Maharashtra Board Class 10 Science Part 1 Chapter 5 Heat PDF Download

5. Heat

Latent heat

Regelation

Anomalous behaviour of water

Dew point and humidity

Specific heat capacity

Can You Recall?

1. What is the difference between heat and temperature?

2. What are the different ways of heat transfer?

In the previous standard, we have learnt about heat and different types of heat transfer. We have also performed few experiments related to expansion and contraction of solids, liquids and gases. We have learnt about the difference between heat and temperature. We have also seen how temperature is measured using a thermometer.

Concepts like latent heat of phase transformation, anomalous behaviour of water, dew point, humidity, specific heat capacity etc are related to certain phenomena experienced by us in our day-to-day life. Let us learn more about these concepts.

5.1 Latent Heat

Try This

1. Take a few pieces of ice in a glass beaker. As shown in figure 5.1.

2. Insert the bulb of a thermometer in ice and measure its temperature.

3. Put the beaker on a stand and heat the ice using a burner.

4. Record the temperature using the thermometer after every minute.

5. As the ice is heated, it starts melting. Stir the mixture of ice and water.

6. Continue the heating even after ice starts melting.

7. Draw a graph of temperature versus time.

You will observe that the temperature of the mixture remains 0 °C till the ice melts completely. If we continue heating, even after conversion of all the ice into water, the temperature of water starts rising and reaches 100 °C. At this temperature water starts converting into steam. The temperature of water remains constant at 100 °C till all water converts into steam. The graph of temperature versus time is shown in figure 5.2.

In this graph, line AB represents conversion of ice into water at constant temperature. When ice is heated it melts at 0 °C and converts into water at this constant temperature. The ice absorbs heat energy during this transition and the absorption of energy continues till all the ice converts into water.

Teacher's Note

When ice melts, it takes heat but temperature stays same. Like when you pay money to buy something, the money goes but its value stays in the item.

Exam Trick

Remember: Latent heat = hidden heat. It is hidden because temperature does not change even though heat is being added. Just like you add money but it is hidden inside the piggy bank.

Points to Remember

Latent heat is heat energy that changes the state of a substance without changing its temperature.
Ice melts at 0 °C and water boils at 100 °C.
During melting, solid becomes liquid but temperature stays the same.
Latent heat of fusion is for melting solids into liquids.
Latent heat of vaporization is for changing liquids into gases.

The temperature remains constant during this transition. This constant temperature, at which the ice converts into water is called the melting point of ice.

Thus, during transition of solid phase to liquid, the object absorbs heat energy, but its temperature does not increase. This heat energy is utilised for weakening the bonds between the atoms or molecules in the solid and transform it into liquid phase. The heat energy absorbed at constant temperature during transformation of solid into liquid is called the latent heat of fusion.

The amount of heat energy absorbed at constant temperature by unit mass of a solid to convert into liquid phase is called the specific latent heat of fusion.

Once all the ice is transformed into water, the temperature of water starts rising. It increases upto 100 °C. Line BC in the graph represents rise in temperature of water from 0 °C to 100 °C. Thereafter, even though heat energy is supplied to water, its temperature does not rise. The heat energy is absorbed by water at this temperature and used to break the bonds between molecules of the liquid and convert the liquid into gaseous state. Thus, during transformation from liquid phase to gas phase, heat energy is absorbed by the liquid, but its temperature does not change. The constant temperature at which the liquid transforms into gaseous state is called the boiling point of the liquid. The heat energy absorbed at constant temperature during transformation of liquid into gas is called the latent heat of vaporization.

The amount of heat energy absorbed at constant temperature by unit mass of a liquid to convert into gaseous phase is called the specific latent heat of vaporization.

Different substances have different melting points and boiling points. The values of melting point, boiling point and latent heat depend on atmospheric pressure.

Use Your Brain Power!

1. Is the concept of latent heat applicable during transformation of gaseous phase to liquid phase and from liquid phase to solid phase?

