Activity

Candle chemistry

Summary
Watch a candle burn, put it out by removing air, and discuss the chemistry of what makes it burn. Also relight a candle from above the wick.
Science content
Chemistry: States of Matter, Properties of Materials (K-7)
Chemistry: Atoms, Molecules (3-7)
Chemistry: Chemical Changes (2, 7)
Physics: Light and Sound (1)
Physics: Heat (3)
Physics: Energy forms, Conservation of Energy (1, 3, 4, 5)
Materials
  • candle in holder
  • lighter
  • vase, or other glass vessel that will fit over the candle and holder
  • optional: molecule models - one black carbon, 4 white hydrogens, 4 red oxygens and 8 grey bonds per set
Procedure

Students can draw the candle while it is burning, to notice the different colours of the flame, the liquid wax below the wick etc.

State changes in the wax:
Solid to liquid to gas. The solid wax melts with the heat of the flame, and the wick draws the liquid wax up by capillary action. Once the wax is a gas it can burn.

Chemical reaction of a burning candle:
Place a jam jar over the candle - it burns for a while then goes out.
The wax needs oxygen from the air to burn. Also note that water condensation builds up on the inside of the glass.
Place differently-sized jars over identical burning candles, to see that the candle with the larger jar will burn longer (as there is more air in it).

The molecules in the candle wax and oxygen from the air combine and rearrange, and release heat and light as they do so.
Optional: use molecule models to show a simple version of the chemical reaction: CH4 + 2O2 → CO2 + 2H2O. Called combustion. The reaction product water is visible on the glass. (Paraffin wax is actually longer chains of C and H atoms, called alkanes, with average 25 carbons. They would combine with more molecules of O2, to release the same products.)
Cover identical candles with differently-sized jars at the same time. Ask students to predict which one uses up oxygen first. (Students need to estimate the relative volumes of the jars.)

Candle trick:
Once the flame of a larger candle has been lit, it can be blown out, then relit from a height above the wick. The wax vapour still in the air ignites, which then relights the wick. The flame must be quite big for this to work.

Additional information on flames:
The flame is a mixture of hot gases, primarily CO2, water vapour, oxygen and nitrogen.
The yellow colour of the flame is due to soot particles glowing because they are hot (black body radiation). Other colours in the flame are from transient reaction intermediates during combustion such as the Methylidyne radical (CH) and Diatomic carbon (C2). These molecules are excited, then emit blue and green visible light (spectral band emission).
https://en.wikipedia.org/wiki/Flame
Try burning copper sulphate or other chemicals to make other flame colours.

For more detailed chemistry of a candle burning see the ChemMatters issue on candle chemistry: http://chicagoacs.net/statefair/CD-2008/Chemmatters/2007_12_smpissue.pdf

Notes

For a lesson on heat convection, only included covering candles with jars to show that they need air to burn.

Try the candle see saw.

Grades taught
Gr 1
Gr 2
Gr 3
Gr 4
Gr 5
Gr 6
Gr 7
Teaching site
Bayview Elementary Science Club
Elsie Roy Elementary
Fraser Elementary
Gordon Elementary Science Club
ingridscience afterschool
Oppenheimer Elementary
Tyee Elementary
Weir Elementary