What you need:
Two identical plastic bottles
Modelling clay
Two drinking straws
Water
What to do:
1. Fill one bottle completely with water. Fill the second bottle with water, but leave about 5 to 6 cm of space at the top.
2. Insert straws in both bottles (they should dip into the water in both bottles) and wrap lumps of clay around the bottles’ mouths to form seals.
3. Blow as hard as you can through the first straw into the bottle that is completely filled with water. Then let it go.
4. Repeat the same thing with the second bottle. Then let that go too.
What happens:
It is hard to blow into the first bottle and nothing happens when you release it. It is easier to blow into the second bottle, but when the straw is released, water splashes out of it.
Why?
The first bottle is sealed and completely full of water. There is no room for air. So you can’t force air in no matter how hard you try.
The second bottle has water and some air (that empty space) inside it. When you blow air into the straw, you can see air bubbles inside the water. This air increases the air pressure inside the bottle, which means that the air molecules inside the bottle compress and make space for new air molecules entering through the straw.
The increased number of air molecules then start pushing on the water (because they find that the space at the top of the bottle is too less to accommodate all of them). Thus, the water gets forced up through the straw and out when you release it.
This shows that when compared, air is more compressible than water. You can push air and force more of it into the sealed bottle. But that technique does not work with water.
Picture Credit : Google