2. Where does the latent heat go during these transformations?

Teacher's Note

Different materials need different amounts of heat to melt. Water needs 80 calories to melt 1 gram of ice, but copper needs only 49 calories. This is like different people needing different times to wake up.

Exam Trick

Remember: Melting point = where solid becomes liquid. Boiling point = where liquid becomes gas. Higher the latent heat value, more energy is needed for the change.

Points to Remember

Melting point is the temperature at which a solid becomes a liquid.
Boiling point is the temperature at which a liquid becomes a gas.
Latent heat of fusion is always less than latent heat of vaporization.
Water has the highest specific latent heat of fusion among common substances.
These values change with atmospheric pressure.

5.3 Regelation

You may have seen the preparation of an ice-ball. First, an ice slab is shredded and then the shredded ice is pressurised around the tip of a stick to prepare the ice-ball. How does the shredded ice convert into solid ice ball? If two small pieces of ice are taken and pressed against each other for a while, they stick to each other. Why does this happen?

Try This

Take a small slab of ice, a thin wire, two identical weights.

Activity

1. Put a slab of ice on a stand as shown in Figure 5.3.

2. Hang two equal weights to the two ends of a metal wire and put the wire on the slab as shown in the figure.

What do you observe?

It is observed that the wire gradually penetrates the ice slab. After some time, the wire comes out of the lower surface of the ice slab. However, the ice slab does not break. The phenomenon in which the ice converts to liquid due to applied pressure and then re-converts to ice once the pressure is removed is called regelation.

The melting point of ice becomes lower than 0 °C due to pressure. This means that at 0 °C, the ice gets converted into water. As soon as the pressure is removed, the melting point is restored to 0 °C and water gets converted into ice again.

Use Your Brain Power!

1. In the above experiment, the wire moves through the ice slab. However, the ice slab does not break. Why?

2. Is there any relationship of latent heat with the regelation?

3. You know that as we go higher than the sea level, the boiling point of water decreases. What would be effect on the melting point of solid?

Can You Tell?

We feel that some objects are cold, and some are hot. Is this feeling related in some way to our body temperature?

Teacher's Note

When you press ice with a wire, the ice melts under pressure. But when you remove the pressure, it becomes ice again. In Kashmir, people make ice balls by pressing snow just like this.

Exam Trick

Remember: Regelation = pressure melts ice. When the wire pushes on ice, it becomes water. When pressure is removed, water becomes ice again. This is the opposite of normal melting.

Points to Remember

Regelation happens when pressure on ice makes it melt and then freeze again.
The melting point of ice decreases under pressure.
Wire can pass through ice without breaking it due to regelation.
Pressure removes the pressure, ice forms again immediately.
This helps us understand why ice skating is possible.

Anomalous Behaviour of Water

In general, when a liquid is heated upto a certain temperature, it expands, and when cooled it contracts. Water, however, shows a special and exceptional behaviour. If we heat water from 0 °C upto 4 °C, it contracts instead of expanding. At 4 °C its volume is minimum (due to contraction). If the water is heated further, it expands and its volume increases. The behaviour of water between its temperature from 0 °C to 4 °C is called anomalous behaviour of water.

If 1 kg of water is heated from 0 °C and its volume is plotted as a function of temperature, we get the graph, shown in fig 5.4. At 4 °C, the volume of water is minimum. It means that the density of water is maximum at 4 °C.

Teacher's Note

Water is special. Most things expand when heated, but water contracts from 0 °C to 4 °C. This is why ice floats on water and fish can live under ice in winter in India.

Exam Trick

Remember: Water is backwards! From 0 to 4 degrees, it shrinks instead of growing. At 4 °C, water is most dense. Cooler than 4 °C or hotter than 4 °C, water takes more space.

Points to Remember

Water contracts when heated from 0 °C to 4 °C, which is opposite to other liquids.
Water has maximum density at 4 °C.
Below 4 °C, water expands again as temperature decreases to 0 °C.
This anomalous behaviour is why ice is less dense than water and floats.
This property helps aquatic life survive in cold climates.

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