Air Pressure: Balloon in a Flask

Put a small amount of water in the bottom of a flask.

Place the flask over a burner or on a hot plate until the water begins to boil.

Turn off the heat and stretch a balloon over the flask opening.

Allow the flask to cool.  You can speed this up by placing the whole flask in ice water. 

The balloon is pushed into the flask.

Why:

When the water is heated, the molecules speed up and many of the escape the flask.  The balloon, placed over the opening, prevents the air molecules from re-entering the flask as it cools.  In addition, there are lots of air molecules on the outside of the flask and balloon, and they no longer meet much resistance, so they push the balloon into the flask.

If you're at home, and don't have a flask or a burner, try this:

Use your tea kettle (or other pan) to heat some water until it begins to boil.  Quickly pour the water into a bottle (you could use plastic since you're nto heating the bottle) - use a funnel - and cap the bottle with a balloon.  Then, proceed as above, cooling the bottle.

Air Pressure: Candle in a Jar

Use a chunk of clay to hold a candle in the bottom of a glass jar (or other container, as seen above).

Pour about an inch or 2 of water into the jar.

Light the candle and place another glass jar (smaller than the first, so it fits inside) upside down over the candle.

What happens:
The fire is extinguished and the water rises.

Why:
The candle burns up the oxygen. Once the oxygen molecules have been consumed, the remaining air takes up less space, so the air outside of the jar pushes the water up.

Some tips:
1 - Make sure you don't set the inverted jar down on the bottom of the larger jar, hold it off the bottom - it will create a seal and the water won't rise!

2 - Colored water may make it easier to see the water level rise - as you can see above, regular, colorless water is difficult to see (it was in person too, not just in the picture).

Cartesian Diver

Cartesian divers  are used to demonstrate the effects of air pressure. 

I used an eye dropper filled part way with water for my diver.  I put colored water in the eye dropper - I think it makes it easier to see what's happening.

Fill a large, plastic bottle with water.  Place the diver in the bottle - the diver should float at this point, make any adjustments you need to to make sure that happens.

Put the cap on the bottle.

Squeeze the bottle and watch the diver dive! 

Why does it dive?

When you squeeze the bottle, you compress the air molecules that are trapped in the bottle, including those inside the eye dropper.  When that air is compressed, additional water enters the dropper (since there's room available).  The additional water (and decreased volume of air) changes the density of the dropper enough to cause the dropper to sink to the bottome of the bottle.

When you release the sides of the bottle, the air trapped in the dropper expands, pushing out the extra water and decreasing the density so the dropper rises. 

A Cartesian diver can be used to explain all kinds of things, like how submarines work, how fish swim bladders work, etc.
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You can also use ketchup (or other condiment) packs as Cartesian divers.  In that case, water doesn't enter the packet, it dives strictly because of the change in density due to the compression of air molecules.

Air Pressure: Straw Fountain

Fill a cup part way with water.

Hold a straw vertically in the water – make sure it’s not touching the bottom of the cup. (You could probably tape the straw to the cup if you’re having a hard time holding on to all of the parts.)

Using another piece of straw, blow across the top of the straw that’s in the water.

What happens:
Water shoots out of the vertical straw.

Why:
Normally there’s a whole lot of air molecules stacked up on top of the surface of the water. When you blow across the top of the straw, you’re pushing some of those air molecules out of the way. There are still air molecules pushing on the rest of the water, and with the pressure over the straw reduced, the water is pushed up the straw.

Tips:
If you're having a hard time getting the water to shoot out of the straw, try some of these tips - they helped me!

1 - Aim the straw you're blowing through slightly up (see picture above).

2 - Use shorter pieces of straw.

3 - Fill the cup with more water.

4 - Practice - it takes a few times to get the technique down.

Air Pressure: How to Blow up a Balloon in a Bottle

Can you blow up a balloon in a bottle?

Go ahead, give it a try. Place a balloon into a bottle (a 2 liter soda bottle works well), leaving a bit sticking out to blow into. Give it your biggest breaths. What happens?

You'll get about this far:


Why can't you get it any more inflated? There's still room in the bottle and the balloon can certainly stretch further than that.

What's wrong?

It's that pesky air pressure again.

The bottle is filled with air. When you start to inflate the balloon, it seals off the opening of the bottle. The air molecules that were in the bottle are stuck in the bottle - they can't get out. And, you're trying to push more air molecules into that space as you inflate the balloon.

Unless you've got lungs unlike any person I've ever known, you don't stand a chance pushing against those air molecules in the bottle.

But, what if you gave those molecules an escape?

Try again, but this time, stick a straw in the bottle with the balloon - the end of the straw needs to be sticking out of the bottle.


Now blow (on the balloon, not the straw!)...

That's more like it!

This post is a part of:

Air Pressure: A Cup, An Index Card & Some Water

This is a pretty well-known, classic experiment when it comes to air pressure.

If you aren't familiar with it, you should be - it's easy to do in any setting.

Fill a cup part way with water. Place an index card on top (make sure the card is large enough to completely cover the cup).

Pick up the cup, placing one hand on top of the index card.


Turn the cup over, holding the index card up with your hand.


Ask your students what they think will happen if you remove your hand that's holding the index card.

Then do it!

There really is water in the cup, I promise! My hand got in the way of the water line...

Air pressure doesn't just push down, it pushes on things in all directions. In this case, air molecules are pushing on the index card. They exert more pressure than the water pushing down does, so the card stays up.

If something breaks the seal, or the index card gets too wet (as it will after enough time has passed), the card will fall and water will gush out!

Air Pressure: Collapse a Milk Jug

Heat some water to near boiling. Pour the hot water into the milk jug and swirl it around to heat the entire container. Pour the water out and quickly cap the milk jug. Within a few minutes, as the air inside the jug cools, the jug will collapse.

The hot water heats the air inside the jug, causing the air molecules to move faster and some of them move out of the jug. By capping the jug, you don't allow any air molecules to re-enter the jug and have created an area of low pressure. The pressure outside of the jug is greater, causing it to collapse.

Air Pressure: Crush a Soda Can

Place a small amount of water in an empty soda can. Place the can on a hot plate and heat until the water boils. While waiting, prepare a large container of cool water. Once the water has begun to boil, use tongs to pick up the can and quickly turn it upside down and place it in the water bath. The air in the can heats up, the air molecules start moving faster and some find their way out of the can, creating an area of low pressure. Turning the can upside down into the water creates a seal in which no air molecules are able to re-enter the can. The pressure outside of the can is greater, and the can is crushed.

To see a “real life” example of this phenomena, check out what happened to this train car when it was washed with hot water and sealed before it was cool.