Many substances dissolve in water. We can observe this with salt or sugar every day. A great part of our body consists of water as well, which contains almost all the substances we need for living in dissolved form. It is the same among plants and animals. Thus, water is the most important solvent in nature.

But there are also water-insoluble substances: fats, plastics and most metals do not dissolve in water, for example.

Whether a certain substance dissolves in water or not depends on many factors.

Dissolving means that the solid is reduced to the small particles of which it is built up, and that these particles are distributed evenly in the water. Depending on their qualities, the particles of some substances mix very well with the water particles, whereas others mix poorly or not at all.

Water consists of molecules with the chemical formula H₂O. That means a water molecule is made up of two atoms hydrogen and one atom oxygen. Those three atoms - as you can see in the picture - are arranged at an angle, and carry electrical charges. The water molecules possess a dipole nature, and therefore they are able to attach themselves to charged particles (ions) in the surface of crystals very well. This property makes water a good solvent.

Salts are among those substances which dissolve in water very well. In comparison to other fluids, water is one of the best solvents for salt-like compounds.

However, the water is only able to take up a limited amount, even of very soluble substances. From a certain concentration, which is different for every substance, the substance does not dissolve anymore when added, but remains as surplus solid sediment. This is called a saturated solution.

You can try this with table salt: How many teaspoons of salt can be dissolved in a small glass of water before insoluble sediment remains?

Water not only dissolves solid substances. Fluids and even gases dissolve in water as well!

Fish are only able to breathe in water, because a little bit of the gas oxygen from the air dissolves in the water.

Acid rain forms when rainwater takes up exhaust fumes from the air, which react acidic in an aqueous solution.

Alcoholic beverages consist of a mixture of the two liquids water and alcohol.

Since water is such a good solvent, water found in nature is never completely pure, chemically speaking, but always contains dissolved substances. Salt from the rocks and gases from the air. Particularly, seawater contains much dissolved salt. In tap water, solid substances are dissolved as well. If you let a drop of tap water evaporate on a glass plate, the dissolved substances remain as a solid coating.