What is the smallest amount that you can smell?

Explore how our olfactory organs work with this smelly practical. 

Equipment

Materials per group

Nice smelling substances – e.g.

• Muguet
• Honeysuckle
• Jasmin perfumes (all mixtures)
• or Ethoxyethane (diethyl ether)
• Ethanol

Equipment per group

• Measuring cylinders, 10 and 25 cm³ 
• Dropping pipettes
• Volumetric flasks, 10 cm³ 
• Pipettes, 1 cm³
• Watch glasses
• Access to a balance
• Safety glasses

Health, safety and technical notes

• Read our standard health and safety guidance here.
• Wear eye protection.
• This is an open-ended problem-solving activity, so the guidance given here is necessarily incomplete.
• Keep away from sources of ignition.
• Ethoxyethane (diethyl ether) is extremely flammable, a respiratory irritant and harmful if swallowed. See CLEAPSS Hazcard HC042.
• Ethanol is highly flammable. See CLEAPSS Hazcard HC040a.

Commentary

There are several different ways of tackling this problem. Each will give a different volume that is the smallest that can be smelt.

This volume is also dependent on the individual performing the experiment. In addition, care is needed because continuously smelling the same substance diminishes the sense of smell. If micropipettes are available, they can be used directly to give the smallest volume that can be smelt. A known amount of the perfume can be dissolved in ethanol, and then diluted until it is just not possible to smell it.

Fresh ethanol must be used to ensure an odourless solvent. This is good practice in performing close sequential dilutions. A known volume of the perfume can be placed on a preweighed watch glass in the centre of an enclosed room of known volume.

When the perfume can be smelt throughout the room (by students already in position in order to minimise air turbulence) then the change in mass of the perfume and the volume of the room can be used to calculate the concentration of perfume vapour. 

Extension

Pure fragrances may be used. An example is cis-3-hexen-1-ol which smells of cut grass.

It is insoluble in water, but can be dispersed by using an odourless detergent such as ‘Tween 40’. From data on the vapour pressure of water and the vapour pressure of pure fragrances, it is possible to estimate the concentration of fragrance molecules in the air above the aqueous solution.

During trialling, it was found that different people have different sensitivities to odours. The differences between people can be a factor of 10 or greater.

Notes

This resource is part of a collection of problem-solving activities, designed to engage learners in small group work. Find out how to use these resources, and obtain a list of suggested ‘junk items’ here.