Why do candles smoke and smell?

Why does a candle start to make more smoke and smell after it's blown out?
04 February 2019
Presented by Jenny Gracie
Production by Jenny Gracie.


The image shows smoke rising from a candle.


Richard was wondering why do candles make more smoke and smell after they've gone out. Jenny Gracie has been sniffing out an answer from Duncan Graham at the University of Strathclyde, and Ricky Carvel from the University of Edinburgh to help shed some light on the question...

In this episode

A lit candle

Why does a candle smoke after it's blown out?

Jenny Gracie has been shedding some light on the problem...

Richard - Why does a candle start to make more smoke and smell when it's blown out?

Jenny - To shed some light on this question I spoke with Duncan Graham a professor of chemistry at the University of Strathclyde, to, first of all, find out how a candle actually works.

Duncan - A candle looks simple enough: a block of wax with the wick, often a piece of string running through the centre. But you need three things to start a fire: fuel, oxygen, and a heat source.

Jenny - I also caught up with Ricky Carvel who teaches combustion and fire dynamics at the University of Edinburgh. He had this to add to how the flame keeps alight.

Ricky - When a candle is burning, the energy from the flames heats up the wax and, while the wick does burn, it's actually the wax that fuels the flame and keeps burning. The melted wax flows towards the wick and is drawn up into the flame. Inside the flame, it's about 800 to 1000 degrees C and the high heat breaks up the big wax molecules into smaller chunks. This process is called pyrolysis. Most of these chunks burn inside the flame and are turned into carbon dioxide and water vapour which are both invisible and have no smell.

Jenny - This process can also be called combustion. Duncan went on to explain it further.

Duncan - If the combustion of the wax is 100% complete, then only light, heat, water vapour, and carbon dioxide will be produced. However, we've all seen the sooty marks that can be left on the edge of a candle holder. On a larger scale, the black marks seen at the back of a fireplace heading up the chimney. This is due to incomplete combustion of the wax, producing small carbon particles which we call soot.

Jenny - So that explains why we see smoke. But what about the smell?

Ricky - The smell you get from a burning candle is due to the tiny proportion of pyrolysis products that didn't burn properly in the flame. When a candle was blown out, the flame stops immediately but the wick and the wax are both still hot, so pyrolysis continues for a few seconds. The solid particles and smelly gases are produced for a moment, but with no flame to burn them, they rise like smoke. This is what we see coming from the wick and this is why the smoke smells more with no flame.

Duncan - It's the same process for burning any type of fuel. By this logic and on a bigger scale if we want a fuel source such as coal to burn more effectively, then we can carefully blow air onto it to increase the amount of oxygen. This, in turn, increases the efficiency of combustion producing a cleaner flame and less smoke.

Jenny - Thanks to Duncan and Ricky for helping us shed some light on that question. Next week we have this question from Leah.

Leah - So why are some people good at imitating accents and doing impressions while others simply aren't? Obviously, you can get better with practice, but why are some people born with the skill of imitation and some aren't?


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