Solutions
Homogeneous Mixtures
Solutions
Particle size: less than 1.0 nm: can
be atoms, ions, or molecules
Heterogeneous Mixtures
Colloids
Particle size: 1-1000 nm: can
be large particles or aggregates
Suspensions
Particle size: over 1000 nm: can
be large particles or aggregates
Do not separate on standing
Do not separate on standing
Particles settle out on standing
Can not be separated by filtration
Can not be separated by filtration
Can be separated by filtration
Do not scatter light
Scatters light (Tyndall effect)
May scatter light, but are not
transparent
Solution: A homogeneous mixture of two or more substances in a single phase: one or more substances
dissolved in another.
Soluble: capable of being dissolved.
Solubility: the amount of substance required to form a saturated solution with a specific amount of
solvent (usually 100.0 g water) at a specified temperature.
Solutions consist of:
1. The Solvent: the substance doing the dissolving: usually the substance of greater quantity.
2. The Solute: the substance being dissolved: usually the substance of lesser quantity.
mass of a solution = mass of solute + mass of solvent
Types of Solutions
1. Gas Solutions: a solution of a gas in a gas; i.e. water vapor in air; air is the solvent and
water vapor is the solute. The solute is usually the smallest mass and the solvent the largest.
2. Liquid Solutions: a liquid solvent with a dissolved solid, liquid, or gaseous solute.
Most solution solvents are water. If the solvent is alcohol, the liquid solution is called a tincture.
a) Miscible: liquids that dissolve in each other.
alcohol (ethanol)/water benzene/carbon tetrachloride
b) Immiscible: liquids that do not dissolve in each other. water/benzene
oil/water
c) “Like dissolves Like” refers to molecular polarity. Polar compounds dissolve polar
compounds; and, non-polar compounds dissolve non-polar compounds.
3. Solid Solutions: mixtures of solids uniformly spread throughout one another.
a) Alloy: a solid solution of two or more metals (the solvent is the metal with the greatest mass).
brass – copper/zinc
bronze – copper/tin
b) Amalgam: alloys in which one of the metals is mercury.
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Properties of Common Homogeneous Solutions
1)
Well Stirred - stirring helps spread the dissolved particles evenly among the liquid particles.
2)
Dissolved particles will not settle out of solution (assuming no evaporation).
3)
The solution is clear and transparent dissolved particles are too small to be seen (0.01-1.0 nm) and a
beam of light passing through the solution can not be seen (no Tyndall effect).
4)
Dissolved particles will pass through a filter due to their small size. Can not use filtration as a
separation method.
5)
Single Phase - considered to be a single phase though one substance may be a solid (sugar) and the
other a liquid.
Solubility and the Rate of Solution
Solubility: the amount of substance required to form a saturated solution with a specific amount of
solvent (usually 100 g water) at a specified temperature. If the solvent is alcohol, the solution
is called a tincture.
Rate of Solution: is a measure of how fast a substance dissolves, i.e. the quantity of solute that dissolves
per unit time.
Factors that determine the rate of solution (how fast a substance dissolves):
1. Nature of solute and solvent:
a) 1.0 g of lead(II) chloride can be dissolved in 100 g of water to form a saturated solution
compared to:
b) 200 g ZnCl in the same amount of water.
2. Particle Size: the smaller the particle size the faster it dissolves. Dissolving takes place only on the
surface therefore the greater the surface area (smaller particles), the faster it dissolves.
3. Stirring: increases the rate of solution by bringing fresh solvent in contact with the solute.
4. Amount of solute already dissolved: the rate of solution (dissolving) decreases with increasing
amounts of solute dissolved.
5. Temperature:
a) For liquids and solids:
1. increasing temperature increases the amount of solute that will dissolve if the heat of
solution is positive (endothermic) but decreases solubility if the rate of solution is
negative (exothermic).
b) For gases:
1. Increasing the temperature decreases both solubility and the rate of solution.
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6. Pressure:
a) For liquids and solids: changes in pressure have almost no effect on solubility.
b) For gases: an increase in pressure increases solubility and a decrease in pressure decreases
solubility.
Solutions and Solubility Curves
Solutions may be:
1. Unsaturated: a solution that contains less dissolved solute than it can hold at a given temperature.
2. Saturated: a solution that contains all of the dissolved solute it can hold at a given temperature.
3. Supersaturated: a solution containing more dissolved solute than it can hold at a given temperature.
4. Dilute: a solution in which a relatively small amount of solute is dissolved in the solvent.
5. Concentrated: a solution in which a relatively large amount of solute is dissolved in the solvent.
Note: saturated and unsaturated are not related to concentrated or dilute. A solution may be saturated
and dilute or unsaturated and concentrated.
Solution Concentrations
Four Methods of Expressing Concentration:
1. Percent by mass: mass of solute present in a given mass of solution.
Percent by mass = mass of solute X 100
mass of solution
The mass of the solution is the sum of the masses of the solute and solvent.
Percent by mass is based only on the mass of the solute and is unrelated to its chemical
formula or molar mass.
2. Molarity: the number of moles of solute in one liter of solution; written as 1M, 2.5M, etc.
Molarity (M) = moles of solute
Liter of solution
a) Preparation – prepared by dissolving the solute in less than one liter of solvent and then filling
up to one liter. Do not dissolve solute in one liter of solvent.
3. Molality: concentration of a solution expressed in moles of solute per kilogram of solvent; written as
1m, 2.5m, etc. molality (m) = moles of solute
kilogram of solvent
a) Preparation: prepared by dissolving the solute in one kilogram of solvent.
Note: When water is used as a solvent (most cases), molarity = molality.
(1 liter = 1 kilogram = 1000 ml = 1000 g)
a) M1V1 = M2V2 use this equation to solve dilution problems.
b) MaVa = MbVb use this equation to solve neutralization and titration problems
(same as “a” above)
4. Normality (N) – the concentration of a solution expressed as the number of equivalents of solute in 1
liter of solution; used primarily in acid-base neutralization calculations.
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Colligative Properties
1. Colligative Properties: properties that depend upon the nature of the solvent and the number of solute
particles (concentration or moles). Colligative properties do not depend on the type or nature of the
solute only on the number of particles/moles present.
(salt, sugar, etc.) Electrolyte vs. non-electrolyte.
2. Colligative properties are proportional to molality and include freezing point, boiling point, vapor
pressure, and osmotic pressure.
3. The effect of the solute on these properties is to raise the boiling point of the solvent (boiling point
elevation), lower the freezing point (freezing point depression), lower the vapor pressure, and increase
osmotic pressure.
Freezing Point Depression
Tf = Kfm
Tf = freezing point depression (change in freezing point)
Kf = molal freezing point depression constant, oC/m
m = molality
Boiling Point Elevation
Tb = Kbm
Tb = boiling point elevation (change in boiling point)
Kb = molal boiling point constant, oC/m
m = molal concentration of the solution
4. Vapor Pressure: the pressure exerted by a vapor over its liquid. Dissolving a solute in the
liquid will lower the vapor pressure.
5. Osmotic Pressure: the external pressure that must be applied to stop osmosis. The greater the
concentration of the solution, the greater the osmotic pressure of the solution.
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