1 The theory of kinetic gas states that molecules in a gas are always moving freely and constantly colliding against the wall of its container.

2 The high frequency of collision between air molecules against the wall of its container will produce a force of pushing against the wall of the container.

3 This force is called air pressure.4 According to scientists, air pressure is force per unit area produced by the collision of air molecules

against the wall of its container.5 Air pressure increases if the frequency of collision between the gas molecules and the wall of its

container increases.

(a)A glass is filled with water until it is full.(b)The glass is then covered with a piece of card.(c)While holding the card, the glass is inverted quickly.(d)When the hand holding the card is withdrawn, water does not flow out.(e)This is because the air pressure exerted on the card is strong enough to support the weight of

the water in the glass.

(a)A tin filled with a little water is heated until the water boils.(b)The mouth of the tin is then covered tightly and cold water is poured over the tin.(c)The tin is seen to collapse slightly.(d)When water boils,' steam molecules push air molecules out of the tin.(e)When the tin is closed tightly and splashed with cold water, the steam in the tin condenses

into water.(f)This causes the air pressure in the tin to be lower than air pressure on the outside.

(g)As a result, the tin wall is forced inwards by air pressure on the outside until it becomes dented.

(a)A ruler is placed on the table with one end lying over the edge of the table.(b)The section of the ruler that is on the table is covered with a piece of newspaper.(c)When the protruding end of the ruler is hit swiftly, the other end of the ruler does not fly upwards.(d)This is because air pressure presses down on the surface of the newspaper.

1 Air pressure depends on two main factors, i.e.(a)volume of gas

If the number of molecules per unit volume of gas is bigger, the frequency of collision of gas molecules against the wall of the container also increases. Therefore, air pressure will also increase.

(b) temperature of gasAt high temperature, gas molecules move at higher speeds. As as result, the frequency of collision of molecules against the wall of its container also increases.

2 Air pressure caused by the atmosphere (layer of air surrounding the Earth) is called the atmospheric pressure.3 Atmospheric pressure is not influenced by the surface area of an object. It depends on the height

of the object fry the sea level.4 The higher the object from sea level, the thinner the atmosphere and the lower the atmospheric pressure.

5 A barometer is an instrument used to measure atmospheric pressure.

!Applications of Principles of Air Pressure1 There are many devices used in everyday life

that operate on the principles of air pressure.

2 For example, a syringe, a siphon, the suction pump of a sprayer and a drinking straw.

3 Syringe(a)A syringe is a simple pump.(b)When the piston is pulled upwards, the

space in the syringe has lower pressure than the atmospheric pressure on the outside.

(c)As a result, the higher atmospheric pressure will push the liquid into the syringe.

(a)A siphon consists of one rubber tube used to transfer liquid from a high level to a lower IeveF.

(b)To transfer liquid from a higher level to a lower level, the rubber tube should be filled with water and both ends of the tube are covered with fingers.

(c)Then, one end of the tube is placed at a higher level while the other end is left at a lower level.

(d)When both the fingers covering the tube ends are removed, water flows from the higher level to the lower level. The

flowing water creates a lower pressure in the tube.

(e)The difference in air pressure forces water in tankX to flow • into the rubber tube to produce a continuous flow.

(a)A sprayer is used to produce very fine liquid spray.

(b)When the piston is pushed, the air in the sprayer will be pushed out. An area of low pressure is created at the nozzle of the sprayer.

(c)The air pressure which is higher in the container forces the liquid into the thin tube.

(d)The liquid that emerges via the thin tube will break up as fine drops as a result of the force on the water.

(a)A drinking straw enables water to be sucked from a container filled with liquid.

(b)When one end is sucked at, air is drawn in and creates an area that has low air pressure.

(c)Higher atmospheric pressure on the outside will push the liquid into the straw.

(d)A drinking-straw will not function if there is a leak anywhere along the section that is not immersed in liquid.

Gases Under High Pressure1 Gas particles can be compressed at higher

pressure because spaces between gas particles are bigger.

2 At high pressure, distances between gas particles are less.

3 Gas particles compressed will undergo changes in state to liquid.

4 A gas under high pressure will change into liquid.5 This enables gases to be stored in liquid form

in tanks or aerosols.6 Since the volume of liquid is smaller than the

volume of gas, the storage of gas in liquid form saves cost and facilitates transportation.

7 Examples of using gas under high pressure:(a)Vehicles that use natural gas have tanks

that are cylinder-shaped to store liquid natural gas.

(b)Liquid oxygen tanks are used by divers and astronauts.

(c)Gas tanks for cooking contain liquid petroleum gas (LPG).

(d)Liquids such as hair sprays and perfumes are kept under high pressure in aerosol cans.

(e)Lighters contain fuel kept in liquid form. When used, the liquid fuel will be converted into propane gas.

will be emptied into an incinerator as they coi.: explode.

