Water is all around us, and so we tend to think of it as normal and perhaps even boring. This In Our Time episode was about the many ways in which water is unusual and interesting. The experts discussing it were Hasok Chang (University of Cambridge), Andrea Sella (University College London) and Patricia Hunt (Imperial College London).
(This is the second time in a week we’ve listened/watched something about water – the fifth episode of Wonders of Life (post) also spent some time discussing water and it’s uniqueness and importance for life.)
They started out with a bit of historical context – before the late 18th Century water was thought of as an element, not as a substance that was made up of other elements. Antoine Lavoisier was the first person to discover that water is made up of hydrogen & oxygen and he is the person who named those elements. Something I didn’t know but that seems obvious now it’s pointed out, is that the word hydrogen means water-maker and is so named because combining it with oxygen makes water. It took a while for this to be accepted by the scientific community as a whole, and took until the mid-19th Century before the proportions of the two elements were known. But it’s now a matter of common knowledge that water is H2O, two hydrogens and one oxygen atom per molecule.
Hunt told us about how that’s right but not the whole story. Each oxygen atom in water bonds to four hydrogens – two with short covalent bonds, and two with longer hydrogen bonds. The short covalent bonds are the bonds that require a chemical reaction and input of energy to break, and these are the two hydrogens that are part of the water molecule per se. Hydrogen bonds form because the water molecule is polarised, Hunt was describing it as the triangle of the water molecule (sitting with the hydrogens at the base and the oxygen at the apex) has two little bunny ears sticking up which are perpendicular to the plane of the hydrogen atoms. So the oxygen is inside a tetrahedral environment with a hydrogen at each of two of the corners of the tetrahedron, and (effectively) an electron at each of the other two. These bunny ears (which are slightly negatively charged) interact with hydrogens on other water molecules (which are slight positively charged. Chang said that in cruder terms this means that the water molecules are “sticky”. Hydrogen bonds are longer than covalent bonds and don’t need a chemical reaction or large amounts of energy to break – Hunt said that they flick on & off every picosecond (which is 10-12 seconds). When pushed she said that that’s not directly observable, but that you do experiments to do with femtosecond (10-15 seconds) bursts of lasers and do calculations involving quantum mechanics to indirectly observe this (this is her area of expertise) and this is the best hypothesis about what’s going on.
They spent a while talking about the properties of water that are unusual. For instance, ice floats on water. We just take this for granted but it’s a unique property – most solids sink beneath the liquid form of the substance. Sella gave olive oil as an example, if you look in the supermarket on a cold day then you see cloudy solid olive oil at the bottoms of the bottles. Water is densest at 4°C while it is still a liquid, and this has to do with how the hydrogen bonds between the molecules push them apart in the solid (I think).
Water is also unique in how high a temperature it freezes & boils, if you compare it to other similar molecules. They used H2S and NH3 as examples of similar molecules that are gases long before water even liquefies. This again has to do with the hydrogen bonds, these hold the water molecules together when otherwise they might drift apart. Chang explained that in the 18th Century there was a certain amount of confusion about what precisely the boiling point of water is, and it turns out that this is justified. Boiling starts with the formation of bubbles of gaseous water which rise to the surface. The surface tension of water (due again to hydrogen bonds) means that it’s very hard or impossible for a bubble to start from nothing. So if the surface of the vessel is very smooth (like a ceramic mug) then the water can be heated past 100°C before it boils – this is called superheated water. He said that in a normal mug you might get to 102°C or 103°C. I followed a link from the In Our Time programme page to some research Chang has done on this – I was particularly struck by his sixth experiment where using degassed water he found that the water gets to a temperature of 108°C without boiling, and then explodes.
Water is a very good solvent. For small ionic compounds (say, salt – NaCl) this is down to the charges on the ions of the compound that’s being dissolved. The positive ones (Na) interact with the oxygen atoms, and the negative ones (Cl) interact with the hydrogen atoms. The way that the hydrogen bonds between different water molecules make the water form a lattice like structure also helps to dissolve some non-ionic compounds. If the molecule is small enough it will fit in the gaps in the lattice, as if it’s in a cage. Hunt then talked about how this makes water very important in life. Partly because it can carry nutrients around the body (in the bloodstream of an animal, in the xylem or phloem of a plant). Water is also an important part of cellular biochemistry. It is the solvent in which the chemistry takes place, and is also involved in helping some of the components of this chemistry (proteins) to fold up into the right shapes. The way water and some things don’t mix (oils, lipids) is how cell membranes work – if you think of oil droplets floating on water then you can see how they could be formed into a shell around a compartment of water.
They also talked a little bit about how there are more sorts of ice than you might expect. At least 15. Ice I is the one that we normally see, and in it all the oxygen atoms are aligned like oranges stacked up in a supermarket. But the orientation of the water molecules is random – so which direction the short covalent bonded hydrogens are in differs randomly between the molecules. If you do things with temperature & pressure to the ice then you get different forms of ice – the oxygens will still be organised the same as Ice I, but the orientation of the water molecules will be ordered in some way or another. For instance all the short covalently bonded hydrogens might be on the same side of each molecule and lined up in rows.
The take home message was that water is much more interesting than one might think, and that chemists are still finding out new things about it. Sella finished up the programme by telling us about one question that’s got the potential to have an impact on everyday life – why is ice so slippery? Apparently the full chemistry & physics behind this isn’t yet known.