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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 (2016 curriculum): 
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)
Science topic (2005 curriculum connection): 
Physical Science: Light and Sound (grade 4)
Physical Science: Chemistry (grade 7)
Lessons activity is in: 
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 draw the candle while it is burning. Help them notice the different colours of the flame, the liquid wax below the wick.

Talk about what is happening to the wax. Solid -> liquid -> 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.
(Older students: a candle is paraffin wax, a mixture of alkanes (hydrocarbon) with average 25 carbons).

What does the candle need to burn? Show what happens when a vase is put over the candle to help students think about it: the candle 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.
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. (Wax molecules are actually longer chains of C and H atoms, and would combine with more molecules of O2, to release the same products.)

Once the flame has been lit, it can be relit, and blown out 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

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: 

Try the candle see saw.

Grades tested: 
Gr 1
Gr 2
Gr 3
Gr 4
Gr 5
Teacher: 
Ingrid
Teaching site: 
Bayview Elementary Science Club
General Gordon Elementary Science Club
ingridscience afterschool
Activity originally developed and delivered: 

Gordon Elementary Science Club