HONORS CHEMISTRY: CHAPTER 16



CHEMISTRY: CHAPTER 16

Solutions

I. Some definitions:

1. Solute – Dissolved particles in a solution; the smaller part.

2. Solvent – The dissolving medium in a solution; the larger part.

3. Solution – Solute + Solvent

4. Solubility – The amount of a substance that dissolves in a given quantity of solvent at specified conditions of temperature and pressure.

5. Saturated Solution – A solution containing the maximum amount of solute for a given amount of solvent.

6. Unsaturated Solution – A solution that contains less solute than a saturated solution.

7. Supersaturated Solution – A solution that contains more solute than it can theoretically hold at a given temperature.

II. Solubility – Why stuff dissolves

The main rule: Things dissolve when it’s more stable for the solute particles to mix with the solvent particles than it is for the solute and solvent particles to stay separated.

• Polar solutes in polar solvents: When you put a polar solute in a polar solvent, the partial charges on the solvent molecules will pull on the partial charges of the solute molecules. Because the solvent and solute molecules interact well with each other, polar solvents dissolve polar solutes.

• Polar solutes in nonpolar solvents: When you put a polar solute in a nonpolar solvent, the solute molecules would rather stick to each other (because of the opposite charges) than the solvent molecules (which don’t have them). As a result, polar solutes don’t dissolve in nonpolar solvents.

• Nonpolar solutes don’t dissolve in polar solvents: Though the nonpolar solutes don’t want to stick to each other, the polar solvents do. Because the polar solvent molecules would rather stick to each other than to the nonpolar solutes, nonpolar solutes don’t dissolve in polar solvents.

• Nonpolar solutes dissolve in nonpolar solvents: Even though nonpolar solvent molecules don’t pull on nonpolar solutes, there’s also not much holding the solute together. As a result, nonpolar solute molecules will eventually mix with nonpolar solutes.

Because things that are polar dissolve in polar solvents and things that are nonpolar dissolve in nonpolar solvents, we use the saying “LIKE DISSOLVES LIKE” to describe the solubility of solids in liquids.

If we find that something does dissolve in a solvent, how can we make it dissolve more quickly?

• Grind the solute particles and make them smaller: Because solutes only dissolve at the point where their molecules hit the surface of the liquid, smaller particles will equal faster dissolving.

• Stir it: When you dissolve a solute the area immediately around the undissolved solute is in contact with very concentrated solution. By stirring it, you put the solute in contact with unconcentrated solution which makes it dissolve more quickly.

• Change the temperature:

o Solids are usually more soluble in hot solvents than in cold solvents because the solvent molecules have more energy to pull them apart.

o Gases are usually less soluble in hot solvents than in cold ones because when energy is added to the gas molecules they are more prone to vaporize than to dissolve.

• If it’s a gas, increase its partial pressure: If you want to dissolve a gas into a liquid, increasing the partial pressure of the gas will allow more gas molecules to come into contact with the liquid and dissolve.

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