What if fire had no oxygen?

Think of a candle on a birthday cake. The little flame flickers and glows.

A flame needs air to burn. What if the air had no oxygen in it?

A flame looks like a thing, but it is really something happening: a chemical reaction. Encyclopaedia Britannica describes combustion, or burning, as a reaction between substances, usually including oxygen, that gives off heat and light. When a match is struck, rubbing heats its tip until the chemicals react and make more heat than can escape.

So what if a fire had no oxygen? This lab is a thought experiment. We follow the evidence about what fire needs, without lighting anything ourselves.

Combustion is a rapid reaction in which a fuel usually combines with oxygen, releasing energy as heat and light. Britannica's example is a match: friction heats the head to the temperature at which its chemicals react and release heat faster than it can escape, and they burn with a flame; that heat then sets the wood and the oxygen beside it reacting.

The question here is what happens when oxygen, one of the reaction's ingredients, is removed. It is a thought experiment built on established facts, not something to try.

Fire needs three things. It needs something to burn, like wood. It needs heat. And it needs oxygen from the air.

Our air has plenty of oxygen.

Firefighters and scientists describe what a fire needs with a simple shape: the fire triangle. Its three sides are fuel (something that can burn, like wood or dry grass), heat, and oxygen. The National Park Service puts it plainly: remove any one of the three and the fire goes out.

Oxygen is easy to forget because it is all around us. The air we breathe is about 21 percent oxygen, and the National Park Service says a fire needs at least 16 percent in the air to start. So in everyday air, there is more than enough.

The fire triangle summarizes the conditions for burning: fuel, heat and oxygen. The National Park Service notes that heat usually starts a fire, from lightning or lava in nature, and teaching materials it publishes add that once burning, a fire produces the heat it needs to keep going.

Air is about 78 percent nitrogen and 21 percent oxygen. The Park Service gives at least 16 percent oxygen as the level needed for a fire to start, so ordinary air supports burning comfortably. Fuels differ too: dead plants, with less water in them, usually burn sooner and hotter than living ones.

In a science class, a teacher can put a glass jar over a candle.

After a little while, the flame goes out! The candle needed more oxygen.

Science teachers have a famous demonstration: they put a glass jar over a burning candle in a lab. No new air can get in, and after a short time the flame goes out. This is something for teachers in a science lab, never something to try at home.

Here is the surprise: the candle does not use up all the oxygen. A study in 1967 found that a candle cannot keep burning once the air has less than about 14 percent oxygen. And when students try two candles of different heights under one jar, the tallest one goes out first.

The candle in a bell jar is a classic classroom demonstration. The Royal Society of Chemistry notes that a 1990 newspaper article offered it as a way to show oxygen being used up in a tightly sealed room, but in 1967 Richard Kempa had already shown that a candle flame cannot be supported in air with less than about 14 percent oxygen by volume. So the flame goes out while oxygen remains, because it is no longer concentrated enough.

The Royal Society of Chemistry also describes student investigations with several candles in one jar: with similar candles of different heights, the tallest goes out first, and wick thickness and wax type also matter.

Burning is a bit like a team. Fuel and oxygen join up and make heat.

No oxygen means no team. No team means no heat, and the fire stops.

In a fire, the fuel joins up with oxygen in a chemical reaction, and that reaction gives off heat and light. The heat warms more fuel, which reacts too, so a fire keeps itself going.

Take the oxygen away and the reaction can't happen. No reaction means no new heat, and the fire goes out. That is why the fire triangle works: each side is one part of the same reaction. In a world where air had no oxygen at all, the flames we know could not burn.

Combustion is self-sustaining while the heat it releases keeps raising nearby fuel and oxygen to the temperature at which they react. Cut off the oxygen, or let it fall below the level a flame needs, and the flame can no longer be supported: Kempa found that level to be about 14 percent for a candle.

Britannica notes that combustion is complicated: chemists now count any reaction in which atoms lose electrons as oxidation, and most flames have a region where reduction reactions happen instead, so the fire triangle is a simplification. This is a thought experiment: there is no model here, and every consequence for a world without oxygen is reasoning, not observation.

Firefighters know about the fire triangle. Water cools a fire. Dirt keeps the air away.

Grown-ups put smoke alarms in homes. Families practice how to get outside.

Firefighting is built on the fire triangle. The National Park Service says water from a fire engine or helicopter removes heat, water or dirt can smother a fire by cutting off its oxygen, and digging a fire line clears fuel from a fire's path. Fire extinguishers remove heat and keep oxygen from a fire. The Park Service also explains stop, drop and roll: rolling on the ground smothers a flame by keeping oxygen away.

At home, families stay safe by planning ahead. Ready.gov suggests smoke alarms inside and outside each bedroom, a fire escape plan with two ways out of every room, a meeting place outside, and practice.

Every firefighting tactic targets a side of the triangle, as the National Park Service explains: water or retardant removes heat and can cut off oxygen, soil smothers, and fire lines or carefully set fires remove fuel. Extinguishers cool fires and deprive them of oxygen.

Prevention and warning matter as much as extinguishing. The U.S. Fire Administration recommends smoke alarms in every bedroom, outside each sleeping area and on every level of a home, tested monthly. Ready.gov's guidance for kids adds an escape plan with two exits from each room, an outside meeting place, regular practice, and staying outside.