Science | Science Experiment of the Week 12 | 330 - Water in a Glass

How can something as light as air hold up a glass full of water?

This Week's Experiment - #330 Water in a Glass

These experiments are from Robert Krampf - The Happy Scientist

This week's experiment has its roots in a phone conversation I had with my friend Mik Jacobs (The Fire Guy). He is on a whirlwind tour, including Hong Kong, California and South Carolina. We were discussing different ways to explain simple science experiments. Later as I was doing the dishes, I followed up some of the ideas and wound up with this one. You will need:

a drinking glass
a sink or bathtub
water
a paper plate or piece of cardboard

We will begin with a common, simple experiment. Fill the glass to the top with water. Place the paper plate firmly over the top of the glass. Holding it over the sink (in case it doesn't work), hold the plate firmly in place and turn the glass upside down. Then release the plate. It should stay in place, and the water should stay in the glass.

Why? It is tempting to think that the plate is holding the water in, but it is not. The plate is keeping the air out. The air pressure outside the glass is holding the water in. How can we show that? Well, first, we can let some air in to take the place of the water. Keeping the glass over the sink, pull down on one side of the plate, just enough to let a little air get in. What happens? The water gushes out. But wait. Doesn't that also fit with the theory that the plate was holding the water in?

To check that, we need to get rid of the plate. Fill the sink most of the way with water. Holding the glass sideways, place it in the water. Once it is totally filled with water, turn it upside down, keeping it under the surface. Lift it slowly upwards. Though most of the glass is above the surface, the water stays inside the glass. As long as the mouth of the glass is under water, no air can get into the glass, and the water can't come out. The external air pressure holds the water in, even without the plate. As soon as the edge of the glass is lifted above the surface, air rushes in, and the water rushes out.

How can something as light as air hold up that much water? The air around you is pressing on everything with quite a bit of pressure. At sea level, it presses with 14.7 pounds on each square inch of surface. If you draw a grid of one inch squares on a piece of paper, you can place the mouth of the glass on the paper and see about how many square inches it covers. Multiply that my 14.7 pounds and you can see that you have quite a bit of pressure pushing upwards. When the glass is full of air at the same pressure, you have the same amount of push downwards as upwards. They balance each other. With the glass full of water, when the water starts to move downwards, there is nothing to take its place. This produces an area of extremely low pressure. The higher pressure on the outside is then plenty strong enough to keep the plate in place.

Have a marvelous week.

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