Pause and Ponder
Q1. What do you think about Thirav's view that ice and water are different substances? How can you find out?
Answer:
Thirav thinks ice and water are different because ice is hard and can be held, while water cannot be held. However, they are the same substance in different states. To find out, we can put water in a freezer—it turns into ice. Then we can leave the ice at room temperature—it melts back into water. This shows they are the same substance.
Revise, Reflect, Refine
Exercises
Think and Answer
Let us think
Q1. Have you ever noticed water in the puddles disappearing? Where does it go?
Answer:
Yes, water in puddles disappears after some time. The water goes into the air as water vapour through the process of evaporation. Some water may also seep into the ground.
Q2. Where else have you seen water disappearing? Can you think of a possible reason why this happens?
Answer:
Other examples: wet clothes drying, mopped floor drying, sweat drying from our body, water left on utensils drying up. The reason is evaporation—water turns into water vapour and mixes with the air.
Q3. If water does not seep through the steel plate, then where has the water gone?
Answer:
The water has turned into water vapour (gaseous state) through the process of evaporation. The water vapour mixes with the air around us.
Q4. 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?
Answer:
Water from puddles disappears due to both (i) seeping into the ground and (ii) evaporation into the air. Some water goes into the soil, and some turns into water vapour.
Q5. Hand sanitiser disappears as you rub it on your hands. What happens to it?
Answer:
Hand sanitiser contains alcohol which evaporates very quickly at room temperature. When we rub it on our hands, it turns into vapour and disappears into the air.
Q6. Suggest possible reasons explaining the appearance of water droplets on the outer surface of the glass tumbler.
Answer:
Possible reasons: (1) Water may be seeping through the glass. (2) Ice may have come out and melted. (3) Water vapour from the air is condensing on the cold surface of the glass tumbler. The correct reason is that water vapour in the air comes in contact with the cold glass surface and condenses into water droplets.
Q7. Where else have you seen water droplets like this?
Answer:
We see similar water droplets on: (1) Dew drops on plants in the morning. (2) The inner side of a steel plate when we cover a boiling water utensil. (3) A cold bottle taken out of the refrigerator.
Q8. Why do we see dew drops more in the morning?
Answer:
In the morning, the temperature is cooler. The cold surfaces of plants cause the water vapour present in the air to condense into tiny water droplets, forming dew.
Q9. When we boil water in a half-filled utensil and cover it with a steel plate, some water drops accumulate on the inner side of the steel plate. Where do these water drops come from? What do you think?
Answer:
These water drops come from the steam (water vapour) rising from the boiling water. When the hot steam touches the cooler steel plate, it condenses back into liquid water droplets.
Q10. Predict what will happen to the mass of cold water kept on the digital weighing balance. Will it increase or decrease or remain the same?
Answer:
The mass will increase. The cold glass tumbler causes water vapour from the air to condense on its outer surface, adding extra water. This extra water increases the total mass measured on the balance.
Q11. Do your findings match with your predictions? Explain your observations.
Answer:
Yes, the findings should match the prediction. The mass increases because water vapour from the air condenses on the cold surface of the glass tumbler, adding weight. The water level inside the tumbler does not decrease, confirming that water is not seeping out.
Check your progress
Let us draw
Q1. Draw a detailed sketch (with labels and caption) about what happens to the water.
Answer:
Draw a puddle of water on the ground. Show arrows going upward from the puddle labeled 'Evaporation - water turns into water vapour'. Show arrows going downward into the ground labeled 'Seeping into soil'. Caption: 'Water from puddles disappears due to evaporation and seepage.'
Let us observe
Q1. 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?
Answer:
We can conclude that ice melts into water at room temperature. This shows that ice and water are the same substance—just in different states (solid and liquid). Ice is the solid state of water.
Let us investigate
Q1. Take a tablespoon of water on a steel plate. Observe whether water seeps through to the other side of the plate or not. Keep observing 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?
Answer:
We observe that water does not seep through the steel plate. The water simply disappears from the surface. This activity is enough to conclude that water does not seep through a steel plate because steel is non-porous. The water must have evaporated into the air.
Q2. What conditions did we keep the same? What did we change in this investigation? What did we measure? (Refer to Activity 8.7)
Answer:
Conditions kept the same: Same amount of water, same room temperature, same time of observation. What we changed: The exposed area of water (bottle cap with small area vs. plate with large area). What we measured: The time taken for complete evaporation.
