Liquefy gaseous chlorine by passing it over a ‘cold finger’ condenser using a dry ice/ethanol mixture and compare it with bromine and iodine in their condensed state as part of a study of the trends in the physical properties of the halogens.

Demonstration

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 17 elements bromine and iodine. The demonstration can be extended to include changes of state for bromine and iodine.

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 Chemicals

Eye protection

Access to a fume cupboard

Protective gloves - for handling pellets of dry ice (-78 oC). Tongs or insulating (not rubber) gloves should be used

Chlorine generator (Note 1)

‘Cold finger’ condenser (Note 2)

Bosses, clamps and stands

Optional:

Boiling tube with cotton wool plug

Test-tube holder

Beaker (100 cm3)

Bunsen burner

Sodium chlorate(I) solution, 14% (w/v) available chlorine (CORROSIVE) about 100 cm3 (also known as Sodium hypochlorite)

Hydrochloric acid, 5M (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) (Can cause serious frostbite if handled without tongs or suitable gloves)

Optional:

Crushed ice - about 100 cm3

Sodium chloride – crushed rocksalt will do, about 100 g .

Bromine liquid (VERY TOXIC, CORROSIVE, DANGEROUS FOR THE ENVIRONMENT), use a sealed ampoule

Iodine (HARMFUL, DANGEROUS FOR THE ENVIRONMENT), a few crystals

Refer to Health & Safety and Technical notes section below for additional information.

Health & Safety and Technical notes

Read our standard health & safety guidance

Wear appropriate eye protection and gloves. 

Sodium chlorate(I) solution (also known as Sodium hypochlorite), NaClO(aq), (CORROSIVE at this concentration) - see CLEAPSS Hazcard.

Hydrochloric acid, HCl(aq), (CORROSIVE at this concentration) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.

Chlorine, Cl2 (g) and (l), (CORROSIVE, TOXIC, DANGEROUS FOR THE ENVIRONMENT) - see CLEAPSS Hazcard and CLEAPSS Recipe Book.

Ethanol, C2H5OH (l), (HIGHLY FLAMMABLE) or industrial denatured alcohol (IDA) (HIGHLY FLAMMABLE, HARMFUL) - see CLEAPSS Hazcard.

Solid carbon dioxide - see CLEAPSS Hazcard. 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 wide-mouth vacuum flask which was unstoppered (unless it is a vented flask specifically designed for cryogenic work). A more powdery form of dry ice can be made using carbon dioxide from a cylinder and a suitable dry ice making attachment. Tongs or insulating (not rubber) gloves should be used for handling dry ice.

Optional items

Bromine, Br2(l), (VERY TOXIC, CORROSIVE, DANGEROUS FOR THE ENVIRONMENT) - see CLEAPSS Hazcard.

Iodine, I2(s), (HARMFUL, DANGEROUS FOR THE ENVIRONMENT) - see CLEAPSS Hazcard.

1 Chlorine generation: see Generation, collecting and testing gases (for chlorine scroll to bottom of page).

2 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.

Procedure

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.

Health & Safety checked, 2016

Credits

This Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.

© Nuffield Foundation and the Royal Society of Chemistry

Page last updated October 2015