Bernard Houde, age 12, of Montreal, Quebec, for his question:

How does water differ from heavy water?

The basic difference is weight. As you no doubt suspect, heavy water is heavier than ordinary water. Its extra weight is contributed by extra particles in some of the atoms in some of its molecules.

Heavy water may contain isotope atoms of oxygen that have extra neutrons. However, most heavy water contains hydrogen isotopes with extra neutrons. The extra weight of heavy water makes it behave somewhat differently from ordinary water.

The quality of heavy water depends upon the percentage of heavy isotopes in its molecules. It may weigh a little more or a whole lot more than ordinary water. But it looks and pours like ordinary water and like ordinary water it is odorless and tasteless. However, the freezing and boiling points of the two are different    and certain living things react to them in different ways.

Ordinary water freezes at 32 degrees and boils at 212 degrees Fahrenheit. On the same temperature scale, heavy water freezes at 38.88 degrees and boils at 214.56 degrees. It takes a little more heat to boil heavy water and less cold to freeze it.

Seeds, as we know, require moisture from ordinary water in order to germinate. For reasons unknown, they cannot germinate in heavy water. Tadpoles and certain other creatures cannot survive in a concentration of heavy water. Biologists are investigating these oddities and also using heavy water to study various biological activities in other living things.

The unusual behavior of heavy water also makes it useful in atomic reactors. In some reactors, it is used as a moderator. In this important safety role it moderates the high energy neutrons created in the radioactive chain reaction. Some reactors use heavy water as a coolant. In this role it absorbs and carries away heat from the core. This operation prevents the core from overheating and also transports the absorbed heat to where it can be used to create steam and electric power.

Heavy water has been used for only a couple of decades and creating it is quite expensive. However, there is nothing new about it, for it always existed in nature    in infinitesimal amounts. There are a few molecules of heavy water in every drop of ordinary water. It is estimated that in about every 5,000 molecules of ordinary water there is one molecule of heavy water.

The extra weight in most heavy water is contributed by isotopes of heavy nitrogen. The nucleus of the ordinary hydrogen atom contains one proton. The nucleus of heavy hydrogen is twice as heavy because it contains the usual proton plus a neutron. Heavy hydrogen is called deuterium. And when heavy water gets its extra weight from deuterium isotopes it is called deuterium oxide.