18.1

Properties of Solutions

• ““Like dissolves like.”Like dissolves like.”

• Materials with similar polarity are soluble in each other. Dissimilar ones are not.

MiscibleMiscible

Liquids that are soluble in each other in all proportions such as ethanol and water.

ImmiscibleImmiscible

Liquids that are not soluble in each other such as hexane and water.

Factors Affecting rate of solution• Nature of solvent ( polar or non or ionic)• Temperature- for most, not all solids in a

liquid, solubility increases with temp.• Agitation- more shaking or stirring, faster

a solute dissolves ( solids in liquids)• Particle size- smaller the particle size,

more surface area, so faster dissolving takes place

All these can be explained by understanding how solvation occurs

Solution of solids

Solubility

• Amount of solute that dissolves in a given amount of solvent at a particular temperature

• Usually expressed in:

g of solute/100g of solvent

Solubility chart shows solubility of different solutes at different temperatures. (pg 504)

Few terms first..

• Saturated- solution is holding max amount of solute for that solvent; if you try to add more solute, it does not dissolve

• Unsaturated – solution is holding less than the max amount of solute

• Supersaturated- solution is holding more solute than it should for that temp. ( we’ll talk about this later)

• At saturation, the solute is in dynamic equilibrium. The concentration is constant.

• Solute species are

• constantly in• motion, moving• in and out of • solution.

The lines represent saturated solutions- solutions which are holding themaximum amount of solute possible for these conditions.

Handout problems

Handout hints

• Make sure you read the axis of the graphs- this one is for 100g of water

• If the problem asks for solubility in 200g of water, then you need to double your reading/or if it asks for solubility in 50g of water then you cut your reading in ½ ; You must always adjust your readings to the amount of solvent in the problem

• If your reading is below the line for your solute, the solution is unsaturated; if it is above the line it, is either supersaturated, or you can calculate how much extra solute will precipitate out ( amount above the line)

Supersaturated Solutions

At higher temperatures, more solute can be dissolved than at a lower temp.

So, if you heat up water and make a saturated solution, then slowly cool the solution down in an uncontaminated env. then more solute will be dissolved than should be for the lower temp- this is supersaturated

As soon as you add an additional crystal of solute, or scratch the glass of the container, the excess solute will precipitate out

Gases and solubility

• Look at the chart on pg.505

Notice- for gases, solubility decreases with temperature

• This is why you seen tiny bubbles when heating up water ( long before it boils)- these are N2 and O2 coming out of solution

• This is why thermal pollution can affect life in waterway- warmer the water, the less Oxygen (DO) can be dissolved in it, less oxygen available for living things

Pressure and solubility of gases

• Increasing the pressure of a gas above a liquid increases the concentration of the gas.

• This shifts the equilibrium, driving more gas into the liquid.

Pressure and solubility of gases

• At constant temperature, the solubility of a gas is directly proportional to the pressure of the gas above the solution.

Henry’s LawHenry’s Law

• S1 = S2

P1 P2

This law is accurate to

within 1-3% for slightly soluble gases and pressures up to one atmosphere.

Henry’s Law problem

If the solubility of a gas in water is .77g/L at 3.5 atm, what is the solubility at 1 atm? Assume temp stays same

.77g/L = S2

3.5 atm 1.0 atm

.77g/L x 1 atm = S2

3.5 atm.22g/L=S2