Solution





In chemistry, a solution is one or more substance (the solute) dissolved in
another substance (the solvent) forming a homogenous mixture. A common
example would be salt dissolving in water (seawater); but also gases may
dissolve into liquids, as well as solids into liquids, and liquids and gases
into themselves.

The solvent is defined as the substance that exists in a greater quantity
than the solute(s) in the solution. If both solute and solvent exist in
equal quantities (such as in a 50% ethanol 50% water solution), the
substance that is more often used as a solvent is designated a solvent (in
this case, water).

Solvents can be broadly classified into polar and non-polar solvents. Common
polar solvents include water and ethanol. Generally polar or ionic compounds
will only dissolve in polar solvents. An excellent test for the polarity of
a liquid solvent is to rub a plastic rod, to induce static electricity. Then
hold this charged rod close to a running stream of the solvent. If the path
of the solvent deviates when the rod is held close to it, it is a polar
solvent.

When a solute is dissolved into a solvent, especially polar solvents, a
structure forms around it (a process called solvation), which allows the
solute-solvent interaction to remain stable.

When no more of a solute can be dissolved into a solvent, the solution is
said to be saturated. However the point at which a solution can become
saturated changes significantly with different environmental factors, such
as temperature, pressure, and contamination. Raising the solubility (such as
by increasing the temperature) to dissolve more solute, and then lowering
the solubility causes a solution to become supersaturated.

In general the greater the temperature of a solvent, the more of a given
solute it can dissolve. However, some compounds exhibit reverse solubility,
which means that as a solvent gets warmer, less solute can be dissolved.
Some surfactants exhibit this behaviour.

Several units are used to express the concentration of solutions:

   * The molar is a mole of solute per liter of solution. See molarity.
   * The molal is a mole of solute per kilogram of solvent. See molality.
   * The molinity unit is moles of solute per kg of solution.
   * The formal is the same as the molal, but is used when it is not clear
     what a molecule of solute is, for instance metals and salts. It is one
     mole of the chemical formula per kilogram.

There are several types of solutions:

   * Solid solution (s): Two or more solid substances are heated to form a
     liquid and then cooled to their solid state
        o Alloy: A solution of metals metallically bonded
   * Gaseous solution (g): Non-reactive gases mix with each other and
     diffuse until the particles are evenly spaced.
   * Liquid solution (l): The solvent is liquid, while the solute may be
     either of the three states of matter.
   * Aqueous solution (aq): A solution where water is the solvent.