Solutions and Solvation

Vocabulary:

1. solvent
2. solute
3. solution
4. polar
5. molecular solvation
6. dissociation
7. electricity
8. electrolyte
9. nonelectrolyte
10. soluble

“Composition VII”, Wassily Kandinsky,

Russian, 1913

11. insoluble

12. aqueous

13. non-aqueous

14. polyatomic ion

15. solubility chart

16. solubility

What is a solution?

Solution: a mixture in which the minor component (the solute) is uniformly distributed within the major component (the solvent).

Solvation

• The process of dissolving a solute in a solvent is called solvation.
• Ionic and molecular compounds undergo solvation in different ways
• Ionic compounds are dissociated by the solvent.
• Molecular compounds undergo molecular solvation.

Water is polar!

• Both types of solvation occur because water is polar.
• This is because oxygen attracts electrons more than hydrogen does. (More about this in a later chapter.)
• The oxygen side of the molecule is slightly negative and the hydrogen side is slightly positive.

Molecular Solvation

• Molecular compounds (those with covalent bonds) dissolve by molecular solvation.
• The positive side of the solute is attracted to the negative side of water and the negative side of the solute is attracted to the positive side of water.
• Click here to take a look.

Dissociation

• Ionic compounds dissolve by dissociation.
• Dissociation is when the oppositely charged ions in an ionic compound are pulled apart due to their attration to the opposite poles of water molecules.

Watch

• Why do some of the substances dissolved in water conduct electricity and some do not?

Dissociation

• When an ionic compound like sodium chloride dissolves in water, the ions dissociate. Take a look:

If the video to the

left does not work,

click on this link.

Dissociation

• The ions are more attracted to the water molecules than they are to each other.

Electricity

• Only solutions that have dissociated ions in them can conduct electricity because electricity is the movement of charges. If there are no charges that can move then there is no electricity. Substances that form ions in solution and conduct electricity are called electrolytes.

So what does this look like at the particle level with something soluble, like Cu(NO₃)_2?

Notice that the ions are separate from one another and are free to move around in the solution. Also, notice there are twice as many NO₃^- ions as Cu²⁺ ions and that the total charge is zero.

But wait!

• If you go back to slide 7 and watch the original video again, you’ll see that some ionic compounds in water like BaSO₄ do not conduct electricity. Why not? Think about it!

Solubility

• Not all ionic compounds dissociate in water! These are called nonelectrolytes; they do not conduct electricity. The ions in the solid are more attracted to themselves than they are to the water molecules.

So what does this look like at the particle level with something insoluble, like PbI₂?

Notice that the ions are bonded as a solid and are not free to move around in the solution. Also, notice there are twice as many I^- ions as Pb²⁺ ions and that the total charge is zero.

Solubility

• So how do you know if an ionic compound dissolves or doesn’t dissolve?

Solubility Rules

• You were given rules that tell you which ionic compounds generally dissolve and which do not.
• Get out the periodic table you were given on the first day of school or borrow one of mine.

Solubility Chart

Dissolves in water

= soluble

= aqueous

Doesn’t dissolve in water

= insoluble

= solid

Practice

• Determine if the ionic compounds on the following slides are soluble or insoluble.

1. AgCl

So silver chloride is NOT soluble. It will remain a solid if placed in water.

2. Cu(NO₃)₂

So copper(II) nitrate is soluble. It will dissociate if placed in water.

3. K₂SO₄

So potassium sulfate is soluble by two different rules. It will dissociate when placed in water.

More practice

• Is (NH₄)₃PO₄ soluble or insoluble?
• Soluble

​

• Is PbI₂ soluble or insoluble?
• Insoluble

​

• Is Na₂CO₃ soluble or insoluble?
• Soluble (remember, sodium is an alkali metal!)

Not all dissolving is equal

• Solubility is the mass of solute that will dissolve in a given amount of solvent at a given temperature.

Temperature effect on solubility

• Temperature is directly related to the solubility of solids. (Higher temp, higher solubility of solids.)
• Temperature is inversely related to the solubility of gases. (Higher temp, lower solubility of gases.)

Some Practice Questions:

1. What properties on the x- and y-axes?
2. What substance has the greatest solubility at 60^oC?
3. How many grams of KNO₃ will dissolve in 100 g of water at 60^oC?
4. What substance’s solubility is least affected by temperature?
5. What substances are most likely gases?