Questions and Answers
Q1. Which of the following best describes condensation?
(i) The conversion of water into its vapour state. (ii) The process of water changing from a liquid into gaseous state. (iii) The formation of clouds from tiny water droplets. (iv) The conversion of water vapour into its liquid state.
Answer: (iv) The conversion of water vapour into its liquid state. Explanation: Options (i) and (ii) describe evaporation. Option (iii) is a result of condensation, but the scientific definition is the change of state from gas to liquid.
Q2. Identify in which of the given processes, evaporation is very important—
(i) Colouring with (a) crayons (b) water colours (c) acrylic colours (d) pencil colours (ii) Writing on paper with (a) pencil (b) ink pen (c) ball point pen
Answer: (i) Colouring with (b) water colours and (c) acrylic colours. Explanation: These paints are wet (liquid based). For the painting to dry and become fixed on the paper, the liquid solvent (water) must evaporate. Crayons and pencils are dry solids.
(ii) Writing on paper with (b) ink pen and (c) ball point pen. Explanation: Inks are liquids. They must dry on the paper through evaporation. If they didn’t evaporate/dry, the ink would smudge immediately.
Q3. 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 is a living plant. Plants release water vapour into the air through a process called transpiration (similar to evaporation). As this water evaporates from the surface of the leaves, it absorbs heat from the surroundings, causing a cooling effect. Plastic grass does not release water, so no evaporation occurs, and the area remains hotter as plastic absorbs sunlight.
Q4. Give examples of liquids other than water that evaporate.
Answer:
- Petrol/Diesel: You can smell them at fuel stations because they evaporate very quickly.
- Nail Polish Remover (Acetone): Evaporates rapidly when the bottle is opened.
- Sanitiser (Alcohol): Evaporates quickly on hands.
- Perfume: Sprayed as liquid but turns to gas to spread fragrance.
Q5. 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: While heat increases evaporation, wind speed is also a critical factor. A fan does not cool the air itself; it moves the air. When wet clothes are placed under a fan:
- The moving air quickly blows away the water vapour surrounding the clothes.
- This makes space for more water molecules to escape (evaporate) from the wet fabric.
- Therefore, even without extra heat, the increased air movement speeds up the evaporation process, drying the clothes faster.
Q6. Usually, when sludge is removed from drains, it is left in heaps next to the drain for 3–4 days. Afterward, it is transported… Reflect upon it and explain how.
Answer: Sludge removed from drains contains a lot of water (it is semi-solid/liquid).
- Drying: Leaving it in heaps allows the water content to evaporate over 3–4 days due to sunlight and wind.
- Weight Reduction: As water evaporates, the sludge becomes lighter and drier (solid).
- Transport Efficiency: It is easier, cheaper, and safer to transport dry solid waste than heavy, runny liquid sludge. It also prevents leaking during transport.
Q7. Observe the activities in your house for a day. Identify the activities that involve evaporation.
Answer:
- Drying Clothes: Wet clothes on a line.
- Mopping: Wet floors becoming dry.
- Cooking: Steam rising from boiling curry or tea.
- Hair Drying: Wet hair drying naturally or with a dryer.
- Salt Making: (If near the sea) Seawater evaporating to leave salt.
- Sweating: Cooling down after exercise.
Q8. How is water present in the solid state in nature?
Answer: Water is present in the solid state in nature as:
- Snow: On mountains and in polar regions.
- Ice: Frozen lakes, glaciers, and ice caps (Antarctica/Arctic).
- Hail: Ice balls falling during storms.
- Frost: Thin ice crystals on grass on very cold mornings.
Q9. Reflect on the statement “Water is our responsibility before it is our right.”
Answer: This statement implies that we cannot simply demand water for our use without first taking care of it.
- Water is finite. The water cycle recycles it, but pollution renders it unusable.
- We must conserve water (prevent wastage).
- We must prevent pollution of water bodies.
- Only by fulfilling the responsibility of conservation can we ensure we have the “right” to access clean water in the future.
Q10. The seat of a two-wheeler parked on a sunny day has become very hot. How can you cool it down?
Answer: You can wipe the seat with a wet cloth.
- The water from the cloth spreads on the hot seat.
- The heat from the seat causes the water to evaporate rapidly.
- Evaporation absorbs the heat from the seat, lowering its temperature instantly (Cooling Effect).