PROPERTIES OF AIR
• Air can be greatly compressed.
• All the air in a space as big as a house can be squeezed into a small tank.
• On the other hand, air can expand almost indefinitely.
• If all the air was removed from the inside of a house and then 1 cubic foot of air was supplied, the air would expand throughout the building.
• Air also has weight.
• And one way we can tell that air exists is by observing some of the things it does.
For example, air can fill a balloon, and when it’s in the form of wind, it can move leaves and blow our hair around.
Let us understand with the help of an experiment:
• Take a lit candle and a glass jar.
• Place a short candle upright, and light it about ten inches behind the jar.
Figure 3: A candle placed ten inches behind the jar
• The flame should be entirely centered behind the jar.
• Now blow hard on the jar on the opposite side of the candle—so that the jar is directly in front of you with the candle directly behind it.
Figure 4: Flame of the candle centered behind the jar
• When air comes into contact with objects, it flows around the contours of the object it hits, creating forces that can lift kites and blow out candles.
• This property is what makes flying a plane possible.
• Because air has weight, it pushes on everything around it, in all directions. We call this air pressure.
Air pressure plays an important role in our day-to-day activities like:
(1) When we make a small slit on one side of the packet of milk it flows out slowly.
(2) If we cut another slit on the other side, the milk flows out rapidly.
(3) This happens because air enters the packet from the second slit and its pressure pushes the milk out of the packet.
• If there was no air pressure, we would not be able to fill up our fountain pens.
• Even doctors would not be able to fill up a syringe.
Figure 5: Syringes (work on the principle of air pressure)