Question:
Aavi has a different opinion from Thirav. What do you think? Why? Do you think Thirav is right? How can you find out?
Answer:
- I think Aavi is correct. Ice and water are the same substance in different forms.
- Why? Because when we freeze water, it becomes ice, and when we melt ice, it becomes water. They are just the same thing in different states.
- Is Thirav right? No, Thirav thinks ice and water are different substances, but they are actually the same.
- How can we find out? We can do a simple experiment. Put water in the freezer and see if it turns into ice. Let ice melt and see if it turns into water. This shows they are the same substance changing forms.
Activity 8.1: Let us observe
Put an ice cube in a cup, leave it on the table, and watch what happens.
Ice gets converted into water.
What can you conclude from the observations?
Does this mean that ice and water are the same substance?
Answer:
- Observation: The ice cube melts and turns into water.
- Conclusion: This shows that ice and water are the same substance in different states.
- Yes, ice and water are the same substance. Ice is solid water.
Question:
“I think the water has been absorbed by the soil on the playground. What do you think about it?”
Answer:
- I think some water is absorbed by the soil, but some water also evaporates into the air.
- Water can disappear from puddles because it soaks into the ground and also because it turns into water vapour and goes into the air.
Question:
Have you ever noticed water in puddles disappearing? Where does it go? Discuss with your friends.
Answer:
- Yes, I have seen water in puddles disappear after some time.
- The water goes into the air as water vapour through evaporation. Some water may also soak into the ground.
Question:
Where else have you seen water disappearing? Can you think of a possible reason why this happens?
Answer:
- I have seen wet clothes drying when water disappears from them.
- I have seen water on the floor drying up after mopping.
- This happens because of evaporation. The water turns into water vapour and goes into the air.
Question:
You might have observed that after washing the utensils, water left on the surface of the utensils dries up after some time. Does the reason you thought earlier to explain water disappearance apply in this case also?
Answer:
- Yes, the reason is the same. The water evaporates into the air.
Question:
Aavi wonders if water has seeped through the surface of the utensils.
Answer:
- Water does not seep through metal utensils.
- The water disappears because it evaporates into the air.
Question:
Design an activity to investigate whose idea is correct.
Answer:
- We can put some water on a steel plate and see if any water comes out on the other side.
- If no water seeps through, then we know that water does not seep through the plate.
- We can observe if the water disappears over time and conclude that it evaporates.
Activity 8.2: Let us investigate
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 conclude that water does not seep through a steel plate?
If water does not seep through the steel plate, then where has the water gone?
Answer:
- Observation: The water on the steel plate disappears after some time. No water is seen on the other side of the plate.
- Inference: Water does not seep through the steel plate.
- Yes, this activity shows that water does not seep through a steel plate.
- The water has evaporated into the air.
Question:
Hand sanitiser disappears as you rub it on your hands. What happens to it?
Answer:
- The sanitiser evaporates into the air.
- It contains alcohol, which evaporates quickly.
Activity 8.3: Let us experiment
Take cold water in a glass.
Add a few ice cubes into it.
Leave it undisturbed for five minutes and observe.
Record your observations and the questions that come to your mind in Table 8.1.
Table 8.1: Observations and Questions
| I observe | I wonder |
|---|---|
| Water droplets form on the outside | Where do these water droplets come from? |
| The glass feels cold on the outside | Is the water leaking from inside the glass? |
Answer:
- Observation: After a few minutes, water droplets appear on the outside of the glass.
- Question: Where do these water droplets come from?
Question:
Suggest possible reasons explaining the appearance of water droplets on the outer surface of the glass.
Answer:
- Possible Reason: The water vapour in the air touches the cold surface of the glass and turns into water droplets. This is called condensation.
- So, the droplets come from the air, not from inside the glass.
Question:
Where else have you seen water droplets like this?
Answer:
- On cold drink bottles taken out of the fridge.
- On car windows on a cold morning.
- As dew on grass and leaves in the morning.
Question:
Why do we see dew drops more in the morning?
Answer:
- Because the air is cooler in the morning, so water vapour condenses more and forms dew drops.
