Lesson 8:
The Chemistry of Solutions

Solubility and Saturation


A variety of units can be used to measure the concentration of solute in a solution.  In general, a concentrated solution is one where the amount of solute in a given volume of solvent is great.  A dilute solution is one where the amount of solute in a given volume of solvent is relatively small.  But can we mix up a solution of any concentration?  For example, can we produce a 90% solution of table salt in water?  How about a 90% solution of table salt in gasoline?

A few substances are infinitely soluble, or miscible, which means that they can be mixed together in any proportion.  Ethyl alcohol and water are an example of miscible substances since they can be mixed together in concentrations ranging from pure alcohol to pure water.  However, most substances are not infinitely soluble. 

Solubility is a term used to describe the amount of solute which can be dissolved in a solvent.  The solubility of two substances will depend on the similarity of the two substances.  As mentioned on the last page, some substances (such as oil and water) have a solubility of close to zero.  Most substances, however, will have a solubility somewhere between complete insolubility and complete miscibility. 



Saturated Solution

Once a solution has reached the limit of the solute's and solvent's solubility, the solution is said to be saturated meaning that it can hold no more solute.  Dissolved particles of the solute are in equilibrium with the undissolved particles. The rate of dissolving is equal to the rate of crystallizing.

If additional solute is added to a saturated solution, the extra solute will settle out, forming a separate layer like the kind you would see when two substances are insoluble. 

Supersaturated salt solution.

A saturated solution of table salt and water.

You can form your own saturated solution of table salt and water as follows.  Add salt to water, stirring constantly until the salt dissolves.  At first, the salt will completely dissolve in the water, discoloring the water slightly but leaving no visible solid residue.  However, once you have added a certain amount of salt to the water, the solution becomes saturated.  When you add more salt past the saturation point, the salt will not dissolve into the water no matter how long you mix the solution.  Instead, the extra salt will settle out in a layer at the bottom of the solution as shown above. 



Unsaturated Solutions

A solutino with any amount of dissolved solute less than the amount required to make a saturated solution is considered to be unsaturated.



Supersaturated Solutions

Whenever a solution contains more solute than it can hold, it is said to be supersaturated. It can be prepared by heating a saturated solution, adding more solute, and then cooling it gently. Excess dissolved solute crystallizes by seeding supersaturated solution with a few crystals of the solute.

Solutions can become supersaturated in a variety of ways, but in every case the supersaturated solution is unstable.  If more of the solute is added or if the conditions change in any way, the extra solute will settle out of the supersaturated solution.  In the water vapor and air solution, dust particles in the air provided the slight change which caused water vapor to settle out, forming clouds and rain. 

If you've ever made sweetened iced tea, you will have taken advantage of the characteristics of a supersaturated solution.  By adding sugar to hot tea, you were able to dissolve much more sugar into the water than you would have been able to dissolve into cold tea.  When the tea cooled, the additional sugar remained dissolved in the tea as a supersaturated solution.  If you tried to add more sugar to the cooled tea, however, the excess solute would drop out of solution and the tea would become less sweet.  A dilute solution contains less solute and a concentrated solution, more.



Standard Solutions

A standard solution is an accurately prepared solution used to determine the concentration of other solutions, which are then known as standardized solutions. In the lab, we can use a standard acid solution to standardize a base solution, and vice versa. The volumetric technique of matching a standard solution and a solution of unknown concentration is known as titration. This process of comparison is known as standardization.


Factors Influencing Solubility

The solubility of two substances depends on several factors in addition to the identity of the substances.  These factors can include characteristics of the environment and the state of the solute and solvent.  In every case, these factors will influence the amount of solute which can be dissolved in a solvent.

Environmental factors which influence solubility include temperature and pressure.  For example, warm air is able to dissolve much more water vapor than cold air can.  You may  have noticed that the air is humid (full of water vapor) only on warm days.  If very humid air cools suddenly, the extra water vapor will fall out of solution and will turn into liquid water.  The liquid water in the air forms clouds or fog.



The solubility of a solute can also depend on its oxidation state, which refers to the amount of electrons found in the substance.   For example, iron and manganese tend to enter water in a low oxidation state, meaning that they have a relatively large number of electrons.  But when these metals are exposed to air, they become oxidized (lose electrons) and are no longer very soluble in water.  So the oxidized metals drop out of solution and cause red or black water complaints as well as stains on sinks and clothes. 

The nature of the solute and solvent play a role as well. Like substances dissolve in like solvents. Inorganic substances dissolve in inorganic solvents and organic substances dissolve in organic solvents. Solubility of a substance depends on two types of forces: attractive forces among the solute particles and attractive forces betwen solute and solvent particles. The former cause lower solubility and the latter, more.


The layering seen in a solution which has passed its saturation point resembles the layering seen when two insoluble substances are mixed.  But, as you can see in the illustration below, these two situations are actually quite different.  One of the layers in the saturated solution contains both solute and solvent while the layers in the insoluble mixture contain only one substance per layer. 

A supersaturated solution and a mixture of two insoluble substances

The extra solute added to a saturated solution may settle either to the top or to the bottom of the solution.  The location of the extra solute depends on its density, a concept we will discuss in a later lesson.
Part 3: Concentration