CBSE Important Questions Class 6 Science Chapter 8 – A Journey through States of Water

Class 6 Science Chapter 8 A Journey through States of Water is part of the new NCERT textbook Curiosity. It teaches students how water changes between its three states, solid ice, liquid water and water vapour. Students learn about evaporation and condensation, the factors that make evaporation faster, the cooling effect of evaporation that we feel when we sweat, and the water cycle that keeps water moving between the land, the sea and the sky.

These CBSE important questions for Class 6 Science Chapter 8 cover why water in an open bowl dries up, how sweating and a breeze cool the body, the four factors that affect evaporation, how refrigerators and air conditioners use evaporation, condensation of steam into droplets, the four stages of the water cycle, and why perfume feels cool on the skin. Every question has a clear step-by-step answer in simple English, so students can practise on their own, parents can check the working, and teachers can use them for class tests. A free printable PDF of these important questions is also available on this page.

Q1. A student leaves a bowl of water outside on a sunny day. After a few hours, the water level decreases significantly. Which of the following factors contributed to the decrease in water level?
(i) High temperature
(ii) High humidity
(iii) Presence of wind
(iv) Low pressure
(a) (i) and (ii)
(b) (i) and (iii)
(c) (ii) and (iv)
(d) (iii) and (iv)

Answer: (b) (i) and (iii)
The water level went down because water evaporated, that is, it changed into water vapour and went into the air. Evaporation becomes faster when the temperature is high and when wind blows over the water. High humidity (a lot of moisture already in the air) slows evaporation down, so (ii) is not a reason.

Q2. Ravi washes a set of equipment in the chemistry lab and notices that water remains on the surface of the equipment. However, after leaving the equipment to dry for some time, he observes that the water has disappeared.
(a) Explain the process that causes the water on the surface of the equipment to disappear.
(b) Discuss the factors that can influence the rate at which the water dries from the surface of the equipment.

Answer: (a) The water disappears because of evaporation. The water on the surface slowly changes into water vapour and mixes with the air. This happens at ordinary room temperature; the water does not need to boil.
(b) The water dries faster when:
• the temperature is higher, because the water gets more heat energy;
• there is air movement, such as a breeze or a fan, which carries the vapour away;
• the air is dry (low humidity), because dry air can take in more water vapour;
• the water is spread over a larger surface area.

Q3. During a summer workout, Rahul notices that he feels cooler as he sweats more. To understand this phenomenon, he measured his skin temperature before and after sweating and observed a significant drop in temperature. Which of the following is the main reason for this sudden drop in temperature?
(a) The sweat releases cold air onto the skin.
(b) The sweat absorbs heat from the skin.
(c) The sweat decreases the humidity around the skin.
(d) The sweat increases the air pressure around the skin.

Answer: (b) The sweat absorbs heat from the skin.
Sweat is mostly water. To evaporate, it needs heat, and it takes this heat from the skin. As the sweat evaporates, the skin loses heat and feels cooler. This is called the cooling effect of evaporation.

Q4. Parth observes that after sweating during physical exercise, he feels cooler when a breeze blows. He then decides to investigate this phenomenon by comparing how he feels in a breeze versus in still air after exercising.
(a) Explain the cooling effect of sweating and how it helps regulate body temperature during physical exercise.
(b) Analyse how the presence of a breeze enhances the cooling effect of sweating and why it makes the student feel cooler compared to still air.

Answer: (a) During exercise the body produces extra heat, so the body releases sweat on the skin. When the sweat evaporates it takes heat from the skin, and the body cools down. This keeps the body temperature from rising too high.
(b) In still air, the air just above the skin soon becomes full of water vapour, so evaporation slows down. A breeze blows this moist air away and brings dry air in its place. This makes the sweat evaporate faster, more heat is taken from the skin, and Parth feels cooler.

Q5. Aisha notices that after running on a hot day, she feels much cooler when she stands under a fan compared to sitting in a room without any air movement. She decides to explore this experience further by comparing her comfort level with and without the fan after her run.
(a) Explain the cooling effect of perspiration and how it helps maintain body temperature during physical activity.
(b) Analyse how the presence of a fan enhances the cooling effect of perspiration and why Aisha feels cooler when the fan is on compared to when it is off.

