STD 7 Basic Science: Chapter 07 Pressure in Liquids and Gases - Questions and Answers | Teaching Manual


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Chapter 07: Pressure in Liquids and Gases - Questions and Answers 
• The teacher who came to inaugurate the Science Club began with an experiment. He placed a wide wooden ruler on a table, some part of it projecting out. A newspaper, folded eight times, was placed on it. He then asked a boy to press quickly on the free end of the ruler with his hand. The boy did so and the paper fell down.
Then he unfolded the same paper and placed it over the ruler on the table. He asked the boy to quickly press on the ruler again. He could neither raise the paper nor make it fall down.
1. Let us also do this experiment.
Isn't the weight of the paper the same when it is folded and unfolded?
Answer: Yes, their weight is the same.
 
•  Why couldn't the paper be lifted up when unfolded?
Answer: When pressed on one end of the scale, the other end of the scale and the paper are raised to a very small extent. There is only a small amount of air between the paper and the table. But the force exerted on the paper by the air is more. So it is not possible to lift the paper.

•  What force is affected so that the paper could not be raised?
Answer: The force of air is affected on the paper. This force can be called air pressure.

2. What are the properties of air?
Answer:
•  Air is present all around us 
•  Air need space to occupy
•  Air possesses weight
•  Air exerts a force
•  Air has no specific shape 
•  Air is transparent

3. Now try some other experiments using the syringe.
After pushing the piston into the syringe, close the open end of the syringe with your finger. Then pull the piston back quickly and release it. 
• Don’t you see the piston moving fast into the barrel? 
Answer: Yes

• Can you explain its reason?
Answer: When the piston is pulled back keeping the open end of the syringe closed, the pressure inside the barrel decreases. In order to equalize the pressure, the air from outside exerts force on the piston. Hence the piston moves inwards.

• After keeping the piston pulled back, close the open end of the syringe with a finger. Now, push the piston in and release it.
In which direction does the piston move now?
Answer: The piston moved outwards.

What is the reason for this movement?
Answer: When the piston is pushed in keeping the open end of the syringe closed, the air inside gets compressed. The pressure of this air is greater than that of the air outside. The pressure of air inside the barrel pushes the piston outwards.

4. Don’t we apply force when we push in and pull out the piston? But when we released the hand, the piston moved inwards in the first case, and outwards in the second case.
• What was the force applied on the piston?
Answer: Atmospheric pressure

• Write down the inferences made from observing the two experiments in your science diary.
Answer: The pressure will transfer from a higher concentration to a lower concentration to equalize the pressure

5. What is meant by the pressure of the gas?
Answer: The force exerted by gas per unit area of the surface is called the pressure of gas. 

6. What is atmospheric pressure?
Answer: The force exerted per unit area by atmospheric air is called atmospheric pressure.

7. The balloon in the bottle
Pour hot water into a glass bottle of more than half a litre capacity. Blow air into a balloon and remove the air from it. Repeat it two or three times.
Pour out the hot water from the bottle and fix the balloon at the mouth of the bottle immediately. Allow the bottle to cool. 
• What will happen to the balloon? 
Answer: 
The balloon inflates into the bottle.

• Now, which is higher - air pressure inside the bottle or outside it?
Answer: When the bottle is cool, the atmospheric pressure inside and outside the bottle is same.

• Write down in your science diary the reason for the inflation of the balloon into the bottle.
Answer: The air inside the bottle from which hot water is removed is hot and expands. When the bottle cools, the air inside the bottle also cools. 
When it gets cooled, the pressure inside the bottle decreased.
The greater atmospheric pressure outside the bottle rushes into the bottle.
Due to this force, the balloon inflates into the bottle.

8. Name the device used to measure atmospheric pressure.
Answer: Barometer

9. Who invented the barometer?
Answer: Torricelli

10. Explain the working principle of the barometer invented by Evangelista Torricelli?
Answer: He formulated the principle of a barometer using mercury, as suggested by Galileo. He discovered that the change in the level of mercury in the tube is due to the change in atmospheric pressure. Accordingly, he made a barometer in 1644.

11. Can you transfer water from a bucket to another vessel using a plastic tube?
Immerse one end of the tube in the water in a bucket and keep the other free end in a small vessel as shown in the figure.
• Does water flow down?
Answer: No the water doesn't flow.

