Balloon in a Bottle
Can you blow up a balloon inside a bottle? It's actually harder than you think, so in this experiment your friends will try as hard as they can with no success. But once you know the science trick you will have no trouble at all inflating the balloon, as well as performing a couple of other cool demonstrations.
What you'll need:
- 2 empty plastic soda or water bottles
- 2 8-10" balloons
Basic Experimental Procedure:
- Prepare one bottle by poking a small hole in the bottom with a nail. If the bottle crushes as you try to do this, try filling it completely full with water and screw the lid on tightl first.
- Put a balloon into the bottle, leaving the neck of the balloon outside.
- Stretch the open end of the balloon around the mouth of the bottle (see photo).
- Repeat steps 2 and 3 with the other bottle, but DON'T poke a hole in this one.
- Try to blow up the balloon inside the first bottle (the one with the small hole). Bet you can't do it.
- Now try to blow up the balloon in the other bottle. This one should be easy.
- To amaze your friends, give them the bottle without the hole while you keep the one with the hole. No matter how hard they try, they can't blow their balloon up, but you have no trouble at all.
Contrary to what you might have thought, the bottles are not quite empty (and we're not talking about the balloons). There is air inside each of them, and even though the air molecules are very small and very far apart (in fact the bottles are almost empty), they are moving around very fast. As air molecules fly around inside the bottle they bump into each other, as well as the inside surface of the bottle. All this bumping creates a pushing force that we call air pressure. This is what makes a balloon inflate when you blow air into it. The more air you add, the more pressure it creates. Before you inserted the balloons, air from outside had already filled each of the bottles with normal atmospheric air pressure (we also call this 1 atmosphere), exactly the same as the atmospheric air pressure outside the bottles.
When you insert the balloon and stretch it around the mouth of the bottle, you seal the opening and trap the air molecules (still at 1 atmosphere of air pressure) between the inside of the bottle and the outside of the balloon. Nothing happens however, because there is also 1 atmosphere pressure inside the balloon (its mouth is open). As long as there is an opening to the outside atmosphere, air molecules are free to move back and forth until the pressure inside is the same as outside, 1 atmosphere.
When you blow air into the balloon you increase the pressure inside and this pushes or expands the balloon. But it also squeezes on the trapped air inside the bottle, increasing its air pressure exactly as much as inside the balloon. No matter how hard you blow into the balloon, the air inside the bottle pushes back just as hard, preventing the balloon from inflating. If the bottle has a hole, however, the air inside is no longer trapped, so as you blow air into the balloon it easily expands by just pushing some of the air inside the bottle out the hole. Or in terms of air pressure, as you blow into the balloon you increase its air pressure (making it greater than 1 atmosphere) and it expands, but the hole allows enough air molecules to escape from the bottle to keep the pressure equal to 1 atmosphere. Thus when you stop blowing, the pressure in the balloon is also 1 atmosphere- which is why it stops expanding- but it's now bigger. [Actually, the air pressure inside the balloon is greater than 1 atmosphere because of the elasticity- or stretching force- of the balloon. The air pressure inside is equal to the atmosperic air pressure outside plus the pressure added by the balloon's elasticity.] If you blow hard enough you can completely fill the bottle with the balloon!
Variations and Related Activities:
When you demonstrate this to your friends, instead of giving them the bottle without the hole, let them try to blow up the balloon in your bottle- but you hold it for them and be sure to cover the small hole in the bottom of the bottle with your finger.
Once you explain the science (and the trick), you can try another cool demonstration. Make sure the hole is open and blow up the balloon inside as big as you can. Before you stop blowing, however, quickly cover the hole completely with your finger. Now when you take your mouth off the balloon, it will stay inflated. You can even look inside the balloon! The balloon won't deflate until you take your finger off the hole. Why is this happening? Think about the air pressure. Inside the balloon is normal atmospheric pressure since the mouth is wide open to the atmosphere, but the air pressure inside the bottle is also atmospheric pressure, because that's what it was just before you covered the hole. Since the air pressure is exactly equal on both sides of the balloon, it won't move either way, i.e. it can't expand or contract. Another way to think about this- if the balloon did contract this would increase the amount of space or volume left inside the bottle, but since the number of air molecules inside remains exactly the same (none can get in or out of the bottle), the air pressure would drop below 1 atmosphere. Since the pressure in the balloon is still 1 atmosphere it's now greater, and the balloon expands back to its starting size. [Strictly speaking, just as noted above, the pressure inside the bottle is actually less than 1 atmosphere, so that this pressure plus the pressure due to the balloon's elasticity is exactly equal to 1 atmosphere.]
One more cool trick. For this one be sure you are outside or over a sink, as you will get wet! Blow up the balloon and cover the hole in the bottle so the balloon remains inflated again. Without releasing your finger from the hole, pour some water into the balloon through the open mouth. The water remains inside. Now take your finger off the hole and watch as the water shoots out, making a little fountain. The air pressure is 1 atmosphere everywhere, but now the elasticity of the stretched balloon is free to squeeze the water out.
Links to more information and activities:
Bill Nye the Science Guy air pressure video:
Air pressure and weather:
More air pressure experiments: