Ryan Dam, of Woodend School, asks :-

We know the speed of light and the speed of sound but what is the speed of smell?

Bryce Williamson, a physical chemist at the University of Canterbury, responded.

The 'speed of smell' is the speed with which 'scent molecules' diffuse through air. This depends on a number of things, as can be demostrated by the following analogy.

A hundred blindfolded children are set lose in a hall to represent air molecules. They mill around a hall, obeying rules that they travel in straight lines, but change direction after collisions. Soon these 'molecules' become reasonably evenly distributed through the hall.

A few blindfolded classmates represent scent molecules. Starting at one end of the hall, they diffuse into the throng of 'air molecules'. The 'speed of smell' can be determined from the average time taken for a 'scent molecules to reach the far end of the hall, which represents someone's nose. Clearly, this speed is much less than the straight-line speed between collisions. Real air and scent molecules travel at hundreds of kilometers per hour but the distance between collisions is only about a thousandth of a millimeter, so they diffuse quite slowly.

With this analogy it is easy to see that the 'speed of smell' depends on such things as the density and temperature (average speed between collisions) of air. It also depends on the size and weight of scent molecules - large, heavy molecules travel slower and undergo more collisions, so diffuse more slowly than small light ones.