.
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.
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.
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.
The heat absorbed or the heat released at constant temperature during a change of phase is known as latent heat.
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’.

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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

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

Results:

1. Tabulation of results.

Time / s	0	30	60	90	120	150	180	210	240	270	300	330	360	390	420
Temperature / °C	25	43	58	70	80	84	87	89	89	89	89	89	89	92	98

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

Before melting, the temperature of the mothballs increases with time.
At a temperature of about 89°C, the mothballs start to melt.
During the melting process, the temperature remains constant although heat continues to be absorbed by the mothballs.
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.

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

Results:

1. Tabulation of results.

Time / s	0	30	60	90	120	150	180	210	240	270	300
Temperature / °C	20	45	65	80	90	96	100	100	100	100	100

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

Before boiling, the temperature of the water increases with time.
At a temperature of about 100°C, the water starts to boil. Bubbles of steam are formed in the water.
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.

Heat Transfer by Convection

May 7, 2022September 17, 2016 by Veerendra

Heat Transfer by Convection

This mode of heat transfer is seen in liquids and gases in which the molecules are less densely packed and are free to move. Thus, convection is the primary mode of heat transfer in fluids.

Let us see the following example and learn how heat is transferred by convection.
The bottom of a vessel containing water is heated. After sometime, the water at the top of the vessel also becomes hot. How does this happen?
Convection-of-Heat

Fig. Hot water (red) rises up and cold water (blue) falls to the bottom.

If a vessel containing water is kept on a flame, the water at the bottom of the vessel gets hot, and as hot water is lighter than cold water, it rises up carrying the heat energy with it (Fig.). The colder and denser water on the top falls to the bottom of the vessel and gets heated in its turn. This process continues and results in a circulating stream of hot and cold water. As a result, the whole water in the vessel gets heated.
This method by which heat is transferred by mass movement of the liquid or gas itself is called convection.

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Practical Applications of Convection

The principle of convection of heat can be used in many applications. When you want to warm a fluid, you should warm it from below. If a room has to be warmed with a room heater, the heater should be placed at a lower level, so that the room gets warmed evenly. Similarly, an air conditioner used for cooling a room should be placed at a higher level.
In earlier times, windows and ventilators were designed and placed so that the buildings were kept cool in summers. Next time you visit old buildings, look out for these special features.

Sea Breeze and Land Breeze
Convection-of-Heat-2
Convection plays a major role in maintaining a moderate temperature in places near the sea. Land masses (beach, coastal town, or city) heat up much faster than water bodies (sea, ocean) during daytime, and cool down much faster during the night. This difference in temperature sets up a wind pattern.
During the day, the air above land rises as it is warm and cooler air from over the sea flows in to take its place. This gives rise to a sea breeze (sometime in the afternoon) which cools the land. In the night, as land cools down much faster than the sea, the cooler air over land flows out to take the place of warmer air over the sea which rises, setting up a land breeze.

Activity

Aim: To show that heat transfer in water is due to convection
Materials needed: A glass beaker made of heat-resistant glass/glass test tube, candle, match box, potassium permanganate crystals/artificial food colouring, and a spoon
Method:

Fill the beaker to the half way mark with clear water.
Take a pinch of the potassium permanganate crystals/food colouring and drop it gently to the bottom of the beaker.
Let it settle for a few minutes.

Convection-of-Heat-1
Observation: You will see that the water at the bottom of the beaker is coloured, but the water at the top is quite clear. Light the candle and pick up the beaker very gently and hold it on top of the candle flame. You will see columns of coloured liquid rise up in the beaker.
Conclusion: This is due to convection. Water molecules at the bottom get heated and rise up, carrying the heat energy with them.

Heat Transfer by Conduction

November 30, 2016September 17, 2016 by Veerendra

Heat Transfer by Conduction

We have seen in earlier examples that metal objects heat up when they come in contact with hot solids or liquids. For example, a stainless steel spoon becomes hot when left in a hot liquid. How do you think this happens?
The heat from the hot liquid moves to the container and from there to the stainless steel spoon.

How Does Conduction Occur?
To understand this, we need to learn what matter is made up of. We know that matter is made up of tiny particles.
You learnt in earlier classes that in solids, the particles are very tightly packed and they cannot move around freely. However, they can vibrate about a fixed point.
particles-of-solid-vibrate-fixed-point

Fig. The particles of a solid vibrate about a fixed point

Let us take the example of a solid rod being heated at one end by a candle flame.
A-solid-rod-being-heated
Heat increases the vibrations of the particles at that end of the rod which is in contact with the flame. These particles collide (bump) with each other and in the process transfer their energy to their more slowly moving neighbours further away from the flame. This makes them vibrate faster and they, in turn, pass the vibration on to their neighbours, even further away from the flame, and so on. In this manner, heat energy is transferred from one particle to the next, although each individual particle remains in its original location. Therefore, we can say that conduction is the mode of heat transfer from a region of lower temperature within a body, or when two bodies are in contact, and it takes place by collisions between neighbouring atoms and molecules of the body.