Question:
When we boil water in a half-filled pot and cover it with a steel plate, some water drops collect on the inner side of the plate. Where do these water drops come from? What do you think?
Answer:
- The boiling water turns into steam (water vapour).
- The steam touches the cool steel plate and condenses back into water droplets.
Activity 8.4: Let us measure
Take a glass half-filled with water and ice cubes. Cover it with a small steel plate. Weigh it on a digital weighing balance.
Observe the reading and record the weight every five minutes for 30 minutes.
Predict what will happen to the mass. Will it increase, decrease, or stay the same?
Do your findings match your predictions? Explain your observations.
Table 8.2: Mass Measurements
| Time | Mass (grams) |
|---|---|
| 0 min | 200 g |
| 5 min | 201 g |
| 10 min | 202 g |
| 15 min | 203 g |
| 20 min | 204 g |
| 25 min | 205 g |
| 30 min | 206 g |
Answer:
- Prediction: The mass will increase because water from the air will condense on the glass.
- Observation: The mass increases over time.
- Explanation: Water vapour in the air condenses on the cold glass, adding to the mass.
Question:
Can we conclude that water is not seeping through the wall of the glass? Can we also conclude that the water collected outside is only due to condensation?
Answer:
- Yes, we can conclude that water is not seeping through the glass because the water level inside does not decrease.
- The water collected outside is due to condensation of water vapour from the air.
Question:
What more can you do to show that water is not seeping from the glass? How would you modify Activity 8.4 to find the answer?
Answer:
- We can mark the water level inside the glass and check if it changes over time.
- If the water level inside stays the same, it shows that water is not leaking out.
Activity 8.5: Let us identify
Put an ice cube in one container and move it to another container of a different shape. What changes do you notice in the shape of the ice cube?
Pour water from one container to another of a different shape. Observe how water behaves compared to the ice cube. What happens to its shape?
Pour water on a clean surface and observe how it spreads.
When water turns into water vapour, how does this water vapour spread? Compare this with how water spreads.
What are the differences in the properties of water in solid, liquid, and gas states?
Answer:
Observations:
- Ice Cube: Keeps its shape when moved to another container.
- Water: Takes the shape of the new container, flows easily.
- Water on Surface: Spreads out but stays together.
- Water Vapour: Spreads out and fills the air.
Differences:
| Property | Ice (Solid) | Water (Liquid) | Water Vapour (Gas) |
|---|---|---|---|
| Shape | Fixed | Takes shape of container | No fixed shape |
| Ability to Flow | Does not flow | Flows easily | Spreads everywhere |
| Ability to Spread | Does not spread | Spreads a little | Fills all available space |
Activity 8.6: Let us complete the diagram
Fill up the blank boxes in the diagram using the words given: Liquid, Freezes, Evaporates, Gas, Condenses.
Answer:
- A: Solid
- B: Liquid
- C: Gas
- 1: Melts (Given)
- 2: Evaporates
- 3: Condenses
- 4: Freezes
Activity 8.7: Let us investigate
Take water in a small bottle cap.
Take the same amount of water in a plate.
Keep both near each other.
Record the time taken for the water to completely evaporate in each case.
What conditions did we keep the same?
What did we change? What did we measure?
Answer:
- Conditions kept the same: Amount of water, temperature, location.
- Condition changed: Exposed surface area (small in cap, large in plate).
- Measured: Time taken for water to evaporate completely.
Observations:
| Exposed Area | Time to Evaporate |
|---|---|
| Small (cap) | 2 hours |
| Large (plate) | 1 hour |
Conclusion:
- Water evaporates faster when the exposed area is larger.
Question:
Reflect on what you did well in this activity.
Answer:
- I carefully measured the same amount of water for both containers.
- I observed and recorded the times accurately.
Question:
Design an activity similar to Activity 8.7 to find out what other conditions can affect how fast water evaporates. What would you change? What would you keep the same? Perform this activity, record the data, and discuss your observations.
Answer:
- Activity: Investigate the effect of temperature on evaporation.
