Gases give rise to particular hazards so great care must be taken when preparing, collecting or testing.

How the gas is to be used will differ from experiment to experiment – it is essential to read carefully the specific instructions given or referred to in the experiment details. This is especially important if the gas needs to be dried.

Gases can be collected by upward or downward delivery or over water. Refer to specific information on each gas.

Read our standard health & safety guidance

Refer to CLEAPSS Laboratory Handbook, section 13.2.2.

Gas preparation (general)

The diagram below shows a typical set of apparatus which can be used to prepare a range of gases.

Gas collection methods


Gas Preparation (specific gases)

Wear appropriate eye protection. The amounts given below are sufficient to generate 1 litre (1 dm3) of each of the named gases.

Carbon dioxide, CO2

42 cm3 of 2 M hydrochloric acid (IRRITANT) is slowly added to an excess of marble chips. Collect gas by downward delivery or over water (slightly soluble).

Refer to CLEAPSS Hazcards and CLEAPSS Recipe Book.  

Hydrogen, H2

28 cm3 of 3 M hydrochloric acid (CORROSIVE) is slowly added to excess zinc granules and 1 g of hydrated copper sulfate (HARMFUL). Collect gas by upward delivery or over water.

Refer to CLEAPSS Hazcards and CLEAPSS Recipe Book.

Hydrogen gas is EXTREMELY FLAMMABLE – ensure there are no naked flames.

Oxygen, O2

50 cm3 of 20 vol hydrogen peroxide (IRRITANT) is slowly added to manganese(IV) oxide powder (HARMFUL). Collect gas over water.

Refer to CLEAPSS Hazcards and CLEAPSS Recipe Book


Chlorine, Cl2

Work in a fume cupboard.

Method 1:

14 cm3 of concentrated hydrochloric acid (CORROSIVE) is added to at least 3 g of potassium manganate(VII) (OXIDISING, HARMFUL and DANGEROUS FOR THE ENVIRONMENT).

Double-check that the acid is hydrochloric and NOT sulfuric. Refer to CLEAPSS Hazcards and CLEAPSS Recipe Book.

Method 2(safer and recommended but slower) :

5 M hydrochloric acid (IRRITANT) is added to 30 cm3 of recently purchased (10–14% available chlorine) sodium chlorate(I) solution (CORROSIVE) with plenty of stirring. Note that sodium chlorate(I) is only available as a solution often called ‘sodium hypochlorite’; it must not be confused with sodium chlorate(V) (sometimes just called ‘sodium chlorate’), which is a white, crystalline solid. School samples often react too slowly because old sodium chlorate(I) is used. This will have less than the required 10% available chlorine (as it applies to both methods). Refer to CLEAPSS Hazcards and CLEAPSS Recipe Book.  


Collect gas by downward delivery. Chlorine is classified as TOXIC, IRRITANT and DANGEROUS FOR THE ENVIRONMENT.

Health and safety checked August 2016


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 September 2016