Liquefying chlorine gas
In this demonstration the gaseous halogen, chlorine, is liquefied by passing it over a ‘cold finger’ condenser cooled using a dry ice/ethanol mixture. Yellow drops of liquid chlorine are collected for comparison with other Group 7 elements bromine and iodine. The demonstration can be extended to include changes of state for bromine and iodine.
Read our standard health & safety guidance
Lesson organisation
This is a class demonstration used to show that chlorine gas is relatively easily liquefied (boiling point -35 °C) by cooling alone. It must be done in a fume cupboard.
The yellow liquid formed can be compared with the other halogens – bromine and iodine – in the condensed state as part of a study of the trends in the physical properties of the halogens.
The demonstration can be done in 5 mins once the chlorine generator is set up and connected to the ‘cold finger’. If the freezing of bromine and the melting and vaporisation of iodine are included allow 15 mins in total.
Apparatus and chemicals
Eye protection
Chemicals required for one demonstration:
Access to a fume cupboard
Protective gloves - for handling pellets of dry ice (-78 oC). Tongs or insulating (not rubber) gloves should be used
‘Cold finger' condenser (see note 1)
Bosses, clamps and stands
Optional:
Boiling tube with cotton wool plug
Test-tube holder
Beaker (100 cm3)
Bunsen burner
Chlorine generator (Toxic, Irritant) (see note 2)
Sodium chlorate(I) solution (14% (w/v) available chlorine) (Corrosive) about 100 cm3
5M hydrochloric acid (Corrosive) about 50 cm3
Ethanol (Highly Flammable) or industrial denatured alcohol (IDA) (Highly Flammable, Harmful) about 20 cm3
A few small pellets of dry ice (solid carbon dioxide) (see note 3) (Can cause serious frostbite if handled without tongs or suitable gloves)
Optional:
About 100 cm3 crushed ice
100 g salt (sodium chloride) – crushed rocksalt will do.
Sealed ampoule of bromine (Very toxic, Corrosive, Danger to the environment)
A few crystals of iodine (Harmful, Danger to the environment)
Technical notes
Chlorine (Toxic, Danger to the environment) Refer to CLEAPSS Hazcard 22A, Recipe card 26,
Sodium chlorate(I) solution (Corrosive) Refer to CLEAPSS Hazcard 89
5M hydrochloric acid (Corrosive) Refer to CLEAPSS Hazcard 47A and Recipe card 31
Ethanol (Highly Flammable) or industrial denatured alcohol (IDA) (Highly Flammable, Harmful) Refer to CLEAPSS Hazcard 40A
Solid carbon dioxide. Refer to CLEAPSS Hazcard 20
Optional items
Bromine (Very toxic, Corrosive, Danger to the environment) Refer to CLEAPSS Hazcard 15A
Iodine (Harmful, Danger to the environment) Refer to CLEAPSS Hazcard 54A
Notes
1 The ‘cold finger’ condenser apparatus should consist of a 1 dm3 Buchner flask fitted with a two-holed rubber bung. One hole in the bung should be big enough to take a test-tube and the other hole fitted with a short length of glass delivery tubing – see diagram.
2 Chlorine generation: see Generation of gases (for chlorine scroll to bottom of page)
3 Dry ice (solid carbon dioxide) can often be obtained from a local university, hospital or industry. Larger chunks can be broken up by enclosing them in a cloth, such as a tea towel, and hitting them with a mallet. The fragments can be stored for several hours in a box made of expanded polystyrene, or in a vacuum flask. A more powdery form of dry ice can be made using carbon dioxide from a cylinder and a suitable dry ice making attachment.
Procedure
HEALTH & SAFETY: Wear eye protection and gloves
a Set up the chlorine generator in a fume cupboard. Make sure it is securely clamped.
b Connect the cold finger apparatus to the generator, using a short length of rubber tubing, and clamp it securely.
c Fill the ‘cold finger’ test-tube about two-thirds full of dry ice chips and slowly add a little ethanol. The mixture will bubble vigorously at first as the solid carbon dioxide sublimes. When the bubbling has settled down, add more ethanol until the test-tube is almost full. (In CLEAPSS instructions for making freezing mixtures, the dry ice is added to the solvent – with the quantities involved here it is unlikely to matter which way round you add them.)
d Generate a gentle stream of chlorine by dripping the hydrochloric acid slowly on to the sodium chlorate(I). The greenish-yellow gas will gradually fill both flasks.
e After about a minute, yellow drops of liquid chlorine begin to condense on the ‘cold finger’ and drop onto the bottom of the flask. At first these drops will vaporise but after a few minutes they will begin to collect as the base of the flask cools down. Continue passing chlorine gas through the apparatus until sufficient liquid chlorine has collected for the class to see. It is helpful to pre-cool the base of the flask with some dry ice or ice/salt mixture. After stopping the flow of chlorine gas, the flask containing the liquid chlorine can be disconnected from the gas generator but should not be brought out of the fume cupboard.
Teaching notes
Liquid chlorine is transported around the country in bulk in rail or road tankers. Here liquid chlorine can be compared with bromine and iodine as part of a study of the trends in physical properties of the halogens.
This demonstration can be extended to include the freezing of liquid bromine (freezing point -7 °C) and the melting and vaporisation of iodine.
Bromine in a sealed ampoule can be solidified by cooling it in a mixture of equal masses of crushed ice and salt (sodium chloride).
A few crystals of iodine gently heated in a boiling tube (containing a cotton wool plug at the mouth of the tube to prevent the escape of iodine vapour) will melt (melting point 114 °C) and then form a deep purple vapour. On cooling, iodine crystals form on the walls of the tube.
Web links
http://funwithchem.hkschoolnet.com/expt3/expt.htm#
Contains video of chlorine liquefaction and solidification (the latter using liquid nitrogen) – commentary in Chinese
Health and Safety checked, October 2007
Updated 29 Oct 2008