• Why doesn't water enter the tube?
Answer: Because there is no pressure difference. The pressure difference is required to water to flow.

• Suck the air out of the tube with your mouth and place it in the smaller vessel. What happens now? Explain.
Answer: Water will flow after sucking the air out of the tube. When one end of the pipe is sucked, there is a partial vacuum created or a region of low pressure is developed. Due to this, there is a pressure difference and this causes the liquid to flow from the high-pressure region to low pressure region.

12. There are other devices which work on the basis of atmospheric pressure. See some examples. Try these devices and write down the method of their working in the science diary.
Answer:
• Syringe: When the piston is pulled up, the pressure of the air inside it decreases. Liquids like medicines enter the syringe into the area of low pressure.
• Straw: When the air inside the straw is sucked in with the mouth, the pressure of air inside it decreases. Liquids rushes into the mouth through the straw.
• Dropper: When the bulb of the dropper is squeezed, the air in the tube goes out and becomes empty. The pressure at the surface of the liquid pushes them into the tube.

13. Some activities are given below.
• Fill a glass tumbler with water. Close its mouth with a paper card and invert it. Does water flow out?
Answer: Water will not flow out. Because of the air pressure pushing up on the card, the card will stay on the glass and the water will not spill out.

• Fix a vacuum hook on a mirror and pull it backwards.
Remember to write down in your science diary the results and inferences of the experiments done.
Answer:
Observation: The vacuum hook firmly adheres to the mirror
* When vacuum hook is pressed on a mirror, the air between them is forced out. Thus the pressure decreases 
* The atmospheric pressure is greater than the air inside. Thus the greater atmospheric pressure pushes the vacuum hook firmly on the mirror.

14. Suspend two identical plastic balls on strings as shown in the figure. What happens when you blow air between the balls? Note down your guess.
Do the experiment and write down the results of your observation. Explain the reasons too.
Answer:
Observation: The balls are pushed closer to each other.
Reason: When air is blown between the balls, the air between the balls moves faster. As a result, the pressure of air between the balls decreases. As the pressure of the air around the balls is comparatively higher, the balls are pushed closer to each other.

15. Can you insert a paper ball into a bottle?
Place a piece of paper crumpled into a small ball at the mouth of a bottle with a narrow neck. Blow air strongly through one side of the mouth of the bottle as shown in the figure.
• What is your observation? What happens to the paper ball?
Answer: The paper ball fell down.

• Blowing a paper up!
Cut a piece of paper 20 cm long and 3 cm wide. Hold the paper piece close to your lower lip and blow air over it.
• What happens to the paper?
Answer: The paper piece moves up

• Ball in a funnel
Arrange a plastic pipe and a funnel as shown in the figure and place a small plastic ball in the funnel. Can you push the ball out by blowing air through the plastic pipe? Invert the funnel while you keep blowing in air. 
• Does the ball fall down?
Answer: The ball does not go out

• Can you put out the flame?
Place a lighted candle on a table. Try to blow air onto the flame using a
funnel. 
• Does the flame extinguish?
Answer: No

• Tabulate the observation results of these experiments.
Experiment Observation resultExplanation
Paper ballThe ball falls downBecause the pressure outside the bottle reduces when air moves out fast.
Blowing a paper upThe paper piece moves upWhen the air above the paper at that piece is blown, the air. place moves faster and the air pressure at that place decreases. As the pressure at the bottom of the paper is comparatively higher. the paper This moves up the paper.
Ball in the funnelThe ball does not go outThe pressure of air below the ball decreases as it moves. The pressure of air above the ball pushes it down.
Can a candle be put out by blowing air through a funnel?The candle flame is not put out. The flame comes close to the funnel The flame moves closer because the pressure there reduces when air moves out fast.
• Do you find any peculiarity in common in all these experiments?
Answer: The objects move towards the area, where the atmospheric pressure is very low.

16. What is Bernoulli's Principle?
Answer: Pressure of air reduces when it moves fast. This principle was
explained by a scientist named Bernoulli. Hence it is called Bernoulli's Principle.

17. Try this experiment. 
Tie a polythene bag firmly on your hand and immerse it in a bucket of water as shown in the figure.
• Explain the result of the observation.
Answer: The polythene bag is sticking to the hand. Because water exerts pressure on it.

