Elsie Edgerton-Till, of Rangi Ruru Girls' School, asks :-
Why is it that two gases (hydrogen and oxygen) can combine to form a liquid (water)?
Trevor Kitson, a chemist at Massey University, responded.
Water is the most wonderful substance on earth! Each molecule consists of two hydrogen atoms bonded to one oxygen atom. The bonds consist of electrons shared between the atoms. Chemists describe oxygen as more `electronegative' than hydrogen. This means that oxygen is very greedy for electrons and pulls the lion's share towards itself and away from hydrogen. The result is that in water the oxygen atom is partially negatively charged and the hydrogens are partially positive; these unlike electrical charges attract each other strongly so that liquid water molecules tend to cling together tenaciously.
In other words, a lot of heat energy is needed to break the molecules away from each other and separate them into a gas, and water boils only at an astonishingly high temperature for such small molecules (100 C).
Hydrogen and oxygen molecules each consist of two identical atoms joined together. Thus in these symmetrical molecules there is not the imbalance of electrical charge that there is in water, and hydrogen and oxygen therefore have very low boiling points (minus 253 C and minus 183 C respectively). We think of water as so ordinary and take it for granted, but if it behaved like other small molecules such as hydrogen, oxygen, nitrogen, methane or carbon dioxide, our bodies (which are at least 60 per cent water) would instantly boil! And if it wasn't for water's fantastically high boiling point, there would never have been any warm cosy ponds for life to evolve in anyway, and the Earth would be dry as dust and utterly dead.