Tag Archives: chemistry

Water as a solvent

Not actually universal

Thinking of solutions often makes me think of water. I’m fascinated by water’s multiple uses in our world. As a cleaning agent, for example. What does it mean, that water cleans things? Well, take the case of dirty dishes. You’ve got some dinner dishes, with small scraps of meat, vegetables, some sauce, some cake crumbs, etc. They’ve been left on the sideboard for a few hours, so that the food scraps have dried out and are stuck to the dishes. Put these few plates, bowls, forks, knives and spoons in a basin of warm water for, say, twenty minutes. You will find that, with a minimal quantity of cleaning agents added – soap is perfectly adequate – you’ll be able to remove all the crumbs and bits of sauce from the plates and utensils easily with your hands. I find hands really great for cleaning, you can feel every lump and bump.

So what’s happening here? Certain chemical processes have occurred. First, The material on the crockery has dried out, over a period of hours. That means it has lost water to the surrounding air. Evaporation of water can occur at any temperature, as the surface water molecules have a higher kinetic energy, explained apparently by the Maxwell-Boltzmann distribution, which I won’t go into here.

So when the dirty dishes are placed in water, reactions occur. Stuff dissolves in water. This is because H₂O is a polar molecule – its oxygen head is electronegative and its two hydrogen tails are electropositive. That’s because the oxygen pulls the electrons it shares with the hydrogen – called covalent bonding – closer to itself, giving it a slightly negative charge, and the hydrogens a slightly positive charge. This polarity attracts water molecules to each other. So if there are any water molecules left on the dirty dishes, and there will be, the basin water will be attracted to them, so softening and breaking up the food particles. And if there is salt in the sauce and sugar in the cake, these will dissolve in the water, because the polar bonds in H₂O are stronger than the ionic bonds in salt (NaCl), so breaking them down, and H₂O will connect with the polar O-H bonds in sucrose (C12H22O11).

I’ve mentioned two other useful factors for cleaning – heat and soap. Any unfortunate who sugars their coffee will know how effective heat is for dissolving their poison. This is simply to do with the energy state of the water molecules. The excited molecules interact more rapidly with the sugar, or salt, causing their rapid dissolution. Soap, and other detergents, act as cleansing agents for a very different reason.

Some substances, particularly hydrocarbons, such as hexane (C₆H₁₄), found in petrol and many glues, are insoluble in water. In our example, think of cooking oil and fats. Greasy stuff. Soap is made up of molecules called surfactants. These lengthy molecules have a water-loving (hydrophilic) head and a grease-loving (hydrophobic) tail, so to speak. Here’s a neat summary of what happens, from Science on the shelves, a website of the University of York:

The head of the molecule is attracted to water (hydrophilic) and the tail is attracted to grease and dirt (hydrophobic). When the detergent molecules meet grease on clothes [or dishes], the tails are drawn into the grease but the heads still sit in the water. The attractive forces between the head groups and the water are so strong that the grease is lifted away from the surface. The blob of grease is now completely surrounded by detergent molecules and is broken into smaller pieces which are washed away by the water.

More detail can be gone into here, but this is a start. The fact that water is such an effective solvent has so many implications for all living organisms it’s hard to know what to turn to next, so I’ll have to give it a think.

I should point out that in researching this piece, which certainly wasn’t hard work, I found at least a dozen good videos describing water as a solvent, and there were countless other videos describing other properties of water. I’m very grateful to be living in the internet age, when so much of this thought-provoking material is so readily available.

some references




Water as a solvent | Water, acids, and bases | Biology | Khan Academy (video)

Properties of Water (video – Amoeba Sisters