Solutions: Forming Solutions: Forming SolutionsSolutions

Unit 10Unit 10

I. What is a solution?I. What is a solution?

A. A. Solution – a homogeneous mixture in which the components are uniformly inter mingled.

1, 1, Solute – substance that is dissolved

2. Solvent – substance that does the dissolving

3. Aqueous solutions – water is the solvent!

B. Solubility – how things dissolveB. Solubility – how things dissolve1. Solubility of Ionic Substances – when an 1. Solubility of Ionic Substances – when an

ionic solid dissolves, it breaks apart into its ionic solid dissolves, it breaks apart into its ions which are dispersed throughout the ions which are dispersed throughout the solutionsolution

2. Solubility of Polar Substances – the polar 2. Solubility of Polar Substances – the polar parts of the molecule are attracted to the parts of the molecule are attracted to the oppositely charged polar parts of the water oppositely charged polar parts of the water moleculemolecule

3. Substances Insoluble in Water – Nonpolar 3. Substances Insoluble in Water – Nonpolar substances do NOT dissolve in water!substances do NOT dissolve in water!

4. How Substances Dissolve4. How Substances DissolveA. Like Dissolves LikeA. Like Dissolves Like

1.) Polar solvents dissolve polar solutes1.) Polar solvents dissolve polar solutes 2.) Nonpolar solvents dissolve nonpolar solutes2.) Nonpolar solvents dissolve nonpolar solutes

C. C. Solution Composition: An Introduction : An Introduction – there is a limit on how much solute can – there is a limit on how much solute can dissolve in a solventdissolve in a solvent11. saturated – when a solution contains as

much solute as will dissolve at that temperature.

2. 2. unsaturated – a solution that can dissolve more solute at that temperature

3. supersaturated – contains more solute than normal for that temperature, very unstable.

Unsaturated, Saturated, Unsaturated, Saturated, Supersaturated SolutionsSupersaturated Solutions

4. 4. Concentrated – a solution in which a relatively large amount of solute is dissolved.

5. Dilute – a solution in which a relatively small amount of solute is dissolved.

(Think strong vs. weak coffee)(Think strong vs. weak coffee)

D. D. Factors Affecting the Rate of Dissolving1. Stirring – rate of dissolving increases

when the solution is stirred!2.2. Temperature –

a. Solids dissolve faster at higher temperaturesB. Gases dissolve faster at colder temperatures

3. Surface Area – the more surface area there is, the faster something dissolves! (Think granulated sugar (large surface area) vs. sugar cubes (small surface area)

Measuring Concentration Measuring Concentration of a of a SolutionSolution

Molarity (M) Moles (n) of solute

dissolved per liter (V) of solution.

M = n/V

Molality (m) moles (n) of solute

dissolved per kg of solvent

m = n/kgsolvent

DilutionsDilutions

M1V1 = M2V2

M = molarity in mol/LM = molarity in mol/LV = volume in LV = volume in L

When diluting a solution, the number of When diluting a solution, the number of moles of solute never changes, just the moles of solute never changes, just the volume.volume.

Colligative PropertiesColligative Properties

are specific properties of solutions. They are specific properties of solutions. They only depend upon the number of particles only depend upon the number of particles in solution in solution NOTNOT the particles’ composition! the particles’ composition!

Boiling Point ElevationBoiling Point Elevation

When a solute is added to pure water, it When a solute is added to pure water, it increases the boiling point of the solution.increases the boiling point of the solution.For example, adding NaCl increases the boiling For example, adding NaCl increases the boiling

point of water to 104point of water to 104°C. WHY?°C. WHY?In order for water to boil, the vapor pressure In order for water to boil, the vapor pressure

(HUH?) of the water HAS TO EQUAL atmospheric (HUH?) of the water HAS TO EQUAL atmospheric pressure, BUT…the solute particles get in the way pressure, BUT…the solute particles get in the way of the liquid water particles joining the gas particles of the liquid water particles joining the gas particles in a bubble & escaping. Therefore, it requires more in a bubble & escaping. Therefore, it requires more heat energy (higher temp.) in order to get the heat energy (higher temp.) in order to get the solution to boil.solution to boil.

Vapor PressureVapor Pressure

In a container, vapor In a container, vapor pressure is the pressure is the pressure of the vapor pressure of the vapor particles above a particles above a liquid.liquid.

Boiling Point ElevationBoiling Point Elevation

ΔΔTTbb = k = kbb•i•m•i•m

ΔΔTTb b = change in boiling point= change in boiling point

KKbb = boiling point elevation constant = boiling point elevation constant i = factor of particles in solutioni = factor of particles in solutionm = molalitym = molality

SolventSolvent KKbb ( (°C/m)°C/m)

Acetic acidAcetic acid 3.073.07

BenzeneBenzene 2.532.53

Carbon tetrachlorideCarbon tetrachloride 4.954.95

WaterWater .512.512

Freezing Point DepressionFreezing Point Depression

Adding a solute also lowers the freezing point Adding a solute also lowers the freezing point of a solution as well.of a solution as well.For example, adding a solute (like salt) to water, For example, adding a solute (like salt) to water,

interferes with the solution forming a regularly interferes with the solution forming a regularly ordered solid. Therefore it takes even lower ordered solid. Therefore it takes even lower temperatures to force the solution to make a solid.temperatures to force the solution to make a solid.

ΔΔTTff = k = kff••ii••mm

ΔΔTTf f = change in freezing point= change in freezing point KKff = freezing point depression constant = freezing point depression constant i = factor of particles in solutioni = factor of particles in solution m = molalitym = molality

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SolventSolvent KKff ( (°C/m)°C/m)

Acetic acidAcetic acid 3.903.90

BenzeneBenzene 5.125.12

Carbon tetrachlorideCarbon tetrachloride 3030

WaterWater 1.861.86