Let me make that more simple.
If you had a sample of RO/DI water and dissolved table salt in it, until it reached saturation and then measured the amount of table salt in the water, you would get x amount. Now, if you made up a batch of seawater and then dissolved a bunch of table salt in it until it reached saturation and then measured all the table salt in it ( don't forget seawater already has lost of table salt in it), the seamix will have more table salt in it than the RO/DI. All of those other ions in seawater help/allow it to dissolve more table salt.
Here's another try at explaining activity coefficients I got from long ago when Randy and I where explaining this on the chem forum to someone.
Me;
One way to look at activity is to think that ions in solution can only do two things; leave solution or stay in solution. If the activity is high they are not happy, they are active and want to go some place. If they can not stay in solution there is only one place to go precip out of solution. If the activity is low, they are not active, they do not want to go anywhere and are happy and stay in solution.
When you add salt to a solution it lowers their activities. Since they are less active they do not want to go anywhere, so they stay in solution.This means you can now add other ions also salts to the mix, which are also happy as their activity has also been lowered. Thus, you can dissolve more of these also which will stay in solution........all to a point of course
Randy;
If you think of Mg(OH)2 dissolving, you are going from a solid where the Mg++ and OH- are very near to each other,to dissolve into Mg++ and OH- that are farther apart. So there is an electrostatic energy of interaction between the positive and negative charges that is lost when those ions are pulled apart.
In very dilute solution, that effect is countered by the energy that is given off when water surrounds each ion that comes off. That is why salts dissolve at all.
So in the end, you reach a balance between the lost electrostatic energy, and the gained hydration energy.
OK, so now look at the situation with a large amount of Na+ and Cl- ions also in solution.
In that case, the Mg++ and OH- ions that go out into solution are not so all alone. An Mg++ can have nearby Cl- ions, and the OH- can have nearby Na+ ions (providing some electrostatic energy). So the electrostatic energy lost is not so large. Since the hydration energy stays largely the same, the overall solubility goes up.
So most ionic salts are more soluble in NaCl solutions than in water. And most acids and bases that tend to ionize will be more prone to be in their most highly charged form. This is one reason why pKa's sometimes shift considerably between freshwater and seawater.
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