Solutions

For understanding of solutions, the following definitions are required.

Solute: Substance to be dissolved. Can be either a solid, liquid, or gas.
Solvent: Substance solute is dissolved in. Typically a liquid (most commonly water).
Solution: Combination of solute and solvent.
Solubility: Measure of the quantity of solute that can be dissolved in a particular solvent at a given temperature.
Saturated Solution: A solution that contains the maximum amount of solute that will dissolve under a given set of conditions.

Solid Hydrate: A solid ionic material that contains a fixed proportion of water in the crystalline matrix. For example, CaSO₄ ^. 2H₂O.

Factors Affecting Solubility

By far the most important factor affecting solubility is the properties of the substances that are being mixed together. In general, molecules of similar polarity tend to dissolve in each other, while polar and non-polar molecules do not. Ionic compounds can be considered as an extreme form of polar molecule, and dissolve much better in polar solvents.

The influence of temperature on solubility is in general difficult to predict. For most solid and liquid solutes disolved in a liquid solvent, solubility increases with increasing temperature. However, this is not always true, and many examples of compounds whose solubility decreases with increasing temperature are known.

Solubility of Gases

For gases dissolved in liquids, it is known that the solubility of the gas decreases with increasing temperature. This is sometimes quantified using Henry's Law.

P_gas = k C

P_gas
k
C

Partial pressure of the gas
A constant for a particular gas in a given solvent
Concentration of the gas dissolved in the solution

The solubility of a gas can be increased by increasing the pressure of the gas 'over' the solvent. At higher pressures, the gas is more soluble in the solvent. If the pressure is decreased, the solubility of the gas decreases. This is the reason that a can or bottle of soda goes 'flat' after it has been opened for a long period of time.

Concentration:

Concentrations are often expressed using the following definitions.

Weight/Weight (w/w)% = { (mass solute) / (mass solution) } * 100%
(This definition is convenient if the molecular formula is not known)
Molarity (M) = (moles solute) / (Liters solution)
(From this it follows that:
M * V = moles

Calculation Hints:

Molarity problems tend to give or request volumes in mL, but most of these equations require that the volume first be converted to liters. Also note the following relationships between mass, concentration, and moles.

grams → moles		concentration → moles
divide mass by M.W.		multiply "M" by volume

Colloids

Colloids are not true solutions, but instead contain 'large' particles suspended in solution. Colloidal suspensions scatter light, which is known as the Tyndall effect.

Colligative Properties

Colligative properties are properties of a solution that depend on how much material is present in a solution, but does not depend on the nature of the solute. Common examples include:

Freezing Point Depression: Addition of a solute lowers the freezing point of a solution. This can be calculated by the equation:

ΔT = k_f * i * m

In this equation, k_f is the freezing point depression constant (1.86 C^o/m for water), m is the molality, and i is the number of particles formed from each formula unit. For dilute aqueous solutions, m is approximately the same as molarity. For molecular compounds, "i" is one. For ionic compounds, "i" is the number of ions present. For example, Na₂SO₄ dissolves in water to give 3 particles (2 sodium ions and 1 sulfate ion).

Osmosis: Osmosis is a result of a solutions tendency to acheive a uniform concentration. Under 'normal' conditions, solute will flow from regions of higher concentration to regions of lower concentration. In osmosis, this 'natural' flow is stopped by a semipermeable membrane. In order to equilize concentrations, solvent flows through the membrane to dilute the more concentrated solution.

One of the most important classes of semipermeable membranes are cell membranes. If a cell (which contains electrolytes), is placed in pure water (a hypotonic solution), water will tend to flow into the cell. If a cell in placed in a concentrated salt water solution (a hypertonic solution, water will flow out of the cell.