In gases and liquids (as opposed to solids) molecules are free to move around. You should think of them as little balls moving in straight lines with some constant velocity until they hit a wall or another molecule. When they hit a wall, they exert pressure on it. The strength of this pressure depends on how many molecules are hitting it (volume), how fast they are moving (temperature) and how big they are (nature of the gas/liquid).

The molecules of a gas, liquid, will move randomly until they reach an equilibrium situation. This means that, when equilibrium is reached, all particles will be moving with similar velocities and they will be distributed homogeneously occupying all the space available to them. In the case of liquids this is more complicated because molecules are interacting with each other keeping them together so they won’t leave the container where they are if it is open at the top. Still, the general principle applies and molecules on a liquid also exert a pressure on the walls of the container by hitting it.

When 2 gases are put in contact, they simply mix occupying both all the space available to them until they reach equilibrium. The same thing happens when you put 2 liquids together. But when a gas and a liquid are put together, what happens then? Then they don’t mix but the pressure they exert in the contact surface must be the same. Think for example of a glass of water. In that case we have water in contact with air (gas). The volume that the water occupies in the gas is not random: it expands until the surface pressure is in equilibrium with the atmospheric pressure, this is the pressure that the air molecules are exerting on the water surface by hitting it.

This is exactly how an old mercury barometer works. In that case you have mercury in contact with air. When the atmospheric pressure increases/decreases the mercury contracts/expands a little bit. Because it is in a very thin tube, one is able to measure exactly these changes in the volume of mercury and infer from them the changes in the atmospheric pressure.

There is one more possible scenario. What happens if we put two liquids/gases (or one of each) together but separated by a membrane in such a way that they can’t mix? If the membrane is non permeable (this means that it does not allow molecules to pass through it), then the gas/liquid with a higher pressure will expand to reduce it deforming the membrane until the pressure at both sides of the membrane coincide.

Sometimes the membrane is semi-permeable. This means that it has small holes allowing small molecules to pass through but no bigger ones. Then, small molecules will move from the side at lower pressure (more diluted) to the side with higher pressure (more concentrated solution) until pressure equilibrium is reached at both sides of the membrane. This process is known as osmosis. The pressure on the semi-permeable membrane is called osmotic pressure.

This is precisely what happens inside cells. A cell is basically a semi-permeable membrane containing a water solution. Since most of the things that we eat are made of cells (vegetables, fruits, animals), osmosis plays a very important roll in the kitchen.

As you can imagine, a cell is actually much more complicated than that. In particular, the membrane in a cell has channels that allow other molecules to go through when necessary. The explanation of how this works at atomic level merited the Chemistry Nobel Prize in 2003 for Peter Agre and Roderick MacKinnon . In the Nobel Prize webpage, you can find a very good explanation for the public on what they did that includes this nice animation:

What is it with osmosis and resurrection then? I learnt about this nice experiment in this post in Martin Lersch’s blog. If you leave salad out of the fridge in a hot day, it will slowly loose the water in its cells by evaporation until it loses its firm structure and looks fluffy. Some of the cells might be permanently damaged, but most of them only need to recover the water they had inside to go back to their original state. If you submerge the salad in water for a some minutes it will flow by osmosis into the cells restoring the initial aspect of the salad. I did the experiment with two leaves of Iceberg. In this series of pictures you can see how they deteriorated after 2 and then 4 hours in the sun and how they recovered after 2 and then 3 hours under water:



The most impresive thing for me was to see how the second leave loses completely its structure becoming flat and then recovers it after only 3 hours under the water.


In his post, Martin Lersch shows this very nice movie he made of the lettuce’s resurrection by photographing it every 60 seconds: