C3bL3 Solubility

Key Words

Crystallisation - formation of crystals

Dissolve - a solid or liquid becomes dispersed as single atoms or molecules in a solvent.

Saturated solution - solution which cannot dissolve any more of the substance.

Solubility - how much substance can be dissolved

Solution - water with a substance dissolved in it.

Solvent - a substance that dissolves another substance.

Grade E

A solvent dissolves a substance to form a solution.  The dissolved substance is the solute.  Solubility is the measurement of how much a particular substance will dissolve in a known amount of water at a given temperature.

While the solubility of copper sulphate increases with temperature,  potassium nitrate becomes very much more soluble.  Solids become a lot more soluble with temperature.

Gases become less soluble as temperature increases.

Water is good at dissolving ionic substances, but is not so good at dissolving covalently bonded substances like oils.  Covalently bonded substances dissolve in organic solvents, like acetone.  Many plastic glues are actually organic solvents.

Grade C

The reason that water is such a good solvent is that it is a polar molecule:

There is a slightly negative region (d-, "delta-negative") and a slightly positive region (d+, "delta positive").  When water dissolves a giant crystal structure like sodium chloride, water molecules surround the sodium ions and the chloride ions, which break away.

The delta negative portions of water molecules are attracted to the positive sodium ion.  For the chloride ions, the delta positive regions are attracted to the negative chloride ions.

A saturated solution is one in which so much substance has been dissolved that no more will dissolve.  If such a solution is left so that the water evaporates, crystals will form, since positive and negative ions will recombine.  The process is called crystallisation.  If the evaporation is slow, big crystals are formed.

Gases are more soluble when the solvents are cool.  The gas molecules don't have as much kinetic energy to escape, so they remain in solution.  If the liquid is heated, the molecules gain enough kinetic energy to escape.  A fizzy drink is a lot fizzier if it's cold.  If it gets warm, it goes flat.

If the gas pressure is increased, more gas molecules will dissolve, because there are more chance of collisions with the water surface.

Grade A

If water in a pond gets too warm, the amount of oxygen dissolved can drop to very low levels, killing fish and other animals.  The effluent from power stations is perfectly clean, but can be warm, causing thermal pollution.  Therefore the temperature has to be monitored carefully.