Q3. What can you conclude from this investigation?
Answer:
We conclude that the rate of evaporation depends on the exposed surface area. A larger exposed area (plate) allows faster evaporation compared to a smaller exposed area (bottle cap) because more water molecules are exposed to air at the same time.
Q4. What would happen if milk is taken instead of water in the above activity?
Answer:
If milk is taken instead of water, evaporation would take longer because milk contains solids (fat, protein) that slow down the evaporation process. Milk would also leave a residue after evaporation, unlike pure water which leaves no residue.
Let us experiment
Q1. Take cold water in a glass tumbler. Add a few ice cubes into it. Leave it undisturbed for five minutes and observe it. Record your observations and the questions that arise in your mind. You can also touch the outer surface of the glass tumbler to feel if there is any change.
Answer:
Observations: Small water droplets appear on the outer surface of the glass tumbler. The outer surface feels cold and wet. The droplets grow bigger and may start dripping down. Questions that may arise: 'Where are these water droplets coming from?', 'Is water seeping through the glass?', 'Is the ice melting and coming out?' The correct explanation is that water vapour from the air condenses on the cold glass surface.
Let us measure
Q1. Take a glass tumbler half-filled with water containing a few ice cubes. Cover it with a small steel plate. Weigh it on a digital weighing balance. Observe the reading on the balance and record the weight after every five minutes for 30 minutes. Predict what will happen to the mass. Do your findings match with your predictions?
Answer:
Prediction: The mass will increase. Findings: Mass increases over time because water vapour from the air condenses on the cold outer surface of the glass tumbler. This extra water adds to the total mass. The level of water inside the tumbler remains the same (if marked), confirming no seepage. Yes, findings match the prediction.
Let us identify
Q1. Put an ice cube in one container and transfer it to another container of different shape. What changes do you notice in the shape of the ice cube?
Answer:
The ice cube does not change its shape when transferred to a different container. It retains its own shape because it is a solid. Solids have a fixed shape regardless of the container.
Q2. Pour water from one container to another container of a different shape. Observe how water behaves compared to the ice cube. What happens to its shape?
Answer:
Water changes its shape to match the shape of the new container. Unlike ice, water flows and takes the shape of the container it is poured into. This is because liquids do not have a fixed shape—they take the shape of their container.
Q3. Pour water on a clean surface and observe how it spreads.
Answer:
Water spreads out on the clean surface, forming a thin layer. It does not stay in a fixed shape but spreads to cover as much area as possible. The volume remains constant.
Q4. When water gets converted into water vapour, how does this water vapour spread? Compare this with the spreading behaviour of water.
Answer:
Water vapour spreads out in all directions to fill the entire available space in the room. It spreads much more than liquid water. While liquid water spreads only on surfaces, water vapour spreads throughout the air, becoming invisible. Water vapour in the gaseous state can reach every corner of a room.
Let us complete the diagram
Q1. Fill up the blank boxes in Fig. 8.5 marked as A, B, C and 1, 2, 3, 4 for conversion of different states of water using the words: Liquid, Freezes, Evaporates, Gas, Condenses. Two words have been filled for you (Melts and Solid).
Answer:
In the diagram: A = Liquid (the middle state between solid and gas). B = Gas (the gaseous state of water). C = Liquid (returning from gas to liquid). 1 = Melts (already filled - solid to liquid). 2 = Freezes (liquid to solid). 3 = Evaporates (liquid to gas). 4 = Condenses (gas to liquid). So the completed diagram shows: Solid ? (Melts) ? Liquid ? (Evaporates) ? Gas. Gas ? (Condenses) ? Liquid ? (Freezes) ? Solid.
Let us investigate (Conditions affecting evaporation)
Q1. Take water in a small cap of a bottle. Take the same amount of water in a plate. The exposed area of water in the bottle cap and the plate are different. Keep both of them near each other. Record the time taken for the water to completely evaporate in each case. What can you conclude?
Answer:
The water in the plate (larger exposed area) evaporates faster than water in the bottle cap (smaller exposed area). Conclusion: Evaporation is faster when the exposed surface area is larger. More water molecules are exposed to air, allowing more molecules to escape into the air as vapour.
Q2. Design an activity similar to Activity 8.7 to find out what are the other conditions which can affect how fast water will evaporate. What would you change? What would you keep the same?
