# Homework Help: Air over water

1. Jun 3, 2007

### Idea04

1. The problem statement, all variables and given/known data
I know with air bubbles in water that they will rise in water because they are lighter and less dense than water.
But if you had lets say a tank of water suspended in the air and at the bottom of the tank there were small holes so small that air can go through the holes but water couldn't.
Would the oxygen pass through the holes and rise in the water, or would the oxygen be blocked by the water molecules.

2. Relevant equations

3. The attempt at a solution
I kind of think that the water would not allow the air to move up because would the water want to move down words and since it couldn't the air will not flow. But I'm not sure on this.

2. Jun 4, 2007

### Andrew Mason

Let the area of a hole = A. Consider a thin disk of water of area A and thickness ds. What is the total force on the thin disk of water covering the hole?

The forces are gravity (down) air pressure (up) and water pressure (down). (we will ignore surface tension).

The downward force consists of the weight of the column of water above the hole + downward air force (air pressure x A) on that column. The upward force is just the air pressure x A. You can see that the atmospheric pressures above and below cancel each other and you are left with the downward weight of the column of water. So which way does the water go?

AM

3. Jun 4, 2007

### Idea04

So if you went to a microscopic level and you had a hole the size of of an oxygen atom and at the bottom there was oxygen atoms and above were water molecules which are larger than oxygen. take in the account that molecule and atoms all have large space between them. Wouldn't some oxygen molecule be absorbed by the water.

4. Jun 5, 2007

### Andrew Mason

In my previous answer I showed that the force required to lift a column of water covering the hole and insert an oxygen molecule is greater than the average force of impact of an oxygen molecule on the water (ie. the air pressure). Is there another way to get an oxygen molecule into the hole?

Because the molecules of oxygen are not all moving at the same speed, occasionally an oxygen molecule with sufficient energy may strike the water over the hole with enough force to penetrate into the water. But there is an equal likelihood that an oxygen molecule will go the other direction (from the water, out the hole).

AM