Unit 9 – Solutions
Solution - a homogeneous mixture that is present in one state
2 parts:
Solute - substance being dissolved
NaCl in water
Solvent - the substance which is used to dissolve something. The most common solvent used is water.
Aqueous solution – a solution in which water is the solvent
The state of a solution is determined by the state of the solvent. (Slide 1)
Solute Solvent
Gas Gas
Gas
Gas
Liquid
Solid
Liquid Gas
Liquid Liquid
Liquid Solid
Solid Liquid
Solid Solid
Air
Oxygen in water
Toxic fumes in activated charcoal (Gas Mask)
Humidity
Alcohol in water (Miscible)
Dental Filing (Hg in Ag)
NaCl in water
Metal alloy
Example
Miscible - being able to dissolve a liquid in a second liquid. The liquid present in the larger amount is considered the solvent.
Basic Solubility Rule: “Like dissolves Like”
Water is highly polar. Ionic and polar covalent compounds tend to dissolve in water, while nonpolar covalent compounds do not dissolve in water.
Solution process for Ionic compounds in water: (Slide 2 - 5)

An ionic crystal is stable due to each ion being surrounded by oppositely charged ions.
Water molecules are attracted to ions and cluster around the ion reducing its attraction to the other ions. When enough water molecules surround the ion to stabilize the charge the ion will separate from the crystal as a free ion.
Solvated – a particle surrounded by solvent molecules. (Slide 5)
Ionic compounds can separate into ions in solution. The process by which a substance separates into ions when in solution is called dissociation. Substances that dissociate in solution are called electrolytes. Electrolytes are capable of carrying an electrical current in solution, or when melted, as a result of the existence of free ions. Substances that are not capable of separating into ions in solution, or when melted are called nonelectrolvtes. Nonelectrolytes cannot conduct electricity.
Example: NaCl
(aq)
 Na 1+(aq) + Cl 1-(aq)
Solubility is explained by collision theory just like reactions.
Different substance will dissolve at different rates, but the rate can be altered by making changes in the solutions conditions.
The factors that affect the rate of solubility are:
Surface area:
If you break a material into smaller pieces, the amount of surface available for collisions increases, resulting in a faster solubility rate.
Stirring of shaking the solution:
Shaking a solution involving a solid causes more of its surface to make contact with the solvent making the solid dissolve faster.
Shaking a solution with a gas as the solute will cause the solute to be driven out of solution, reducing the amount of material dissolved. A classic example of this effect is shaking a can of soda, causing the carbon dioxide to leave the soda caring much of the soda out of the can.

Changing the temperature:
In general heating a solution involving a solid solute will cause the solid to dissolve faster. This can be explained in terms of particle motion, when particles are heated they move faster and have more energy. If faster particles collide with the solid there is more chance for a piece of the solid to break off and dissolve.
However, since a gas is already at a temperature above its boiling point it will boil out of any solution that is heated, so heating a solution with a gaseous solute will usually make the gas leave the solution, lowering the concentration of the gas.
The solubility rules we have been using to determine precipitates in a reaction are slightly over simplified. i.e. – Calcium hydroxide is insoluble according to rule #6. However, limewater is a solution of Calcium hydroxide in water. Calcium hydroxide is slightly soluble in water, and so some will dissolve when mixed with water.
What is more useful for a chemist is a Solubility Curve. A Solubility Curve is a graph used to determine the amount of solute that will dissolve in a specific solvent, at various temperatures.
You can also use this table to determine the temperature needed to dissolve a specific amount of solute and to compare the solubility of different compounds, for instance
KC1 is less soluble than NaC1 at temperatures below thirty five degrees, but above thirty five degrees KC1 is more soluble.
The Solubility Curve allows us to classify solutions based upon the amount of solute that has been dissolved at a specific temperature. (Slide 6)
These classifications are: (Slide 7; Analogy Slide 8)
Saturated Solution - A solution which is holding as much solute as it is able to hold. A saturated solution is one which has conditions that meet directly on the solubility curve.
Ex. 44 g of KC1 dissolved in 100 ml of water at 80 o Celsius.

Unsaturated Solution - A solution which is holding less solute than it is able to hold.
The conditions used to describe an unsaturated solution would meet below the curve for that substance.
Ex. 30 g of KC1 dissolved in 100 ml of water at 80 o C.
Supersaturated Solution - A solution which is holding more solute than it is supposed to be able to hold. The supersaturated solution is recognizable because the conditions describing it meet above the curve for that substance.
Ex. 50 g of KC1 dissolved in 100 ml of water at 80 o C.
A supersaturated solution is made by making a saturated solution at a higher temperature and letting it cool slowly, or by letting the solvent evaporate slowly. If done right the solute will remain in solution until disturbed.