Water as a Solvent

We can differentiate between three basic kinds of solute (molecule dissolved in a solvent), these are "neutral & non-polar", "neutral & polar" and "ionic".


A representative analogy for the three different types of solute molecule

my black and white cat Thia, she looks like a neutral polar molecule
A neutral non-polar solute has an overall charge of zero, ie it is neutral, and inside there are no strong regions of charge polarisation. An analogy would be to say the molecule has an overall "grey" colour distributed evenly over the whole volume of the molecule.

A neutral polar solute has an overall charge of zero, ie it is neutral, but inside the charges within the molecule are not evenly distributed although they balance out overall to zero. An analogy would be to say the molecule has sploges of black and white distributed around the molecule, but that the black and white parts match exactly (a bit like a zebra or black-and-white cat).

An ionic solute has an overall charge, this charge can be distributed evenly or unevenly within the molecule. The over-riding consideration in this case is the total charge. An analogy would be to say the molecule has sploges of black and white, but now there is an overall perponderance of one colour, ie more black (negative charge) OR more white (positive charge).

Water, because it is polar, can dissolve both neutral-polar and ionic solutes. Water (alone) cannot dissolve neutral non-polar molecules. Thus water can dissolve table salt (which is made up from Cl- and Na+ ions), and it can dissolve honey (which is neutral but polar) but it cannot dissolve oil (olive oil).

Honey is a mostly a mix of sugar molecules (ie fructose and glucose). Sugar has polar parts (see the OH units on the molecule below) which the water is attracted to and thus honey dissolves in water.

Honey has a melting point around 45 °C and so honey is actually an everyday "super cooled liquid" ie a substance that is a liquid when it "should" be a solid.


A "sugar" molecule in 3d, the oxygen atoms are red, the carbon atoms are grey and the white atoms are H. Next is the chemical structure of sugar, the OH units shown in blue allow honey to dissolve in water.

Oils, such as plant oils, are composed of molecules built up from glycerol and an acid component (in olive oil this is oleic acid), see the chemical structure below. The long carbon chains in the oil are non-polar and neutral and when oil is mixed with water, the water "repells" the oil forcing all the oily molecules together, eventualy the globules of oil all combine for form a seperate layer of oil above the water (see the video to the right).

The oleic portion is called an unsaturated fat because there is one double bond between the carbon atoms of the chain (where the chain "kinks").


table salt dissolves in water

honey dissolves in water

oil does not dissolve in water

An "oil" molecule, the part which makes olive oil insoluble in water is coloured blue, the glycerol component is shown in red, the oleic acid part is the black+blue components
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