A physical change that many find confusing is the solution process. When salt is sprinkled into water, it seems to disappear, but letting the water evaporate reveals the salt still there. A solution is a homogenous mixture (made the same throughout). If you let salt water sit overnight (or longer), the salt will not settle out.
A solution is composed of a solvent (the greater component, commonly the liquid) and one or more solutes (the lesser components). There is no requirementg that the solvent be a liquid, although this is the most familiar situation. The table below shows some other possibilities and examples
| solid in solid | alloys | bronze (Sn in Cu), brass (Zn in Cu) |
| liquid in solid | hydrated crystals | CuSO4·5H2O |
| gas in solid | catalysts | H2 in palladium |
| solid in liquid | salt water | salt in water |
| liquid in liquid | mixtures | antifreeze and water |
| gas in liquid | carbonated drinks | CO2 in water (soda) |
| liquid in gas | the atmosphere | water in air |
| gas in gas | the atmosphere | N2, O2, Ar, CO2 in each other |
Solubility is a measure of the maximum mass of solute that can be dissolved in a given amount of a specific solvent. This is commonly measured in grams of X per 100 grams of water. For example, the solubility of lithium chloride is 45.4 grams per 100 grams of water (written 45.4 g/100g). Other solvents may be useful, and would be mentioned in the label.
If a solute can dissolve in a given solvent, it is said to be soluble,
if not it is insoluble. If two liquids can form a solution,
we say that they are miscible with each other, if they are
not (like oil and water) we say that they are immiscible.
When a solvent has dissolved the maximum amount of a given solvent under specific
conditions, that solution is said to be saturated. In the case
of salt water, some amount of undissolved salt will remain at the bottom of
the container. In actuality, this remaining solid is continually being exchanged
with the dissolved salt, atom by atom, but the description still applies.