- Conditions kept the same: Amount of water, exposed area.
- Condition changed: Temperature (one in sunlight, one in shade).
Observations:
| Condition | Time to Evaporate |
|---|---|
| In sunlight (warm) | 30 minutes |
| In shade (cool) | 1 hour |
Conclusion:
- Water evaporates faster at higher temperatures.
Activity 8.8: Let us explore
Place one bottle cap with water in sunlight and another in shade.
Observe and record the time taken for water to evaporate in each case.
What conclusions can you draw?
Answer:
- Observation: Water in sunlight evaporated faster than water in shade.
- Conclusion: Heat from sunlight increases the rate of evaporation.
Activity 8.9: Let us make a model
Make a pot-in-pot cooler using two earthen pots and sand.
Allow 4-5 hours for it to cool down.
Observe how it creates a cooling effect.
Keep vegetables and fruits in it and observe for a week.
How many days can the vegetables and fruits stay fresh? What conditions affect this? What else can be used instead of sand for better cooling?
Answer:
- Vegetables and fruits can stay fresh for several days, up to a week.
- Conditions like temperature, humidity, and keeping the sand moist affect how long they stay fresh.
- Using materials like wet cloth or sawdust instead of sand might improve cooling.
Activity 8.10: Let us engage in a group activity
Add a small burnt piece of newspaper into water in a bottle and squeeze it for a few minutes.
What do you observe?
Answer:
- Observation: A hazy cloud forms inside the bottle.
- Explanation: Dust particles from the burnt paper help water vapour to condense into tiny droplets, forming a cloud.
Activity 8.11: Let us understand the process
Label the diagram using the words: Cloud, Lake, Ocean, River, Groundwater, Evaporation, Condensation, Rain, Snow.
Answer:
- Water evaporates from the Ocean.
- Water vapour condenses to form Clouds.
- Rain or Snow falls from clouds.
- Water collects in Rivers and Lakes.
- Some water becomes Groundwater.
- Water returns to the Ocean, continuing the Water Cycle.
Question 1:
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.
Question 2:
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) (b) water colours – Evaporation helps the water to dry, leaving the colour on the paper.
(ii) (b) ink pen – Evaporation helps the ink to dry on the paper.
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 releases water into the air through transpiration.
- Evaporation of this water cools the air around the grass.
- Plastic grass does not release water, so it does not cool the air.
Question 4:
Give examples of liquids other than water, which evaporate.
Answer:
- Petrol
- Alcohol
- Perfume
- Nail polish remover (acetone)
- Kerosene
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:
- Fans increase air movement, which helps carry away water vapour.
- This speeds up evaporation, even if the air is cool.
- Evaporation still takes heat from the surroundings, so clothes dry faster under a fan.
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:
- Drying the sludge reduces its weight as water evaporates.
- This makes it lighter and cheaper to transport.
- Dry sludge is safer to handle because harmful germs may die in sunlight and air.
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:
- Activities involving evaporation:
- Drying clothes
- Cooking (boiling water)
- Sweeping and mopping floors
- Ironing clothes (steam evaporates)
- Understanding evaporation helps us:
- To dry clothes faster by placing them in sunlight or under a fan.
- To cool things down using evaporation, like using wet cloths.
- To conserve water by knowing how it evaporates.
Question 8:
How is water present in the solid state in nature?
Answer:
- As ice in glaciers and ice caps.
- As snow on mountains.
- As frost on cold mornings.
Question 9:
Reflect on the statement “Water is our responsibility before it is our right.” Share your thoughts.
Answer:
- We should take care of water resources and not waste water.
- We must keep water clean and safe for everyone.
- We have a duty to protect water before we use 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:
- We can sprinkle water on the seat.
- As the water evaporates, it cools the seat.
- We can also cover it with a wet cloth to cool it down.
Learning Further
Question:
Wet one hand with water and leave the other dry. Blow air across both hands and feel the cooling effect. Find out the reasons for it.
Answer:
- The wet hand feels cooler because the water evaporates, taking heat away from the skin.
- The dry hand does not have water to evaporate, so it doesn’t cool as much.