hydrated. The separation of the solute particles from one another prior to dissolving is an endothermic process for both solvent and solute (steps 1 and 2), but when the solute and solvent combine with each other, this is an exothermic process (step 3). If the energy released in step 3 is greater than the energy absorbed in steps 1 and 2, the solution forms and is stable.
The term solubility refers to the maximum amount of material that will dissolve in a given amount of solvent at a given temperature to produce a stable solution. By looking at the plot of solubilities below, you can see that most solids increase in solubility with an increase in temperature.
Gases, however, decrease in solubility with an increase in temperature.
Degrees of Saturation
When referring to solutions, there are three degrees of saturation—unsaturated, saturated, and supersaturated. If a solution is unsaturated, the solvent is capable of dissolving more solute. When the solution is saturated, the solvent has dissolved the maximum amount of solute that it can at the given temperature. At this point we say that the solution is in a state of dynamic equilibrium—the processes of dissolving and precipitation are happening at the same rate. A supersaturated solution is one in which the solvent contains more solute than it can theoretically hold at a given temperature. Supersaturated solutions are often formed by heating a solution and dissolving more solute, then cooling the solution down slowly. These solutions are unstable and crystallize readily.
Concentration Terms
Solutions are often referred to as being concentrated or dilute. These two terms are very general. While concentrated indicates that there is a lot of solute dissolved in the solvent (perhaps the solution is near to being saturated) and dilute indicates that a small amount of solute is dissolved in the solvent, we often need to be exact with quantities in chemistry. The units of concentration that you should be familiar with for the SAT II exam are reviewed below.
Molarity (M )
The molarity of a solution is a measure of the number of moles of solute per liter of solution. This is the most common concentration unit used in chemistry. For instance, you might see an expression that looks like this:
[NaCl] = 0.75
which means that 0.75 mole of NaCl is dissolved per 1.00 L of solution. The brackets around the number indicate that the concentration is expressed in terms of molarity. Let’s now run through how you calculate the molarity of a solution.
Example
Calculate the molarity of a solution prepared by dissolving 20.0 g of solid NaOH in enough water to make 100 mL of solution.
Explanation
Convert grams to moles:
