CHAPTER- 8 A JOURNEY THROUGH STATES OF WATER
NOTES
1. When water changes into the gaseous state, it is called water vapour. Water vapour is another state of water.- The process by which water changes into the vapour state is called evaporation or vaporization.
- The process of evaporation continues constantly even at room temperature.
2. Condensation— When the water vapour present in the air comes into contact with a cold surface, water droplets are formed. The process by which water vapour changes into the liquid state is called condensation.
3. Water is a substance that is found in all three states-
- In the solid state, it exists as ice.
- On heating, ice melts and changes into the liquid state.
- On further heating, water changes into the gaseous state.
4. Ice— Ice (solid state) retains its own shape no matter which container it is placed in, whereas water takes the shape of the container into which it is poured. Ice neither flows nor spreads.
5. Water— Water (liquid state) flows and changes its shape. It has no fixed shape and takes the shape of the container in which it is kept.
6. Water Vapour— Water vapour (gaseous state) shows the property of spreading throughout all the available space. A gas does not have a definite shape. It is present in the air around us.
7. The process by which a solid changes into the liquid state is called melting.
8. The process by which a liquid changes into the solid state is called freezing.
9.If the amount of water present in the air is already high, then water evaporates slowly.
10. Rainfall— As air rises upward from the Earth’s surface, it gradually becomes cooler.
- At a certain height, the air becomes so cool that the water vapour present in it changes into tiny droplets of water.
- These tiny water droplets remain suspended in the air and appear in the form of clouds.
- Many water droplets join together to form large drops of water, which become heavy and start falling downward. These falling drops are called rainfall.
11. Water Cycle— Water from oceans and the Earth’s surface evaporates into the air as water vapour and returns as rain, hail, or snow. Eventually, it flows back into the oceans. This continuous circulation of water is called the water cycle.
LET US ENHANCE OUR LEARNING
Question- 1. Which of following best describes condensation?
1) The conversion of water into its vapour state.
2) The process of water changing from a liquid into gaseous state.
3) The formation of clouds from tiny water droplets.
4) The conversion of water vapour into its liquid state.
Answer- 4) The conversion of water vapour into its liquid state.
Question- 2. Identify in which of the given processes, evaporation is very important-
1) Colouring with
a) Crayons, b) water colours, c) acrylic colours, d) pencil colours
2) Writing on paper with
a) pencil, b) ink pen, c) ball point pen
Answer- 1) Colouring with b) water colours
2) Writing on paper with b) ink pen
Question- 3. We see green coloured plastic grass at many places these days. Space around natural grass feels cooler than space around the plastic grass. Can you find out why?
Answer- Natural grass contains moisture, which evaporates due to the heat of the sun. During the process of evaporation, heat is absorbed, which lowers the temperature of the surrounding area and gives us a feeling of coolness. On the other hand, plastic grass does not undergo evaporation because it does not contain moisture. For this reason, the area around it feels hotter.
Question- 4. Give examples of liquids other than water that evaporate.
Answer- Other liquid substances besides water that can evaporate include:-
- Ethanol (alcohol)
- Petrol
- Perfume
- LPG, CNG
Question- 5. Fans move air around, creating a cooling sensation. It might seem strange to use a fan to dry wet clothes since fans usually make things cooler, not warmer. Normally, when water evaporates, it requires heat, not cold air. What do you think about this?
Answer- When a fan is switched on, the airflow increases, causing the water in wet clothes to evaporate more quickly. The air from the fan speeds up the process of evaporation. Therefore, clothes dry faster with fan air, even though it gives a feeling of coolness. Cooler air increases the rate of evaporation because it can hold more moisture, which speeds up the drying of clothes.
Question- 6. Usually, when sludge is removed from drains, it is left in heaps next to the drain for 3-4 days. Afterward, it is transported to a garden or a field where it can be used as manure. This approach reduces transportation cost of the sludge and enhances the safety of individuals handling it. Reflect upon it and explain how.
Answer- When sludge is removed from drains and left for three to four days, the water in it evaporates. Due to evaporation, the sludge becomes lighter, making it easier to transport from one place to another, and it also occupies less space. As the sludge dries, the number of harmful microorganisms decreases, reducing the risk of adverse effects on the health of the workers. This method is not only economically beneficial but also enhances the safety of the workers.
Question- 7. Observe the activities in your house for a day. Identify the activities that involve evaporation. How does understanding the process of evaporation help us in our daily activities?
Answer- Household activities that involve evaporation:-
- Boiling water– When water is heated, it evaporates as steam.
- Drying clothes– When washed clothes are hung in the sun, water evaporates from them, causing the clothes to dry.
- Cooking in a pressure cooker– When food is cooked in a pressure cooker, steam comes out, which is an example of evaporation.
- Drying dishes after washing– When dishes are left to dry after washing, the remaining water evaporates, leaving them dry.
- In cooking– Because of evaporation, we need to control the amount of water in a vessel while cooking, which prevents food from burning.
- In household cleaning– Due to evaporation, spilled water can dry on its own if not immediately cleaned, but it is better to clean it before it dries to avoid stains.
- Water management– Water kept in the open gradually evaporates, so it should be covered to conserve it.
By understanding the process of evaporation, we can manage time and resources more efficiently in our daily life.
Question- 8. How is water present in the solid state in nature?
Answer- In nature, water is found in solid form in several ways:-
- Ice– This is the most common solid form of water, found in mountains, glaciers, etc.
- Hail– These are small pieces of ice that fall during rainfall.
- Layer of snow/ice– In winter, a thin layer of ice forms on the surface of lakes, ponds, and rivers.
- Icebergs– These are large pieces of ice that float in the sea in polar regions.
Question- 9. Reflect on the statement "water is our responsibility before it is our right." Share your thoughts.
Answer- This statement tries to make us understand that water is extremely important for our life, and conserving and using it wisely is our foremost responsibility.
Water is a limited resource, and wasting it will not only affect us but also impact future generations.
If we use water wisely, we can preserve it and enjoy its benefits. If we misuse it, it will not only fail to meet our needs but its availability will also become limited. Conserving water is our responsibility, and only by using it properly can we truly claim our right to it.
Question- 10. The seat of a two-wheeler parked on a sunny day has become very hot. How can you cool it down?
Answer- A two-wheeler seat parked in the sun can be cooled using the following methods:-
- Parking in the shade– Placing the vehicle in the shade reduces the impact of the sun’s rays on the seat.
- Covering the seat– Covering the seat prevents it from coming in direct contact with sunlight, keeping it from heating up.
- Using a wet cloth– Placing a wet cloth on the seat for a while helps reduce its heat.
- Sprinkling water– Sprinkling cold water on the seat cools it down quickly, as evaporation helps the seat lose heat faster.
By using these methods, we can quickly cool a seat heated by the sun and use it comfortably.