Answer:
Activity design: Take two plates with the same amount of water and same exposed surface area. Keep one plate in sunlight (hot condition) and the other in shade (cool condition). Keep everything else same (amount of water, container, time). Change only the temperature condition. Measure time for complete evaporation. Observation: Water in sunlight evaporates faster. Conclusion: Higher temperature increases evaporation rate. Other conditions to test: Wind (with fan vs. without fan) - wind increases evaporation. Humidity (dry day vs. humid day) - higher humidity slows evaporation.
More to do!
More to know!
Do you know?
LET US ENHANCE OUR LEARNING
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.
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) Evaporation is very important for water colours (b), since the water in water-colour paint must evaporate for the painted colour to dry and set on the paper; it is not important for crayons, acrylic colours in the same way, or pencil colours, since these do not primarily rely on the evaporation of a liquid to dry (though acrylic colours also dry partly through evaporation of water, the process most centrally dependent on evaporation among the choices is water colours). (ii) Evaporation is very important for writing with an ink pen (b), since the liquid ink must evaporate/dry for the writing to set on the paper; it is not relevant for pencil (a solid, no liquid involved) or ball point pen (uses a thicker, oil-based ink that dries mainly by absorption rather than evaporation).
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 contains water and undergoes continuous evaporation of water (and also transpiration) from its blades, and this evaporation draws heat from the surroundings, producing a cooling effect around it. Plastic grass, being an artificial material with no water content, cannot evaporate any water, so it does not produce this cooling effect, and the space around it feels comparatively warmer.
Q4. Give examples of liquids other than water that evaporate.
Answer:
Examples of other liquids that evaporate include perfume, nail polish remover (acetone), petrol, diesel, alcohol/spirit, kerosene, and hand sanitiser.
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:
A fan does not add heat to the clothes; instead, it speeds up evaporation by continuously moving away the air that has become saturated with water vapour near the wet cloth's surface and replacing it with fresh, drier air. This keeps the rate of evaporation high, since evaporation slows down when the surrounding air already has a lot of water vapour in it (higher humidity near the surface). The heat needed for evaporation still comes from the surroundings (including the clothes and the air itself), and the fan's cooling sensation on our skin is a separate effect caused by faster evaporation of our own sweat; the fan's real role in drying clothes is increasing air movement, not adding heat.
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 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:
Sludge removed from drains contains a large amount of water. Leaving it in heaps for 3 to 4 days allows much of this water to evaporate into the air, which significantly reduces the weight and volume of the sludge. This makes the sludge lighter and easier (and cheaper) to transport to gardens or fields. It also makes it safer to handle, since wet, heavy sludge is more difficult and messier to work with, and reducing its water content through evaporation makes the material more manageable and less likely to cause spillage or contact-related hazards during handling and transport.
Q7. 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:
Activities in a household that involve evaporation include drying of washed clothes, drying of a mopped or washed floor, drying of washed utensils, evaporation of sweat while doing physical work, cooking processes (such as boiling and reduction of liquids), and use of items like perfume, sanitiser, or nail polish remover. Understanding evaporation helps us in daily life by allowing us to choose the best time, place, and method to dry clothes or floors faster (for example, using sunlight, spreading clothes for a larger exposed area, or increasing air movement with a fan), and helps us understand cooling effects, such as why we feel cooler when sweating in a breeze or using devices like earthen pots and pot-in-pot coolers to keep things cool without electricity.
Q8. How is water present in the solid state in nature?
Answer:
In nature, water is present in the solid state as ice, snow, hail, glaciers, ice caps at the polar regions, and frost, particularly in cold regions and at high altitudes such as mountain peaks.
Q9. Reflect on the statement "Water is our responsibility before it is our right." Share your thoughts.
Answer:
Although access to clean water is indeed a basic right for every person, water is a limited natural resource, since only a small portion of the water available on Earth is actually fit for use by plants, animals, and humans (most being in the oceans, which cannot be used directly). Because of this, it is everyone's responsibility to use water wisely, avoid wasting it, and keep water bodies free from pollution, so that this right to clean water remains available and sustainable for present and future generations; without responsible use and conservation, the right to water itself would be threatened.
Q10. The seat of a two-wheeler parked on a sunny day has become very hot. How can you cool it down?
Answer:
The hot seat can be cooled down by sprinkling or wiping it with a small amount of water; the water absorbs heat from the seat as it evaporates, producing a cooling effect on the seat's surface, similar to the cooling effect seen with sprinkling water on floors or roofs during summer.