[JennyG (OP)]

Richard asks,

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

What do you think?

[evan_au]

The high temperature and ordered structure of the candle flame ensures that oxygen diffuses into the burning region from the atmosphere, and vaporised candle wax diffuses outwards from the wick. In between, they encounter high enough temperatures to ensure all of the candle wax is broken down into carbon dioxide and water vapor. Both of these chemical residues are colorless and odorless.

However, when you snuff out the flame, temperatures are (temporarily) high enough to break down the candle wax, but it does not support complete combustion, so a variety of chemical products are produced, some of which are visible as soot, and some of which smell.
See: https://en.wikipedia.org/wiki/Soot

[Petrochemicals]

The fuel becomes vapourised until the wick cools beneath its vapourisation temperature. Just like if you put a camp fire out you get smoke, put a candle out you get smoke.

[evan_au]

As a sidenote:
In a candle flame on Earth, Gravity+Convection plays an important role in taking away the hot products of combustion above, and feeding in cool oxygen from below, vaporising the wax and sustaining the flame.

However, in microgravity on the ISS, heavy fuel molecules must passively diffuse away from a wick, while light oxygen must passively diffuse from outside, producing a sphere of flame far from the wick. Deprived of heat from the flame, the wax stops vaporising, and the products of combustion smother the flame...

A recent experiment on ISS showed a new form of invisible "cool" combustion.... cool!

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