Lynn Reynolds, age 11, of Omaha, Nebraska, for her question:

Is mercury rated as a liquid or solid?

This question was sparked by a class discussion. The majority rated mercury as a liquid, but one student insisted that it is rightfully a solid. It is true that mercury can appear in a solid, a liquid, or a gaseous state. But under normal, every¬day circumstances this most fascinating silvery metal is a liquid. For convenience, we judge the state of matter under normal temperature and pressure at the earth's sur¬face. And under these conditions, mercury is without doubt a liquid.

Judging solid, liquid and gaseous substances seems like a rather simple science project. So it is    until curiosity prods us to wonder what goes on behind the scenes. Why, for instance, does liquid mercury become solid when the temperature dips to minus 37.97 degrees Fahrenheit? And why does it remain a fluid liquid up to a temperature of 673.84 degrees Fahrenheit, when it boils into a vaporous gas?

These strange goings on are called the three states of matter. The sudden changes from solid to liquid to gas are triggered by a tug of war between two opposite energies. Since mercury is a chemical element, all its atoms are alike. They have a mutual attraction for each that strives to hold them together. This energy succeeds in the solid state and fairly well in the liquid state. It is present in the same degree, even when it fails to hold the atoms together in gaseous mercury.

Opposing forms of energy are injected from outside. For example, all atoms and molecules use heat energy to move faster, and their speedy motions strive to weaken and finally break the mutual bonds that strive to hold them together. Mercury is a dense, metallic element and a quart of it weighs :pore than 28 pounds. Compared with other substances, the natural attraction between its atoms in the solid state is merely so so.

The critical temperature at which mercury melts or freezes is minus 37.97 degrees F. This small helping of heat provides the precise amount of energy needed to tip the balance between solid and liquid mercury. Below this temperature the mutual attraction of the atoms is strong enough to hold them together in a rigid structure.

At this point, the mercury in a thermometer freezes solid, expands and breaks the glass tube. However, less than one degree of heat provides enough heat to tip the scales again. The frozen atoms get enough speed partially to sever their bondage and the mercury changes from its solid to its liquid state of matter.

If and when the temperature soars to 673.84 degrees Fahrenheit, mercury atoms zoom around fast enough to break apart from their mutual attraction and go their separate ways. At this point, liquid mercury changes to its gaseous state. Our weather rarely dips to the freezing point of mercury and never, thank goodness, soars to its boiling point. Hence, we usually encounter this fascinating metal in its liquid state. So we rate it as a liquid  that is, under normal conditions.