A solvent, like water, is the medium we use to dissolve and "hold" many things. For example, if you take a teaspoon of salt and stir it into a glass of water, you have created a salt solution, water is the solvent and the salt is the solute. Solvents are important in that they stablise a substance without reacting with it, they provide a surrounding medium to which we can add other things.

To act as a solvent in our everyday lives there needs to be a great deal of the solvent. Water is abundant, I can't think of anything else that is liquid and available in the quantites required. What else could possibly be a solvent for life? Examples of less ubiquitous solvents include oils, honey, petrochemicals all of which have to be grown (sunflower oil), made (by honey bees) or dug up and refined (like petrol).

Water is a fantastic solvent, but how does it work?

Water molecules are very small and they can arrange themselves around another molecule (or solute) very easily.

The diagram to the left shows the original picture of water and next to it is a new simplified depiction of water. The "bunny ears" of electron density are represented by the "delta minus" δ− near the O-atom, and the H-atoms now have δ+ meaning they are slightly positively charged. We use the delta symbol because this is not a whole unit of charge, but some amount less than this. We now reserve the plus or minus symbol inside a circle to represent a whole unit of charge, +1 or -1.

Water molecules are polar, the positive end is attracted to negative charges in the solute and the negative end is attracted to positive charges in the solute. You can see this in the diagrams to the left. Here the slightly positive H-atoms point at the negative Cl anion, and the slightly negative O-atoms point at the positive Na cation. These are ionic interactions they involve charges rather than a sharing of electrons.

Remember also that these are "cartoon" images and the water actually surrounds the ion in 3-dimensions, the water forms a rough sphere around each ion. We call this the first solvation sphere.

Also in tese cartoons I've only drawn in the water molecules interacting with the ion, there are still more water molecules out beyond the ones I've explicitly represented. Once we move to 3d the "picture" becomes much more complex, as shown in the image to the right.

Then, don't forget that water is a dynamic solvent and the water molecules are always moving, breaking and forming H-bonds, this is represented in the animation below, which is taken from a simulation of water around a chloride anion. The white dashed lines which apear and disapear are the H-bonds forming and breaking over time.