Heather Mcdonald of Macandrew Intermediate asks :-

I lit two tea candles, one red and one uncoloured. The red one burnt longer. Why?

Kathryn McGrath, a chemist at Otago University, responded.

Did you check that your two candles were indentical in every respect but colour?

For a true comparison the candles must be identical in every other respect i.e. same mass, same shape and size, be made from exactly the same type of wax, have identical wicks and be burning at the same time and under the same conditions. If all of these things are the same then the comparison being made is what effect the colour of the candle makes.

You could ask a craft candle maker what dyes are used in candles and how much. Could you make candles identical except for different amounts of red dye? Any effect due to colour should show up here. Or you could use similar amounts of different colour dyes.

There are many other investigations you could do. It may be more fun if a whole class work in small groups on slightly different problems.

For example, candles of identical shape, mass and wick could be made from various waxes, eg beeswax and vegetable wax, to determine which burns longer or brighter. (If the latter, can you think up a way to compare them?) Remember that the Romans used animal fat so that could be included as one type of material.

Other possible investigations which come to mind include:- Does the shape of the candle affect the rate of burning? Which material is best for a wick?

Using this example we can see that by taking the time to design the initial experiment you can then change things in a controlled way allowing different scientific questions to be asked and answered.