Answer: (a) Running makes Aisha's body warm, so her body sweats (perspires). Sweat is mostly water, and when it evaporates from the skin it takes the heat it needs from the body. This cools the body and stops it from overheating.
(b) When the fan is off, a layer of moist air stays close to the skin and the sweat evaporates slowly. The fan keeps blowing this moist air away and brings dry air, so the sweat evaporates much faster. Faster evaporation removes more heat from the skin, which is why Aisha feels much cooler under the fan.

Q6. Evaporation is a process in which a liquid turns into a gas. This process occurs all around us, such as when water dries up from a wet surface or when sweat evaporates from our skin. Several factors influence the rate at which evaporation occurs. Explain the key factors that affect the rate of evaporation. Discuss how each of these factors contributes to the process, providing examples from everyday life.

Answer: 1. Temperature: At a higher temperature the water gets more heat, so more of it changes into vapour. Clothes dry faster on a hot day than on a cold day.
2. Surface area: When the liquid is spread out, more of it touches the air, so it evaporates faster. Water in a wide plate dries sooner than the same water in a narrow glass.
3. Humidity: If the air already has a lot of water vapour (high humidity), evaporation is slow. On a humid day clothes take a long time to dry; on a dry day they dry quickly.
4. Wind: Moving air carries the vapour away and brings dry air, so evaporation speeds up. Clothes dry faster on a windy day.

Q7. How does the cooling effect of evaporation play a role in the design of cooling systems, such as air conditioners and refrigerators?

Answer: Air conditioners and refrigerators work on the rule that evaporation takes in heat. Inside them a special liquid called a refrigerant flows through pipes. It takes heat from the room or from the inside of the fridge and evaporates into a gas, which makes that space cold. The gas is then squeezed and cooled back into a liquid outside the appliance, where it gives out the heat it had taken in. This cycle keeps repeating, so the inside stays cool.

Q8. The water which is sprinkled on a 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.
Answer the following questions based on the passage given above:
(a) Discuss the role of temperature in the process of evaporation and how it affects the rate at which water evaporates.
(b) How does the process of condensation relate to the formation of water droplets from steam?
(c) Why does evaporation occur continuously, even at room temperature, and what factors influence its rate?

Answer: (a) The higher the temperature, the more heat energy the water gets, and the faster it changes into vapour. That is why water sprinkled on a hot pan turns into steam at once.
(b) Condensation is the opposite of evaporation. When steam meets cooler air it loses heat, and the vapour changes back into tiny drops of liquid water. These are the droplets we see.
(c) Even at room temperature some water at the surface always has enough energy to escape into the air, so evaporation never stops completely. Its rate depends on temperature, surface area, humidity and air movement.

Q9. List the factors affecting the process of evaporation.

Answer: The rate of evaporation depends on:
1. Temperature: higher temperature, faster evaporation.
2. Surface area: larger surface, faster evaporation.
3. Humidity: less moisture in the air (dry air), faster evaporation.
4. Wind: more air movement, faster evaporation.

Q10. Describe the main stages of the water cycle.

Answer: The water cycle has four main stages:
1. Evaporation: The heat of the Sun changes water from oceans, lakes and rivers into water vapour, which rises into the air.
2. Condensation: High up, the vapour cools and changes into tiny water droplets, which form clouds.
3. Precipitation: When the droplets in the clouds become big and heavy, they fall to the ground as rain, snow or hail.
4. Collection: The water flows into rivers, lakes and oceans, or soaks into the ground, and the cycle starts again.

Q11. Explain how surface area affects the rate of evaporation and describe a real-life situation where this is important.

Answer: When a liquid is spread over a larger area, more of its surface touches the air, so more water can escape as vapour at the same time. Hence a larger surface area makes evaporation faster.
Real-life example: Wet clothes dry much faster when they are spread out on a clothesline than when they are left in a bunched-up heap, because spreading them out increases the surface area.

Q12. Why do we feel cool when we apply perfume on our skin?

Answer: Perfume is a liquid that evaporates very quickly. To change from liquid to vapour it needs heat, and it takes this heat from our skin. As the skin loses heat, we feel cool. This is the cooling effect of evaporation, the same reason why sweating cools our body.

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