NCERT Book Class 6 Science A Journey through States of Water PDF Download

Chapter 8: A Journey through States of Water

On a bright summer afternoon, Aavi and Thirav enjoy their shikanji (lemonade). After looking at the ice in the lemonade, Thirav wonders about the nature of ice and water.

Aavi has a counterview to Thirav. What do you think? Why?

Do you think Thirav is right? How can you find out?

Ice feels hard to touch and we can hold it in our hands, whereas, water cannot be held in the same way. So, they must be different substances.

No, these are the same substances.

We can put water in the freezer of a refrigerator and check if it gets converted into ice.

Yes, I know when water is left in the freezer, it gets converted into ice but probably something gets added to the ice in the freezer.

Activity 8.1: Let us observe

• Put an ice cube in a cup, leave it on the table and observe.

Ice gets converted into water.

What can you conclude from the observations?

Does this mean that ice and water are the same substance?

Yes, ice and water are the two forms of the same substance. These forms are also called states. These different states of water show many differences in their behaviour. Water flows but ice does not. Water splashes but ice does not.

Teacher's Note

Remember that state means the physical form of a substance, not a change in what the substance is. When ice melts into water, you still have H₂O - nothing has been added or taken away. Only the form has changed. This is why Aavi is correct: ice and water are the same substance in different states.

8.1 Investigating Water's Disappearing Act

It is a rainy morning. While going to school, Aavi and Thirav observe that there are a lot of water puddles in the playground. That evening, when they go to play, they are surprised to observe that some of the water in the puddles had disappeared.

Have you ever noticed water in the puddles disappearing? Where does it go? Discuss with your friends.

Where else have you seen water disappearing? Can you think of a possible reason why this happens?

I think the water has been absorbed by the soil on the playground. What do you think about it?

You might have observed that after washing the utensils, water left on the surface of the utensils, dries up after some time. Does the reason you thought earlier to explain water disappearance apply in this case also?

Aavi wonders if water has seeped through the surface of the utensils.

Thirav thinks that water does not seep through the surface of the utensils. Design an activity to investigate whose idea is correct.

Activity 8.2: Let us investigate

• Take a tablespoon of water on a steel plate as shown in Fig. 8.1.
• Observe whether water seeps through to the other side of the plate or not.
• Keep observing this at regular intervals until the water completely disappears.

What do you infer? Is this activity enough to come to the conclusion that water does not seep through a steel plate?

If water does not seep through the steel plate. Then, where has the water gone?

This water gets converted into gaseous state called water vapour. The water vapour is another state of water. Let us think of another observation where you notice the water disappearing.

While making dosa, we sprinkle some water on the hot pan and it disappears. Where does it go?

[Figure 8.1: Steel plate with a tablespoon of water, See in your textbook]

The water which is sprinkled on the hot pan gets converted into steam. Steam is actually water vapour, some part of which converts into water droplets.

The process of conversion of water into its vapour state is called evaporation.

The process of evaporation takes place continuously, even at room temperature. Can you think of other examples of evaporation?

Drying of wet clothes, mopped floor, and sweat on our body are some examples of it.

Now what do you think is the reason for the disappearance of water from the puddles? Is it due to: (i) seeping of water into the ground or (ii) evaporation of water or (iii) both of these?

Hand sanitiser disappears as you rub it on your hands. What happens to it?

Do you know?

Water vapour is actually invisible but the presence of tiny droplets of water in the steam makes it visible.

Teacher's Note

Evaporation happens at all temperatures, not just when water is hot. This is a key point many students miss. The puddles disappear on a cool morning, wet clothes dry indoors at room temperature, and sweat evaporates from your skin - all because evaporation is constantly occurring. The higher the temperature, the faster it happens, but it never stops.

Let us draw

Draw a detailed sketch (with labels and caption) about what happens to the water.

Key Points

• Ice and water are two different states of the same substance; when ice melts, nothing is added or removed, only the form changes.
• Evaporation is the process where water changes into water vapour (an invisible gas), and it happens continuously even at room temperature, not just when water is heated.
• Water vapour is invisible, but steam (which we see coming from hot pans or kettles) contains tiny water droplets mixed with water vapour, making it visible.

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