Saturated solution

[reising1]

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Let's say we have an unsaturated solution of silver chloride and we want to turn it into a saturated solution.

We can do this at least three ways, and I'm trying to understand why.

1. Lowering the temperature - ?
2. Adding sodium chloride - from the common ion effect, having more ions will cause more of the silver chloride to go into solution, becoming saturated
3. Evaporating some of the water - ?

Can anyone help with (1) and (3)?

 

[Chillbo Baggins]

#1. It's kind of a Le Chatelier's principle thing. If you were to add heat to the reaction (dissolution), the equilibrium would shift to the right. Conversely, lowering the temperature will reduce solubility.

#3. Less solution + same number of moles of solute = more saturated

 

[reising1]

In particular, I don't understand #3. If the solution is not currently saturated, doesn't that mean there isn't enough water for all the molecules to bind to? So I would think we need to add more water.

 

[Chillbo Baggins]

The silver chloride dissociates because of interactions with the water; if there's less water, it's harder for the molecules to dissociate, up to the point where they won't dissolve anymore.

 

[reising1]

Right, it's harder for them to dissociate, so how can they become saturated with water? They must dissociate to become saturated, right?

 

[quechua]

Right, it's harder for them to dissociate, so how can they become saturated with water? They must dissociate to become saturated, right?

No... You're overthinking it.

A saturated solution has undissoicated particles. Think about adding an extreme excess of sugar to lemonade. You can see the sugar at the bottom right?

An unsaturated solution is the opposite.

That's just an oversimplified example.

 

[reising1]

Right, think about lemonade with some sugar particles at the bottom. If you suck more water out, there will now be even more sugar particles at the bottom because the sugar is no longer dissolved into the water that you evaporated.

 

[quechua]

Right, think about lemonade with some sugar particles at the bottom. If you suck more water out, there will now be even more sugar particles at the bottom because the sugar is no longer dissolved into the water that you evaporated.

Yes with one correction. The water has to evaporate out of solution... Not simply sucked out. Do you ever boil eggs and add salt to the water? If you have you'll notice that salt forms around the ring of the pot as the water evaporates out as a gas.

 

[reising1]

So when the water comes out, solute comes out too. So if this is the case, how does removing water free up more space for solute to become saturated. For instance, assume we started with 50 mol water and 1 mol salt where 0.5 mol salt is saturated (bonded to water) and 0.5 mol salt is not (sitting on the bottom of the container, for example). So, for every 50 mol water, 0.5 mol of this salt can be saturated.

Now, if we remove some water (say 10 moles), and if as you said, solute is removed as well (0.1 mol), then we are left with 40 mol water, 0.4 mol saturated salt. Back to normal, still with 0.5 mol salt unsaturated.

So what is wrong in this logic?

 

[Chillbo Baggins]

If you're evaporating the water, the solute doesn't come along with it. Unless it's something with a really low boiling point.

 

[pyrrion89]

Let's say we have an unsaturated solution of silver chloride and we want to turn it into a saturated solution.

We can do this at least three ways, and I'm trying to understand why.

1. Lowering the temperature - ?
I think of this as the reverse of a certain purification technique involving crystallization. You can increase the solubility by heating, so naturally if you cool a solution then you reduce its solubility.

2. Adding sodium chloride - from the common ion effect, having more ions will cause more of the silver chloride to go into solution, becoming saturated
Yes.

3. Evaporating some of the water - ?
I like to think of solvation as a communal dance. For a given solute molecule, it has a set number of group dance partners with which it must dance in order for there to be solvation (i.e., the molecule is saturated and not precipitated). Let's say it's 5 water molecule partners for every one solute molecule. If there's 20 solute molecules, you need at least 100 water molecules (20 x 5 = 100). During evaporation, the number of water molecules will decrease but the solute molecule population will remain constant.

Let's say 40 molecules of water leave. So now only 60 water molecules remain, but there's still 20 solute molecules. Given that our dance rules are still applied, there's only enough dance partners to satisfy 12 solute molecules (12 x 5 = 60). Because we have 20 solute molecules, 8 solute molecules are not solvated and thus are precipitate. The solution is now beyond full saturation whereas before it was fully saturated. This means is because its ability to solvate has been reduced because there are fewer dance partners. Its solubility has been reduced.

Thus, not every solute molecule will have enough partners so some solute molecules will precipitate. This means the solution is less soluble now. This example illustrates going from full saturation to hypersaturation, but it can also elucidate how you go from unsaturated to saturated (i.e., too many dance partners to just enough).

Can anyone help with (1) and (3)?

Hope that helps.

 

[reising1]

Oh, so the whole point is that when you remove some water, there is more leftover solute (maybe as precipitate) such that the solution is saturated (because a solution with precipitate means it's saturated)?

 

[Chillbo Baggins]

Oh, so the whole point is that when you remove some water, there is more leftover solute (maybe as precipitate) such that the solution is saturated (because a solution with precipitate means it's saturated)?

Basically yes. The definition of saturation is the point at which no more solute can dissolve.