In case of solid metals or liquid metals, heat is transferred by freely moving electrons.
Conduction is the primary mode of heat transfer through a solid. Conduction of heat energy can occur within a body or between two bodies when they are in contact with each other. However, the rate of conduction of heat is different in different materials. Also, some materials allow heat energy to flow through them easily while others do not.

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Conductors and Insulators

If you observe the utensils in your kitchen, you will notice that though most of them are made of metals, their handles are made of wood or hard plastic.
Why aren’t the handles made of metal as well? This is because metals get heated whereas materials like plastic and wood do not. It would be difficult to hold the handles made of metal while cooking.

conduction
Substances that conduct heat easily are called conductors. For example, metals are conductors of heat.
Substances such as wood, straw, clay, rubber, glass, and Bakelite (a kind of plastic) do not conduct heat very well and are called insulators.
Air and water are insulators. Some materials such as wool, fur, and bird feathers are insulators because they trap air between their fibres.

Practical Applications of Conduction

Both conductors and insulators are useful to us, depending on our needs. We use good conductors (metals) to make cooking utensils, and heat-resistant plastic to make the handles for these vessels.
We wear woollen clothes in winter because wool is a bad conductor of heat. Thus, it helps in retaining body warmth. The wool fibre has a series of curls and these are called ‘crimps’.
These crimps create small air pockets. The greater the number of crimps, the greater is the number of air pockets which can hold and trap air.
Thus, a thin layer of air is created which insulates us from the cold weather and also prevents body heat from escaping into the surroundings.
cooking pan

Fig. Cooking Pan

Activity

Aim: To demonstrate that the rate of conduction of heat is different for different materials Materials needed: Rods of equal diameters and equal lengths made of aluminium, copper, and iron (you will find these in your school lab), two wooden slabs, small metal balls, wax, and flame
Method:

Take two rods at a time, say, the copper and aluminium rods.
Glue the metal balls on to the rods at equal distances using wax.
Arrange the rods as shown in the figure.
Light the burner and make your observations.
Repeat the experiment with iron and aluminium rods.

conduction-2

Observation: You will see that the balls drop fastest for copper, then aluminium and slowest for iron.
Conclusion: Of the three metals, the rate of conduction is highest for copper and the lowest for iron.

What is Heat

November 30, 2016September 17, 2016 by Veerendra

HEAT – A FORM OF ENERGY

Have you stirred hot tea or milk with a stainless steel spoon? Next time you do this, leave the spoon in the hot liquid for a little while. Now, touch the handle of the spoon. What do you notice? The handle of the spoon would have got a little warm. Why did the handle of the spoon become warm?
When there is a difference in temperature between two bodies, a certain type of energy flows between them. This energy is called heat energy.

When this heat energy flows into a body, it warms the body. When it flows out of the body, it cools the body. In other words, when a hot body and a cold body are in contact, the hot body loses heat energy while the cold body gains heat energy.

The point to note is that, left to itself, heat energy flows from a region of higher temperature to a region of lower temperature and never in the other direction (Fig.). Heat flow continues till the temperatures of both the hot and the cold body become the same (whose value will be somewhere in between the temperatures of the hot body and the cold body).
Direction-of-flow-of-heat-energy

Fig. Direction of flow of heat energy

Measuring Heat
Just as we measure length in centimetres and metres, heat energy (and any other form of energy for that matter) is measured in calories or joules. In SI system (which you know is the international standard for the system of units), heat energy is measured in joules. The symbol used to represent ‘joule’ is ‘J’.

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Preventing Loss of Heat
A thermos flask (Fig.) is a very good example of how heat loss by all three modes of heat
transfer, namely, conduction, convection, and radiation is minimized. Let us see how this is done.
thermos-flask

Fig. Diagrammatic representation of a thermos flask.

Heat loss due to conduction is minimized by using insulating materials (like plastic) for the outer casing and the cap of the thermos flask. The inner jar is a double-walled bottle made of glass or stainless steel. The space between the two walls is a vacuum, so that heat loss due to conduction is minimized.
This also reduces heat loss due to convection as there are no air molecules to carry the heat away. Heat loss due to radiation is minimized by making the surface of the jar highly reflective, so that heat radiations are reflected back into the jar

Activity

Aim: To show that heat energy flows from a hot body to a cold body Materials needed: A coin, tongs, boiling water, cold water, and two glasses

Method:

Take two glasses and fill them to three-fourths of the capacity with cold water.
Boil water in a vessel and drop a coin in the boiling water. Leave it for some time so that the coin gets really hot. Ask an adult to use the pair of tongs to pick up the hot coin and drop it in one of the glasses filled with water.
After about 2 minutes, dip your finger in the two glasses, first in the glass without the coin and then in the glass with the coin.

HEAT-ENERGY-1

Observation: You will observe that the water in the glass in which the hot coin was dropped will be warmer.
Conclusion: Heat energy has been transferred from the hot body (coin) to the cold body (water).