The short answer is no; oxygen is not flammable. However, it is a vital component of the combustion process. Combustion is a chemical reaction that involves a fuel and an oxidizer, resulting in the production of heat and light. Oxygen is the most common oxidizer in combustion processes on Earth.
- Oxygen is not flammable. A spark in a pure oxygen environment does not produce a fire.
- However, oxygen is an oxidizer. It supports combustion. If you have heat and fuel and introduce oxygen, you get fire.
What Makes a Substance Flammable?
A substance is flammable or combustible if it ignites easily and burns rapidly in the presence of an oxidizer, such as oxygen. Several factors determine flammability, including the substance’s ignition temperature, volatility, concentration, chemical composition, and physical state. Common flammable substances include gasoline, alcohol, and wood.
Flammable vs. Oxidizer
Understanding the difference between being flammable and being an oxidizer is crucial in the context of combustion:
- Flammable Substances: These are materials that catch fire and burn in the presence of an oxidizer. They are the fuel in a combustion reaction.
- Oxidizers: These substances, like oxygen, do not burn themselves but support the combustion of flammable materials. They cause the fuel to ignite and maintain the burning process by providing the necessary oxygen atoms for the chemical reaction.
Examples of Oxidizers and Flammable Substances
While oxygen is not the only oxidizer, it is the most familiar one. Fuels include obvious materials, like wood and propane. However, even metals become fuels under the right conditions.
- Oxidizers: Oxygen (from air), chlorine, and fluorine are common oxidizers. They facilitate the burning of fuels but do not combust independently.
- Flammable Substances: Gasoline, natural gas (mostly methane), and ethanol are examples. They ignite and burn when they come into contact with an oxidizer.
Reactions Between Oxidizers and Flammable Substances
When a flammable substance meets an oxidizer like oxygen, it can ignite, leading to combustion. For example, when gasoline vapors mix with air and are exposed to a spark, the gasoline ignites, rapidly combining with oxygen in a combustion reaction that releases energy as heat and light. The heat provides the activation energy that makes the reaction self-sustaining.
The “No Open Flames” Warning
Hospitals and other locations prohibit smoking or open flames near oxygen tanks precisely because, while oxygen itself isn’t flammable, it greatly enhances the combustion process. Here’s a brief explanation:
- Oxygen Supports Combustion: Oxygen is a key element in the fire triangle, which also includes heat and fuel. It doesn’t burn by itself, but it is an oxidizer that supports the combustion of other materials. The presence of oxygen makes it easier for materials to ignite and burn.
- Increased Risk of Fire in Oxygen-Rich Environments: In environments with a high concentration of oxygen, such as near an open oxygen tank, flammable materials ignite more easily and burn much faster and hotter than they would in normal air. This increases the risk of fire and the severity of any fire that does occur.
- Safety Precautions: Prohibiting smoking and open flames near oxygen tanks is a critical safety precaution. The presence of a heat source (like a cigarette or flame) in an oxygen-rich environment quickly leads to a fire or explosion, especially if there are any flammable materials nearby.
- Vulnerable Settings: Hospitals and similar settings often have patients who are more susceptible to harm from fires and smoke inhalation. The safety rules about oxygen tanks are even more stringent in these environments to protect these vulnerable individuals.
Combustion Without Oxygen
Interestingly, materials can burn without oxygen, in a process called anaerobic combustion. This occurs in environments where oxygen is absent, but other oxidizing agents are present. An example is combustion in chlorine gas, where a fuel burns in chlorine instead of oxygen. Another example is burning of metals like magnesium in an atmosphere of carbon dioxide (although carbon dioxide or CO2 contains oxygen). Such reactions often occur at higher temperatures and can be more vigorous than combustion in oxygen.
Then, there are reactions like fusion in stars that are not combustion, yet produce the heat and light we associate with burning.
Demonstration: Oxygen Is Not Flammable
The demonstration involving balloons filled with oxygen, hydrogen, and a mixture of oxygen and hydrogen is a classic experiment that illustrates the properties of these gases, particularly their roles in combustion. Because the demonstration involves flammable gases and a risk of explosion, it should only be conducted by experienced individuals with appropriate safety measures in place.
- Only conduct this project outdoors in a well-ventilated, open area.
- Wear safety goggles and fire-resistant gloves.
- Keep a fire extinguisher nearby.
- Ensure that all spectators are at a safe distance.
- Do not conduct this experiment in dry, windy, or fire-prone conditions.
- Three balloons
- Pure oxygen gas
- Pure hydrogen gas (make it yourself)
- A mixture of oxygen and hydrogen gas (commonly in a 2:1 ratio, hydrogen:oxygen)
- A long stick or pole
- A lit candle or a lighter attached to the end of the stick
- Safety goggles
- Fire-resistant gloves
- A clear, open outdoor area with no flammable materials nearby
- A fire extinguisher
- Hearing protection, such as earplugs
- Inflate one balloon with pure oxygen, another with pure hydrogen, and the third with a mixture of oxygen and hydrogen.
- Securely tie the balloons.
- Oxygen Balloon:
- Attach the balloon filled with oxygen to the end of a stick.
- Using another stick with a lit candle or a lighter, carefully bring the flame close to the balloon.
- The balloon pops loudly, but there is no significant flame since oxygen supports combustion but is not flammable itself.
- Hydrogen Balloon:
- Repeat the process with the balloon filled with hydrogen.
- When the flame touches the balloon, the hydrogen ignites with a loud pop and a visible flame, demonstrating that hydrogen is highly flammable.
- Hydrogen-Oxygen Mixture Balloon:
- For the balloon with the hydrogen-oxygen mixture, exercise extreme caution as this is the most dangerous part of the demonstration.
- When ignited, this balloon creates a louder explosion and a more intense flame, as the mixture of hydrogen and oxygen creates an optimal environment for a rapid and energetic combustion reaction.
Again, this demonstration involves risk. A safer option is watching someone else perform it:
- Best, Nicholas W. (2015). “Lavoisier’s ‘Reflections on Phlogiston’ I: Against Phlogiston Theory”. Foundations of Chemistry. 17 (2): 137–51. doi:10.1007/s10698-015-9220-5
- Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
- Weast, Robert (1984). CRC, Handbook of Chemistry and Physics. Boca Raton, Florida: Chemical Rubber Company Publishing. ISBN 0-8493-0464-4.
- Werley, Barry L., ed. (1991). ASTM Technical Professional training. Fire Hazards in Oxygen Systems. Philadelphia: ASTM International Subcommittee G-4.05.