What makes a candle burn faster?

The speed of a candle burning probably depends on many things. Just by thinking around, we could guess that it depends on the width of the wick, the rate at which the liquid wax flows into the wick (which depends on the wax viscosity and the shape of the candle), and the rate at which oxygen can be supplied to the flame. This, in turn, depends on the dynamics of the airflow around the flame, which probably depends in part on the energy density of combustion of the liquid wax.

I would guess that the scent added to candle wax influences the viscosity of the liquid wax and its energy density of combustion. I'm sure, with sensitive enough instruments, you could measure the effect of different scents on candle combustion. But would you ever notice this effect in everyday life? That depends on the relative proportion of scent to wax in a candle.

I'm guessing that it doesn't take much scent to make a candle smell. Even though are noses aren't as good as dog noses, we can still detect tiny concentrations of scent molecules in the air. Also, most scents are volatile organic molecules, and their addition in large proportions to wax might prevent it from forming a solid. So, is there enough scent in a scented candle for you to notice its effect on the burn in everyday life? Probably not.

If there are particular candles you're curious about, then you can answer this question for yourself. Do an experiment! :) Experiments are fun, and this one is also really easy. Take two candles that are identical--except for the scent-- And then put them next to each other, and light them at the same time.

You could do this experiment anywhere. You don't have to worry too much about the air in the room being still, because the candles will be next to each other, and they will experience almost the same variations in airflow. So set the candles up in your living room or kitchen. You can even put them out when you go to bed, just be sure to relight them at the same time.

Have fun!