Maharashtra Board Class 7 Science Chapter 3 Properties of Natural Resources PDF Download

Properties Of Natural Resources

Let's Recall

1. Which are the gases present in air? Why is air called a homogeneous mixture?

2. What are the uses of the various gases in air?

Properties Of Air

Air is all around us. We cannot see it, yet we feel the presence of air. When we inhale, air is taken in through the nose. If we blow on our hand through our lips, we can feel the air.

Try This

1. Take a stick from a broom or a paper or plastic straw. Tie a thread at its centre and hang it so that it remains horizontal.

Tie two rubber balloons of the same size at the two ends of the stick. See to it that the stick remains horizontal. Now remove one balloon, inflate it and tie it again at its original position. Does the stick remain horizontal? The stick goes down on the side of the inflated balloon. This means that air has weight. Air is a mixture of gases. Therefore, like all other matter, it has mass and weight.

2. Take an injection syringe without the needle. Pull its piston and observe it as you do so.

The piston can be pulled out easily. The piston then remains in that position even if you let it go. Now, close the inlet hole of the syringe tightly with your thumb, pull the piston and let it go. Is the force required to pull out the piston more or less than before? Does the piston remain as it is after you let it go.

The molecules of the gases in the air are in constant motion. When these molecules strike a body, they create pressure on that body. This is the pressure of air that we call atmospheric pressure.

On pulling the piston with the inlet hole of the syringe closed, more space becomes available to the air in the syringe, and it becomes rarified. As a result, the pressure of the air in the syringe is lowered. Comparatively the pressure of the outside air is very high. That is why, when the pulled out piston is released, it is immediately pushed in. If this experiment is repeated, holding the syringe in different positions like vertical, horizontal or inclined, the piston is found to go in to the same extent in all the cases. From this, we can infer that atmospheric pressure is the same in all the directions.

Teacher's Note

Air is everywhere around us. In India, you can feel wind blowing from the sea during summer, which is due to air pressure differences.

Exam Trick

Remember: Atmospheric pressure means air pushing on us from all directions equally. Like when you swim underwater, water pushes on you from all sides.

Points to Remember

Air has mass and weight.
Air molecules move in all directions.
Atmospheric pressure is the same in all directions.
Air pressure changes when its density changes.

Find Out

Is there any atmospheric pressure on the moon?

Do You Know?

Under ordinary conditions, atmospheric pressure at sea level is about 1,01,400 Newtons per square metre. It can be measured with the help of a barometer. As we go higher above sea level atmospheric pressure decreases.

A Little Fun!

Place a piece of cardboard on the mouth of a glass filled completely with water. Holding the cardboard firmly in place with one hand, invert the glass quickly. Take your hand away from the card board. What do you see?

Great Scientists

In 1726, the Swedish scientist Daniel Bernoulli put forth the important principle that the pressure of air decreases when its velocity increases while the pressure of air increases when its velocity decreases. If a body is moving through air, the air pressure decreases perpendicular to the direction of the motion and then the air in its surroundings flows rapidly from higher pressure to the lower pressure.

Observe And Discuss

Take some water in a plastic cup and hold a straw upright in it. Hold a small piece of another straw near the upper end of the first straw and perpendicular to that straw. Blow hard through the small piece of straw. You will see a fountain of water. Why did this happen?

On blowing through the straw, the air at its open end is pushed away and hence the pressure of the air at the opening goes down. As the pressure of the air near the upper end of the erect straw becomes less than atmospheric pressure, the water in the cup is pushed from where the pressure is higher to where it is lower, that is, in the upward direction, and the water comes out in the form of a fountain. The more forcefully you blow the higher will be the fountain. This straw fountain works on Bernoulli's principle.

Teacher's Note

This principle explains how aeroplanes fly. In India, the Bernoulli principle is used in many engineering applications.

Exam Trick

Remember: Lower pressure pulls things. Like when you sip through a straw, the lower pressure inside pulls water up.

Points to Remember

Velocity of air increases, pressure decreases.
Air pressure difference creates wind and breeze.
Bernoulli principle works in fountain experiment.

Use Your Brain Power!

What is the effect of increased temperature on the pressure of air?

When there exists a difference in the air pressure at two different places, the air starts flowing from the place of higher pressure to the place of lower pressure. At such times, we feel a breeze or a wind blowing. Thus, winds are an effect of the difference in air pressure. You will learn more about this in the lesson called 'Winds', in Geography.

Try This

Take ice-cubes in a glass upto three-fourths of its height. Now observe what happens. How did the water droplets appear on the outside of the glass?

Due to the ice cubes in the glass, the air surrounding the glass cools down. As the water vapour in this air cools down, it condenses and gets transformed into water. It is this water that collects on the outer surface of the glass.

The level of humidity is different in different places. Similarly, the humidity levels also change from time to time during the period of a single day.

The level of humidity of the air is determined by its capacity to hold water vapour. During the night or at dawn, when the temperature of air is low, its capacity to hold the vapour is less. At such times, the excess vapour is transformed into water droplets. This is what we call dew.

In the afternoon, when the temperature of the air is high, the capacity of the air to hold the moisture also increases. Then, compared to its full capacity to hold water, the proportion of moisture in the air is less and we feel that the air is dry.

The proportion of water vapour in the air is high during the monsoons and in coastal areas. As a result, we feel the dampness or humidity.

In summer, wet clothes dry quickly, but in the rains, they do not. Why is this so?

Use Your Brain Power!

1. Dip an uncorked inverted empty bottle in a slanting position into the water in a wide container. What do you observe?

2. What change takes place in a balloon on filling air in it?

We come to know from the activities above, that air has properties like occupying space, having a certain volume, having mass and weight.

Air is a mixture of very fine particles of some gases, dust, smoke and moisture. When rays of light fall on these minute particles, the particles spread the light in all the directions. This natural phenomenon is called scattering of light.

Teacher's Note

Scattering of light makes the sky blue. In India, you can see this clearly during sunrise and sunset.

Exam Trick

Remember: Particles in air scatter light. This is why the sky looks blue during the day but red during sunset.

Points to Remember

Air has volume and mass.
Light scatters due to particles in air.
Water vapour in air affects humidity.
Dew forms when air cools down.

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