Reef Chemistry Question of the Day #245 Osmosis

[sghera64]

I’m not saying your wrong, I just can’t see why the tap water would force itself to leave and become salty. Can you explain this better for me?

I like Randy’s explanation. But I’d like to try to give you a different one to see if it helps.

Let’s say you take a small tank (10 gal) of aquarium water at 78 degrees. You carefully lay an acrylic tray of RO/DI water on top of that tank water (It’s not totally full so it floats - - an fresh water is less dense anyway). In the center of the tray is a 1 in hole that you kept plugged.

You let both bodies of water become perfectly still before removing the plug.

Ask yourself, does the less dense fresh water drain down into the salt water? Does the salt water rush up into the fresh water? Do the two simply “mix”?

The answer is they mix?

The water and ion molecules diffuse as they vibrate about, but the water and ions become more and more uniformly distributed as time goes on. This is the nature of nature. Things go from orderly (pure water) to less orderly when barriers are removed. Examples: Dissipating smoke, the big-bang of the universe.

As Randy was saying, it takes energy to put things into order (purity them) and energy is released when they “decay” or become less orderly - - the nature of things. Even in social systems, it takes energy to create “exclusivity” such as clubs, classes, clans. Nature tends to promote (in many cases rewards) integration [just an analogy there].

 

[IonicBond]

The balloon gets smaller

 

[Grey Guy]

Thanks

 

[Grey Guy]

Sure, I can try, but explaining some processes can be complicated.

Suppose you have air in half of a box with a glass divider down the middle. On the other side is vacuum. The concentration of air is much higher on the side with the air, and if you remove the divider, lots of air flows out of the one half and into the empty half. That continues until the concentration of air is the same on both sides. It makes sense and fits with everyday experience.

Liquid water does the same thing. It moves until the concentration of free water molecules is the same on both sides.

In seawater, there are about 49 water molecules for each ion, such as sodium or chloride. Each ion takes up a space similar to a water molecule, so there is less water in a liter of seawater.

If you have a box with salt water on one side and fresh on the other, purely from a concentration perspective, water wants to move into the salty side because the concentration is lower. Worse yet, the water molecules crowd around each ion. Maybe 3-10 water molecules depending on the type and size of ion. They are tightly attached and no longer free and the water “concentration” is even lower than it would seem because these don’t count as much.

The liquid water can’t really flow into salty liquid water , but it tries to, and that is the “osmotic pressure”. As it tries to force its way in, it exerts pressure.

This part seems much less fitting with everyday experience, but it is clearly established with more sophisticated chemistry, both in theory and in practical testing.

Thank you. You make it clear how this all works. I now understand why the experiment works the way it does. My supposition that salt water would migrate to fresh, was incorrect, but of course, now I see the opposite is true, and it makes sense. You are a good teacher, and thanks for submitting the original question.

 

[Randy Holmes-Farley]

Thank you. You make it clear how this all works. I now understand why the experiment works the way it does. My supposition that salt water would migrate to fresh, was incorrect, but of course, now I see the opposite is true, and it makes sense. You are a good teacher, and thanks for submitting the original question.

Thanks.

FWIW, if the membrane were permeable to salt, it would migrate from the salt water to the fresh water. That's what causes salt creep in a shut off RO membrane.