• What force presses the bag to your hand?
Answer: Liquid pressure

18. What is Liquid pressure?
Answer: The pressure exerted by a liquid is called liquid pressure.

19. What are the properties of Liquid pressure?
Answer: 
• Liquids exert pressure in all directions.
• Liquid pressure increases with depth.

Activity 1
Fill air in a balloon and fix pieces of cellotape at different parts of the balloon.
Make small holes using a needle on the balloon at the portions where the pieces of cellotape are fixed. Repeat the experiment done earlier using this balloon.
• Does water gush out in all directions?
Answer: Yes, the water gush out in all directions

• What inference do you arrive at from this?
Answer: Liquids exert pressure in all directions

Activity 2
Make a few small holes of the same size around a plastic bottle, 3 cm from the bottom. Pour water into the bottle.
Observe water flowing out through the holes.
What are the peculiarities of the water flowing out?
Answer: water will flow in all holes with the same pressure and quantity.

Make 3 holes in equal distance from each other upwards from the bottom of a long plastic bottle. Fill the bottle with water keeping the holes closed. Observe the flow of water coming out.
• Does the water coming out through all the holes fall at the same distance?
Answer: No, the water coming out through all the holes fall at a different distance. The distance of falling water increases with the increase in depth.

• What is the change in liquid pressure towards the bottom?
Answer: The liquid pressure increases with the increase in depth. Liquid pressure is higher at bottom levels.

• Is there any change in the flow of water when its level in the bottle falls?
Answer: The water from the bottom level falls with higher pressure and distance than other levels. Liquid pressure and distance increase with the increase in depth

• Take a vessel full of water. Keep one end of a thin plastic tube at the bottom of the vessel. Blow air gently through the other end. Is there any change in the size of the bubbles rising from the bottom to the top? What might be the reason?
Answer: Yes, there is a change in the size of the bubbles rising from the bottom to the top. The size of bubbles increases as their distance from the plastic tube increases towards the top. As depth increases, pressure increases. Thus, when a bubble rises from below the surface it encounters less pressure. This causes the volume to increase and the bubble rises in size as it rises from a depth.

20. Can you explain the reason for providing wider basements while constructing dams?
Dams are broader at the base than at the top. Why?
Answer: We know that the liquid pressure increases with the increase in depth. So, the liquid pressure will be highest at the basement of the dams and the pressure of the water in it may break the base and may result in floods. Increasing the area of the basement will reduce the pressure. Providing a wider basement for dams will reduce the pressure.

21. What is Pressure gauge?
Answer: Pressure gauge is a device used to measure liquid pressure.

22. Take a bucket full of water. Hold the funnel in the water at different depths. Observe the change in the level of water in the tube and write it down in the table.
 Position of funnel  Level of water in the tube (cm) 
  At the surface of water  2
  At the middle of water in the bucket  5
  At the bottom of water in the bucket  10
Analyse the table: What are the inferences you can arrive at?
Answer: Liquid pressure increases with the increase in depth.

Let us assess
1. Find the situation that helps us to understand that gas exerts pressure in all directions
(a) Drinking water using a straw
(b) Blowing air into a balloon
(c) Blowing of wind
(d) Rising up of steam
Answer: b. Blowing air into a balloon

2. It was not possible to drink water using a straw with a hole on its side. Why?
a. Water flows out through the hole in the straw.
b. Air pressure inside the straw increases.
c. Air moves out through the hole in the straw.
d. Air pressure inside the straw does not decrease.
Answer: d. Air pressure inside the straw does not decrease.

3. Why do sea divers wear special costumes?
Answer:  The pressure exerted by water deep under the sea is much greater than at sea level. Hence, the deep sea divers wear special suits which protect them from the extreme pressure of water.

4. Haven't you seen an injection needle fixed at the top of a drip bottle when a drip is administered to patients in hospitals? Why?
Answer: If an injection needle is not fixed at the top of the drip bottle, the medicine does not flow into the patient's body. Because the pressure inside the drip bottle is less than that of the atmospheric pressure. When an injection needle is fixed at the top, air moves inside the bottle through the hole and the medicine flows into the body.

5. Haven't you noticed sheets, tiles etc. fixed on the roofs of buildings rising up when a strong wind blows? Why does this happen?
Answer: When a strong wind blows, the air on the upper part of the roof moves faster. As a result air pressure at that place decreases. As the air pressure in the surrounding area is comparatively higher. So the roofs rise up.





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