January 2007

A chlorate candle is a device that produces oxygen when breathable air is scarce. Traditional wax candles spread soft haloes of light as they burn, a process that consumes oxygen from the air. Chlorate candles also burn, but their function is to release oxygen for emergency breathing.

First-responders must be prepared for many dangerous situations. Lack of oxygen is an immediate risk that can lead swiftly to death. That's when a chlorate candle can be a life-saver. These candles can be counted among the equipment carried by firefighters and mine rescue crews. They can also be found on submarines and air- and spacecraft.

Storing Oxygen

Oxygen is one of the gases in Earth's atmosphere that is critical to life. Humans and animals must have oxygen to live, and we get it by breathing air into our lungs where it is absorbed by the blood. Chlorate candles store oxygen for breathing emergencies. But rather than store it as a gas, oxygen in a chlorate candle is locked up in the form of a solid chemical compound, sodium chlorate, NaClO3. In that form, the oxygen is very stable and easy to store: chlorate candles may last for many years.

Chlorate Candles: Chemistry to the Rescue

Chemistry is the key to unlocking oxygen in a chlorate candle from the solid to the gaseous form. Chlorate candle ingredients are packed into a cylindrical container that resembles a large candle. Into the container go sodium chlorate mixed with iron powder. The "rescue" reaction is:

Iron powder is added because it burns at a high temperature--around 600 degrees C. Just enough heat is supplied by the burning iron to break down NaClO3 and release O2 at a steady rate. Sodium chloride, common table salt, is the other product of the rescue reaction.

How much O2 is produced from a chlorate candle? For each kilogram of the sodium chlorate-iron mixture, enough O2 is released to allow one average-sized adult, breathing normally, to breathe for about six and one-half hours. The precious oxygen released by this chemical tool can help preserve life in many emergency situations.

Diatomic Gases in Nature

Oxygen, the life-sustaining gas produced by the chlorate candle, is known as a diatomic molecule because it consists of two (designated by the prefix di) oxygen atoms bonded to one another. Atoms of hydrogen (H2), oxygen (O2), nitrogen (N2), chlorine (Cl2) and fluorine (F2) combine in pairs in nature to form simple molecules known as diatomic gases.

Diatomic gases dominate Earth's atmosphere the blanket of gases surrounding the planet. The atmosphere consists of approximately 80 percent N2 and 20 percent O2, with very small amounts of other gases. Because of the pull of gravity on these gases, the atmospheric blanket is thickest closest to the Earth's surface, and thins out as one approaches space.

Hydrogen is the lightest of all elements, and because it is lighter than air, early hot air balloons and airships were filled with flammable H2 gas. (Helium was substituted after it was found hydrogen was too risky.) Astronomers tell us that the element hydrogen probably makes up 75 percent of the mass of the universe and that great clouds of diatomic hydrogen gas in space may be areas where new stars are being generated.

Follow-up Questions:

  1. How much oxygen could be produced by a 3.5 kg chlorate candle?
  2. Draw and label the parts of Earth's atmosphere, showing the height above the Earth to scale.
  3. What are the minor gases of Earth's atmosphere and their relative abundances?


For a list of previous "Chlorine Compound of the Month" features, click here.

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