Boyle's law
By Boyle's law describes the behavior of gases under constant temperature. It states that the pressure of a given amount of gas is inversely proportional to its volume, as long as the temperature remains constant. This means that as the volume of a gas decreases, its pressure increases proportionally, and vice versa. Mathematically, \begin{align} p \propto \frac{1}{V}. \end{align}
Balloon in a Syringe
Take a large (30-40 ml) disposable plastic syringe. Remove its plunger. Blow a small diameter 1 cm baby balloon. Put the baby balloon in the syringe tube. Insert the plunger. Close the nozzle of the tube tightly with your left hand finger and push the plunger slowly with the right hand thumb. The baby balloon will slowly shrink in size.
Now keep the nozzle closed and slowly retract the plunger. The baby balloon will slowly expand.
When the balloon shrink in size, it shows that if the pressure increased inside the balloon its volume decreases. When the balloon expand, it shows that as pressure in the balloon decrease, its volume increases.
Hence, in both the cases \(pV\) remains constant. Take care that temperature in the syringe remains constant.
Filling bottle using a funnel
It is generally observed that funnel is kept slightly above the bottle when it is filled with a liquid. The pressure of the air increases when it is compressed.
Place the funnel on the bottle and fill the bottle with water. Observe that water enters the empty bottle. Place the funnel again and put flour dough in the space between the funnel stem and the bottle mouth in such a way that air cannot go inside the bottle or come out of it. Now, try to fill the bottle with water. You will observe that after a little water has gone in, no more water goes in. Make a hole in the dough. Observe that water starts filling again into the bottle.
There was air in the bottle before it was filled with the water. When water is poured without the dough, the air inside the bottle was connected with the air outside it through the gap between the funnel and bottle mouth. Hence, when water is poured into the bottle, the air could come out through the gap. But when the dough was put, the air could not come out. After a little water had entered the bottle, the air inside the bottle get compressed and the pressure increased (Boyle's law). This increased pressure does not allow water to enter the bottle.
When a hole was made in the dough, a connection was established between the outside and the inside of the bottle. Hence, as water enters into the bottle, the air comes out through the hole.
Gravity defying test tube
You need two test tube of slightly different dimeters so that one can slide inside the other (like a piston).
Fill the larger test tube three-fourth with water. Push the smaller test tube half way in it. Keep holding the outer test tube and invert the setup. As water starts coming out through the gap between the two test tubes, the smaller test tube rises.
Water falls down through a very narrow gap between the test tubes leaving no space for air to enter. This creates a low pressure region inside the larger tube. The atmospheric air, which is at higher pressure, forces the inner test tube to move up.
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