The sublimation of air-freshener
Pieces of solid air freshener are heated in a hot waterbath and the vapour caught by cooling with ice. No liquid will be observed so students will be able to appreciate that a solid has turned directly to a gas without passing through the liquid phase. A fume cupboard, or other method of preventing escape to the air, is required for this experiment.
Read our standard health & safety guidance
Lesson Organisation
This experiment is best done as demonstration. As it can take several minutes for anything to happen, it would be advisable to have another activity for students while they wait.
Most of the substances in the air fresheners are harmful. This is not a problem in day-to-day use as the vapour pressure and hence the amount which is in the air is low. However, heating them causes them to sublime quickly and they could reach harmful levels in the air so a fume cupboard or other method of preventing escape to the air is necessary.
Apparatus and Chemicals
Eye protection
Gloves (for those with sensitive skin)
Beakers, 100 cm3, 2
Clamp and stand
Shallow dish
Thermometer
Access to:
Fume cupboard
Hot water
Solid air freshener, few lumps (see note 1)
Technical Notes
p-dichlorobenzene (Harmful, Dangerous for the environment) Refer to CLEAPSS Hazcard 23
1 Solid toilet bowl cleaners work best; if possible use a coloured one. If cheap ones containing p-dichlorobenzene (see below) are used, handle them with tongs in a fume cupboard. Gel-type air fresheners will not work.
Procedure
a Wear eye protection. Place a few lumps of air freshener in the bottom of one of the 100 cm3 beakers. Fill the other beaker three quarters full of ice.
b Clamp the beaker with ice carefully in position on top of the beaker of air freshener.
c Put the shallow dish or pan into the fume cupboard and fill it about one-third full of water which is hotter than 45 °C.
d Put the stacked beakers into the dish – see diagram.
e Observe what happens to the solid. Be patient as it may take a while.
Teaching notes
Sublimation is the vaporisation of a solid. The opposite process; the formation of a
solid directly from a vapour, is called deposition. The heat from the water bath causes
the solid air freshener to sublime. The cold beaker causes the vaporised air freshener to
re-form the solid.
If possible use a coloured air freshener and notice that the material that collects on the cold beaker is white. The dye does not sublime because it is not chemically a part of the compound that does sublime. Vapour deposition is an important industrial process for separation and purification.
It is possible to use other materials that sublime including iodine, naphthalene and dry ice (carbon dioxide).
• If iodine (Harmful, Dangerous for the environment) is used, use only a few crystals and do the activity in a fume cupboard. Refer to CLEAPSS Hazcard 54A.
• Naphthalene (Harmful, Dangerous for the environment) mothballs must be heated to near 70 °C to sublime. Refer to CLEAPSS Hazcard 46.
• Dry ice sublimes at -78.5°C and above. Handle with tongs or thermal gloves. You would not be able to watch this re-form the solid but it is great for observing the change from solid to gas. Refer to CLEAPSS Hazcard 20.
Health & Safety checked, April 2008
Updated 8 Apr 2008
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