The sublimation of air freshener

Pieces of solid air freshener are heated in a hot water bath 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.

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.

Equipment

Apparatus

• Eye protection
• Access to a fume cupboard
• Gloves (for those with sensitive skin)
• Beakers (100 cm³), x2
• Stand, boss and clamp
• Shallow dish
• Thermometer (-10–110 °C)
• Kettle (for hot water)

Chemicals

• Solid air freshener (HARMFUL), a few lumps

Health & Safety and Technical notes

• Read our standard health & safety guidance
• Wear eye protection and gloves. Work in a fume cupboard.
• Air freshener – solid toilet bowl cleaners work best; if possible use a coloured one. If cheap ones containing 1,4-dichlorobenzene (para-dichlorobenzene), C₆H₄Cl₂(s), are used, handle them with tongs in a fume cupboard. Para-dichlorobenzene is HARMFUL and DANGEROUS FOR THE ENVIRONMENT – see CLEAPSS Hazcard HC023. Gel-type air fresheners will not work.

Procedure

1. Wear eye protection and work in a fume cupboard. 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.
2. Stand the beaker containing the air freshener in a shallow dish.
3. Carefully, clamp the beaker containing the ice in position on top of the beaker of air freshener. See diagram.
4. One-third fill the shallow dish with warm water (hotter than 45 °C).
5. 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 – see CLEAPSS Hazcard HC054.

If naphthalene (HARMFUL, DANGEROUS FOR THE ENVIRONMENT) is used, do the activity in a fume cupboard. Naphthalene mothballs must be heated to near 70 °C to sublime – see CLEAPSS Hazcard HC046b.

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 – see CLEAPSS Hazcard HC020a.