Cindy Dincher, age 10, of Williamsport, Pennsylvania, for her question:

Is fire a solid, liquid or gas?

Young science students are told that everything in our world is made of solids, liquids or gases. This may be a bit misleading, because it does not include happenings. Things that happen use energy    and we cannot say that anything is made of energy. Solids, liquids and gases are atoms and molecules of matter. Chemicals use energy in all sorts of ways to model and remodel their molecules. One of these chemical operations is fire.

Coal and wood are solids. In order to burn, they must use up oxygen. And oxygen is a gas. As they burn, most of the solid fuels are changed into other gases. They also give off water vapor and this may turn to steamy moisture. So, when a fire burns, there are solid fuels and ashes, filmy gases and often a trace of liquid water. Yet the fire itself is none of these things. It is a chemical happening that uses energy to change solids into gases and other substances.

The use of energy makes fire very active. It devours or consumes its fuel. Scientists call it a chemical reaction because chemicals are changed and remodeled. Fire consumes wood and coal and they disappear. Or so it seems. Actually, the solid fuel is broken apart into small particles. Most of the pieces are tiny, invisible gas molecules that zoom off and mingle with the other gases of the air. Some of the leftovers are gritty ashes that the fire cannot break into smaller pieces.

Tiny molecules of wood and coal are neat bundles of atoms. The atoms in each package are attracted to each other. This attraction is the energy that ties the package together. To break the package apart, stronger energy is needed from outside. In a fire, this energy is provided by heat. Alighted match provides enough energy to break up some of the molecules in a piece of paper. When they break apart, the energy that held them together is set free. This provides more heat to keep the paper fire blazing away. Some substances need more heat to start burning than others.

A wad of burning paper provides enough heat to break up molecules of wood. The wood burns with still more heat    enough to start coal burning. As the fire burns, more and more fuel molecules burst their bonds. Some fragments zoom off separately as gas molecules of carbon dioxide and water vapor. Fragments of carbon swirl off in sooty plumes of smoke. The fire uses oxygen from the air to keep its burning energy going.

Fire is just one kind of chemical reaction and there are many others. Some give off heat as their molecules are remodeled. Others use up heat. Trees use the energy of sunlight to tie assorted atoms into molecules. This energy from the sun is stored up in wood and coal. When these fuels are burned, the energy that held the molecules together is freed. It heats the coals red hot and lights flames in the blazing gases.