3 Do not dent or puncture aerosol containers. Since their contents are in high pressure, they ma. explode if they become dented or punctured.

4 Gas tanks must be kept in an open and cool place that has good air circulation.

5 The gas valve must be turned off each time afteuse.

6 The valve of the gas tank, rubber hose and gas sto,ei must be examined as often as possible to ensu-e that there is no leakage.

7 Do not shake the liquid in the aerosol can. Shaki-; can hasten the reaction between the chemica substances in the tin and may cause an explosion

1 Gas containers normally used to store gas under hi:-pressure are aerosol tins.

2 Substances usually stored in aerosol cans are pain= perfumes, hair cream, etc.

(a) When the button is pressed downwards, the valve of the aerosol can opens.(b) The liquid in the container will spray out in the form of gas because the air pressure is lowered.

(c) The liquid in the container is pushed out because the gas pressure in the aerosol can is higher.

1 Fill in the blanks with the words from the box below.

Name the following appliances that use the principle of air pressure.

5 The plastic container above is filled with air. It is connected to a vacuum pump. The air in the container is pumped out. What is the observation? Write a suitable conclusion.

(a)Observation • ________________________________________________________________

(b)Conclusion

4 Figure 4 shows an apparatus set up to show the effect of air pressure.

(a)What happens to the balloon when the ® Bunsen burner is turned on?

[I mark]

(b)Give a reason for your answer in (a).

[I mark]

(c)What conclusion can you make from this experiment about the relationship between temperature and air pressure?

[ 1 mark]

(d)If the flask is uncorked, do you think the Ea balloon will expand? Explain.

[2 marks]

1. Figure 1 shows the steps carried out in an experiment.

(a)What is observed when the glass is inverted and the hand is removed?

(b)Explain your answer in 1(a).

(c)In Figure 1(b), mark with arrows to show the direction of air pressure acting on the cardboard.

(d)What is the conclusion of this experiment?

(e)State three factors that affect the air pressure in an enclosed container.

1 .

2 .

3 .

2. In Figure 2, some water was boiled in a can. Steam was allowed to escape for a short while. The can was closed tightly and cold water was poured over the can.

(a) What happens to the can when cold water is poured onto it?

(b)Briefly explain your answer in (a).

(c)(i) In the space provided below., draw a diagram to represent the condition of the can at the end of the experiment.

(ii) In your diagram, mark the area of high pressure with 'H' and the area of low pressure with T.

(d)Why is cold water poured over the can during the experiment?

(e)Write a conclusion for this activity.

3. Figure 3 shows a rubber sucker with a hook stuck onto a wall.

(a)To stick a rubber sucker onto the wall, it is pressed until it is flat on the smooth wall and then released . Explain why this is done.

(b)In Figure 3, mark the area of high pressure with 'H' and the area of low pressure with T.

(c)What modification can be made to the rubber sucker to ensure that it sticks more firmly onto the wall?

(d)What will happen if there is a hole at X on the rubber sucker?

(e)Give a reason for your answer in (d).

4. Two tins of condensed milk are inverted over two cups as shown in Figure 4. Tin P has one hole while tin Q has two holes.

(a) From your observation, can the milk flow out easily from

(i) tin P? _________________________________________________________________________

(ii) tin Q? _________________________________________________________________________

(b) Explain your answers in (a)(i) and (a)(ii).

(i) Tin P :.________________________

(ii)Tin Q :___________________________________________________________________________

(c) What conclusion can you make from this experiment?

5. Figure 5 shows the apparatus of an experiment on air pressure.

(a)(i) What happens to the balloon when the water in the beaker is heated? (ii)

Explain why this happens.

(b)(i) What happens to the balloon when the flask is placed in a beaker containing ice cubes?

(ii) Give a reason for your answer in (b)(i).

(c) Based on the results of the experiment, state the relationship between temperature and air pressure.

•

6. Table 6 shows some daily problems related to air pressure. Complete the table by naming the devices which can be used to solve these problems. Use the words aiven in the box below.

7. Figure 7.1 shows the apparatus set-up in an experiment.

(a)State the function of the vacuum pump.

(b)(i) What happens to the balloon after the pump is switched on? (ii)

Explain your answer.

(c)(i) What will happen when the pump is switched off and then removed?

(ii) Explain your answer.

(d)What is the conclusion of this experiment?

(e)Figure 7.2 shows a hard-boiled egg placed at the mouth of a conical flask. There are some burning nieces of caner in the flask.

What happens to the egg after some time? Explain your answer.

8. Figure 8 shows two types of gas tanks.

(a) Name two methods which can convert gases into liquids.

1.

2.

(b) State two advantages of storing gases in the liquid form.

1.

2.

(c) Name two examples of gases which are stored in gas cylinders under high pressure.

1.

2.

(d) State three safety measures when using gas under high pressure.

1.

2.

3.

3. Mark the place of high pressure with X and the place of low pressure with Y of the following devices that use the principle of air pressure.