Jodi Cranston, age 11, of San Diego, Calif., for her question:

WHAT IS CARBON DIOXIDE?

You'll find carbon dioxide used in many different ways. Cakes, for example, rise in an oven because baking powder or yeast in the batter releases carbon dioxide. Carbon dioxide also produces the sparkle or fizz in your soft drinks. There are even some fire extinguishers that use carbon dioxide because it will not burn and it can also put out fires.

Carbon dioxide is a tasteless, colorless, odorless gas.

When human beings and animals breathe in oxygen, they exhale carbon dioxide that is produced by the oxidizing or burning of food in their bodies. Green plants take carbon dioxide from the air and give off oxygen when light shines on them. Plants combine carbon dioxide with water to make food.

Carbon dioxide itself is not poisonous. It can, however, be dangerous to humans and animals if it collects in the bottom of wells, silos, caves and mines, since a concentration of carbon dioxide will suffocate them. Because it has a higher density than air, it can collect in some places.

Carbon dioxide can become a solid when it is cooled at atmospheric pressure to minus 109.3 degrees. In this form it is called dry ice.

It is given this name because when it melts, it will not turn to liquid as frozen water will. Instead it changes back into a gas.

Carbon dioxide molecules are made up of one atom of carbon and two atoms of oxygen. The gas was discovered by a Belgian physician and chemist named Jan Baptista van Helmont in the 1600s.

Then in 1781 a French chemist named Antoine Laurent Lavoisier discovered the composition of carbon dioxide. This scientist also wrote the first modern textbook of chemistry. Carbon dioxide is produced as a waste as humans and animals breathe and also when wood, oil, gasoline or any fuel containing carbon burns with a large supply of oxygen.

When fuel without enough oxygen burns, it will produce carbon monoxide. Carbon monoxide is an odorless and colorless gas that is extremely poisonous.

Carbon dioxide is also sometimes called carbonic‑acid gas. The reason for this is that it produces a weak acid when dissolved in water.

The process of dry ice melting directly from a solid state into a gas without first becoming a liquid is called sublimation.