Which of the following best describes condensation?
- The conversion of water into its vapour state.
- The process of water changing from a liquid into gaseous state.
- The formation of clouds from tiny water droplets.
- The conversion of water vapour into its liquid state.
Answer
The conversion of water vapour into its liquid state.
Reason — The process of conversion of water vapour into its liquid state is called condensation. Options 1 and option 2 describe evaporation, while option 3 refers only to the formation of clouds and not to the process of condensation itself.
Identify in which of the given processes, evaporation is very important—
(i) Colouring with
- crayons
- water colours
- acrylic colours
- pencil colours
(ii) Writing on paper with
- pencil
- ink pen
- ball point pen
Answer
(i) Water colours and acrylic colours
Reason — These colours use water as the medium. After colouring, the water evaporates and the colour dries on the paper. Crayons and pencil colours are solids and do not involve evaporation.
(ii) Ink pen
Reason — the ink is a watery liquid that dries by evaporation after writing. A pencil leaves behind solid graphite and a ball point pen uses a thick, oily ink, so evaporation is not important for them.
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, being a living plant, contains water and continuously loses water from its surface, which evaporates into the air. Evaporation causes a cooling effect, so the space around natural grass feels cooler. Plastic grass is artificial and contains no water and cannot evaporate, so it does not produce any cooling effect.
Give examples of liquids other than water that evaporate.
Answer
Some liquids other than water that evaporate are hand sanitiser, spirit, petrol, kerosene, nail polish remover and perfume.
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
A fan does not make the air colder; it only increases the movement of air around the wet clothes. With greater movement of air, water evaporates faster, so the clothes dry quickly. The fan therefore dries clothes by speeding up evaporation, not by supplying cold air. The cool feeling we get from a fan is also because the moving air makes the sweat on our body evaporate faster.
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 the sludge is left in heaps for 3–4 days, much of the water present in it evaporates. As a result, the sludge becomes drier and lighter, so less effort and cost are needed to transport it. Drying also makes the sludge less wet and less likely to spill or splash, making it safer and easier for people to handle.
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
Some activities at home that involve evaporation are drying of washed clothes, drying of a mopped floor, drying of washed utensils, drying of wet hair, evaporation of sweat from our body, and cooling of water kept in an earthen pot (matka).
Understanding evaporation helps us in daily life. We dry clothes faster by spreading them in sunlight or in a breeze, keep drinking water cool in earthen pots during summer, sprinkle water on floors and roofs to cool them, and use fans and coolers more effectively.
How is water present in the solid state in nature?
Answer
In nature, water is present in the solid state as ice and snow. It is found as snow on high mountains and as ice in glaciers. Under suitable atmospheric conditions, water may also fall as snow or hail.
Reflect on the statement "Water is our responsibility before it is our right." Share your thoughts.
Answer
Only a small part of the water on the Earth is fit for our use, and most of it lies in the oceans which cannot be used directly. As the population grows, the demand for water keeps increasing and many places face a shortage of water. So, before treating water as our right, it is our responsibility to use it wisely, avoid wasting it, and keep our water bodies free from pollution. By conserving water and preventing its pollution, we make sure that enough clean water is available for everyone now and in the future.
The seat of a two-wheeler parked on a sunny day has become very hot. How can you cool it down?
Answer
We can cool the hot seat by sprinkling or wiping a little water over it. The water spread on the seat takes heat from it and evaporates. Since evaporation causes a cooling effect, the seat cools down.