Sam Darling, of Ettrick, asks :-

Why is water a liquid, when it is made up of hydrogen and oxygen, which are both gases?

Daniel Schumayer, a physicist at Otago University, responded.

There are eighty stable elements naturally occurring on Earth, but we experience an abundance of different substances. How is this possible? This is similar to the concept of an alphabet; we have finite number of symbols, the letters, from which we build up the plethora of words. We do this by "gluing" one letter to another. Elements can do the same, but the "glue" is called a chemical bond. Not only atoms, but molecules may also establish bonds with other molecules, similarly to how we form sentences from the individual words. The answer to your question, therefore, must lie in the ability of water molecules to bond together.

Each atom has a positively charged core surrounded by a cloud of negatively charged electrons. Although the water molecule is electrically neutral, the electrons are not distributed uniformly; the oxygen becomes slightly negative, leaving the hydrogen atoms slightly positive. If there is another water molecule nearby, the slightly negative oxygen of the first molecule will attract the hydrogen atoms of the other molecule, since those are slightly positively charged. This electrical attraction forms a bond, called a hydrogen bond, between the two molecules. But the hydrogen atoms of the first molecule are still "free", and therefore able to bind with a third molecule in the very same way. The molecules thus end up forming large clusters in which they are bound to one another.

Hydrogen bonding explains the unusual properties of water. Since the molecules have strong tie to their neighbours, they cannot freely rush around in the entire volume available to them like in a gas, but they can only move by tumbling over each other, i.e. flowing. The hydrogen bonds bind water molecules so strongly, that water has a much higher boiling temperature, 100 Celsius, than compared to other molecules also containing hydrogen bonds; e.g., hydrogen-fluoride boils at 20 Celsius, and hydrogen-chloride does so at minus 85 Celsius. Inter-particle forces, in general, are responsible for whether a substance is solid, liquid or gaseous. Hydrogen itself becomes liquid if it is cooled down to minus 253 Celsius, or solid below minus 259 Celsius. Oxygen undergoes the same transitions; liquid below minus 183 Celsius and solidifies below minus 219 Celsius.

Isn't it fascinating that our lives, which so strongly depends on water, may only exist because of these relatively strong hydrogen bonds?