Flames go out when their oxygen supply is cut off, so what is happening when you blow a candle out?
Jonathan Wallace
Newcastle upon Tyne, UK
Advertisement
For a flame to be sustained, three things need to be present: fuel, oxygen (or an alternative oxidising agent) and heat. This is known as the triangle of combustion, and if any of the three sides of this triangle are removed, the fire will be extinguished.
Blowing on a candle extinguishes it because the sudden rush of air provided is sufficient to rapidly cool the point of combustion. In addition, vaporised wax is blown away from the point of combustion, removing fuel. The combined effect of knocking out both heat and fuel means that the flame will go out despite the excess of oxygen available.
Pat French
Telford, Shropshire, UK
To blow a candle out, your breath has to be sudden so that the blast of air carries the hot, burning gas away from the fuel-soaked wick. This also cools any remaining fuel and gases below the ignition temperature so that combustion stops and the candle goes out. Jet engines and explosives have the same effect on oil well fires.
Trick candles are made with heat-retaining or heat-generating substances added to the wax so that it stays hot and can’t easily be blown out.
Paul Whiteley
Bittaford, Devon, UK
A candle wick isn’t actually burning. Although it chars, it is acting as a reservoir of liquid fuel that is drawn up by capillary action and heat, which vaporises the liquid wax. The vapour then ignites due to the heat given off by the flame.
You can see this by bringing a lighted match close to the wick of a recently extinguished candle. The flame leaps across the gap to ignite the vapour. Sometimes a candle might reignite of its own accord after it has been blown out because there is sufficient heat still in the wick to vaporise the wax and take it beyond its flash point.
Some fire brigades use giant fans to extinguish flames and remove flammable vapour from burning buildings.
Ben Crossley
Wigston, Leicestershire, UK
Although exhaled breath contains some carbon dioxide, there is still plenty of oxygen to sustain combustion. That is why blowing gently on glowing kindling will encourage it to burn bright.
I recommend the 1861 book The Chemical History of a Candle by Michael Faraday. I also suggest wetting your fingers and pinching out candles, as blowing them out will make the room stink of wax.
To answer this question – or ask a new one – email lastword@newscientist.com.
Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise. We reserve the right to edit items for clarity and style. Please include a postal address, daytime telephone number and email address.
91É«ÇéƬ retains total editorial control over the published content and reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format.