In general, SOLUBILITY is an ability of a substance to dissolve. In the process of dissolving, the substance which is being dissolved is called a solute and the substance in which the solute is dissolved is called a solvent. A mixture of solute and solvent is called a solution.
To put it in simple words:
When we insert sugar into water it will dissolve. In this process:
- sugar is the solute
- water is the solvent
One of the characteristics of table sugar is its solubility in water
That was a definition of solubility as it is used in a common language. Now let's see solubility as chemists understand it:
Chemist's understanding of Solubility
A chemist understands solubility as a measure. A chemist would say that:
SOLUBILITY is understood as a maximum amount of solute that dissolves in a solvent at so called equilibrium. In chemistry an equilibrium is a state where reactants and products reach a balance - no more solute can be dissolved in the solvent in the set conditions (temperature, pressure). Such a solution is called a saturated solution.
To put it in simple words: If you take one litre of water and you start dissolving table salt in it (chemical formula of salt is NaCl) and:
- temperature of water is 25oC
- pressure is 1 ATM (Atmosphere - standard pressure in the open air on Earth)
you should be able to dissolve exactly 357.00 grams and not a gram more. The rest of the salt will stay on the bottom as residue and will not dissolve. Solubility of salt in water is therefore 357.00g/L. When this amount of salt is dissolved the solution reaches its equilibrium. Every chemical substance which dissolves in water has a fixed solubility. If it does not dissolve - its solubility is zero. Many of these solubilities have been measured and special charts are produced displaying solubility of many substances at once.
To complete our introduction to solubility, we will describe two groups of substances in case of which solubility measure cannot be applied. These are miscible and immiscible substances
Miscible and immiscible substances
Some substances, like water and alcohol, can be mixed together and create a homogenous phase in any proportion. A solubility measure cannot be applied to such two substances. Such substances are called miscible. On the other hand if two substances cannot be mixed together (like water and oil), they are called immiscible.
Now, when you know what solubility really is, you can check out 'why do things dissolve', where we explain in detail why some things dissolve and some do not.