Solutions

12.1

A solution is a homogenous mixture of 2 or more substances

The solute is(are) the substance(s) present in the smaller amount(s)

The solvent is the substance present in the larger amount

Solutions

Solvent: greater quantity (water)

Solute: smaller quantity (sugar)

Immiscible: two liquids do not mix

miscible: two liquids can mix alcohol in water (in any quantities)

Parts of a Solution

Solute Solvent Example

solid solid

solid liquid

gas solid

liquid liquid

gas liquid

gas gas

A saturated solution contains the maximum amount of a solute that will dissolve in a given solvent at a specific temperature.

An unsaturated solution contains less solute than the solvent has the capacity to dissolve at a specific temperature.

A supersaturated solution contains more solute than is present in a saturated solution at a specific temperature.

Sodium acetate crystals rapidly form when a seed crystal isadded to a supersaturated solution of sodium acetate.

12.1

12.2

Three types of interactions in the solution process:• solvent-solvent interaction• solute-solute interaction• solvent-solute interaction

Hsoln = H1 + H2 + H3

“like dissolves like”

Two substances with similar intermolecular forces are likely to be soluble in each other.

• non-polar molecules are soluble in non-polar solvents

CCl4 in C6H6

• polar molecules are soluble in polar solvents

C2H5OH in H2O

• ionic compounds are more soluble in polar solvents

NaCl in H2O or NH3 (l)

12.2

Concentration Units

M =moles of solute

liters of solution

Molarity (M)

Molality (m)

m =moles of solute

mass of solvent (kg)

12.3

Temperature and Solubility

Solid solubility and temperature

solubility increases with increasing temperature

solubility decreases with increasing temperature

12.4

Temperature and Solubility

Gas solubility and temperature

solubility usually decreases with

increasing temperature

12.4

Pressure and Solutions

P Solubility (gas in liquid)Henry’s law

Pressure and Solubility of Gases

12.5

The solubility of a gas in a liquid is proportional to the pressure of the gas over the solution (Henry’s law).

c = kP

c is the concentration (M) of the dissolved gas

P is the pressure of the gas over the solution

k is a constant (mol/L•atm) that depends onlyon temperature

low P

low c

high P

high c

Calculating the solubilityIf we want to make an accurate prediction about whether something will dissolve in water or not, we need to know four key factors:

•1 the type of solute 3 the volume of solvent

•2 the mass of solute 4 the temperature

•We can say, for example, that the solubility of salt in water at 20 °C is 35 grams per 100 cm3 of water.

•This means that at 20 °C you can completely dissolve 35g of salt in 100cm3 of water.

•In general:  Solubility = mass of solute / volume of solvent

How Much Will Dissolve?

• The table shows how much potassium nitrate (a type of fertiliser) and potassium chloride (the main ingredient in ‘Lo-Salt’) dissolve in water at different temperatures. The numbers tell you how many grams of solute dissolve in 100 cm3 water.

Temperature (°C) Solubility of potassium nitrate

Solubility of potassium chloride

10 20 32

20 30 35

30 44 36

40 60 38

50 78 40

60 100 42

Graph showing the solubility of potassium nitrate and potassium chloride.

0

15

30

45

60

75

90

105

10 20 30 40 50 60

Temperature C

So

lub

ility

PotassiumNitrate

PotassiumChloride

Questions1. How does the solubility of the substances change as the

temperature changes?

2. Which substance is the most soluble at 10°C?

3. Which substance is the most soluble at 40°C?

4. Copy and complete this sentence: ‘As the temperature rises, the solubility …’

5. Which of the two solids dissolves best a at 10 °C b at 40 °C?

6. Use your graph to estimate the solubility of potassium chloride at 55 °C

A colloid is a dispersion of particles of one substance throughout a dispersing medium of another substance.

Colloid versus solution

• collodial particles are much larger than solute molecules

• collodial suspension is not as homogeneous as a solution

12.8

Suspensions

• These are mixed, but not dissolved in each other

• Will settle over time

• Particles are small

Mayo