solutions

In chemistry or biology, a __solution__ is defined as a mixture of a **solute** and a **solvent**.



Some examples of solutions would be a glass of Kool-Aid, salt water, or even blood.


 * The //solute// is the substance that is being dissolved or mixed into another liquid, and is usually present in smaller amount than the solvent.
 * The //solvent// is the substance that is accepting and dissolving or mixing with the solute, and is usually present in greater amount.

When an ionic compound dissolves in water, the resulting mixture is considered //aqueous//.

Follow this link to watch an animation of salt dissolving in water.

For example, solid sodium chloride (the solute) will dissolve in water (the solvent) to produce an aqueous sodium chloride solution.

NaCl (s) --(H 2 O)--> NaCl (aq)

In reality, the sodium chloride in solution is no longer a molecule of NaCl, but is broken apart by water into Na + and Cl - ions: NaCl (aq) --> Na + (aq) + Cl - (aq)

This is called a **total ionic equation.**

Here is an example:


 * Molecular equation:** NaCl (aq) + AgNO 3 (aq) --> NaNO 3 (aq) + AgCl (s)

A more accurate depiction of this reaction is the **total ionic equation:**

Na + (aq) + Cl - (aq) + Ag + (aq) + NO 3 - (aq) --> Na + (aq) + NO 3 - (aq) + AgCl (s)

Note that the solid (not soluble) does not get broken apart in the equation (nor in the solution). The same concept applies to liquids and gases. **//Only aqueous substances get broken into their constituent ions.//**

To simplify the chemical equation, common ions on both sides of the reaction arrow (called **spectator ions**) are 'canceled out' to provide the **net ionic equation**:

Na + (aq) + Cl - (aq) + Ag + (aq) + NO 3 - (aq) --> Na + (aq) + NO 3 - (aq) + AgCl (s)


 * net ionic equation:** Cl - (aq) + Ag + (aq) --> AgCl (s)

Solubility
Many solids will not dissolve easily in water, as you have seen when dealing with solubility predictions.
 * Solubility is a property of how strong the ionic bonding is within a solid. As ionic bonding strength increases, the solubility of the solid will decrease. However, no substances are completely insoluble - tiny amounts of the solid can still dissolve.
 * Solubility can change with temperature (see below).
 * //Agitation// (stirring or swirling) can speed up the dissolving process, but will not allow more solute to dissolve.
 * Dissolving is a //physical change//, since the chemical bonds within the molecules or ions are not being broken and re-arranged.
 * As more and more of a solute is dissolved in the solvent, it becomes more difficult to dissolve. At some point, the saturation point is reached.
 * The rate of dissolving (dissolution) is also affected by surface area. A powdered solid will dissolve more readily than a large crystalline solid.

When a solvent can no longer dissolve any more solute, it is called a __saturated solution__. This saturation level can be modified by heating or cooling the mixture.
 * If it is heated, the molecules and ions will move faster, expand slightly, and allow more solute to dissolve.
 * If the mixture is cooled, then the opposite occurs, and the solution becomes __supersaturated__, and crystallization of the solute can occur upon cooling (think about how you make Jell-o).



The chemical composition of a solute also has an affect on solubility:



The amount of a solute dissolved in a volume of solvent is called its //concentration//. Concentration is defined as **molarity (M):**

M = moles of solute/L of solution, or **M = mol/L**

Go here for more on solution concentrations.

Try your knowledge of solutions using this [|online quiz]. +