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Colourful electrolysis^54
An interesting introduction to the electrolysis of brine (sodium chloride solution). Students use Universal Indicator to help them follow what is happening during the reaction.
Lesson organisation
This experiment works well if students are directed to make detailed observations and then attempt to explain for themselves what they think is happening.
The main issue is likely to be the availability of sufficient U-shaped test tubes.
Apparatus and chemicals
Eye protection
Each working group will require:
U-shaped test tube Clamp and clamp stand Carbon electrodes and electrode holders, 2 of each Electrical leads, 2 Power pack Beaker (100 cm^3 ) Spatula Stirring rod
Students will need access to:
Sodium chloride (salt) Universal indicator (Highly flammable) Distilled water
Technical notes
Hydrogen (Highly flammable) Refer to CLEAPSS® Hazcard 48 Chlorine (Toxic, Dangerous for the environment) Refer to CLEAPSS® Hazcard 22A and 47B. Sodium hydroxide (Corrosive) Refer to CLEAPSS® Hazcard 91 Universal indicator solution (Highly flammable) Refer to CLEAPSS® Hazcard 32 and Recipe card 36
If electrode holders are not available, another suitable means of securing the electrodes could be used. Do not use bungs because the products are gases.
If distilled water is a problem, then tap water could be used. But it may affect the colours produced, especially in areas with hard water.
Procedure
HEALTH & SAFETY: The products produced by this reaction are all more hazardous than the reactants. Hydrogen is Extremely flammable, chlorine is Toxic and Dangerous for the environment, and sodium hydroxide is Corrosive. Ensure that the current is turned off a soon as a trace of chlorine is detected. Chlorine (Toxic, Dangerous for the environment) can be a problem for asthmatic pupils. If the directions in the procedure notes are followed then very little chlorine is produced. Sodium hydroxide is Corrosive. Ensure that students wear eye protection, especially when they are clearing up the experiment. a Put about 75 cm 3 distilled water into the beaker. Add about 2 heaped spatulas of sodium chloride. b Stir until the salt dissolves. Then add several drops of Universal Indicator solution. Stir to mix thoroughly. You need enough indicator to give the water a reasonable depth of green colour. c Pour coloured salt solution into the U-shaped test tube and clamp it as shown in the diagram.
power supply set to 10V
electrical leads with crocodile clips
graphite electrode
clamp
u-tube solution of salt and Universal indicator
d Wash the carbon electrodes carefully in distilled water and then fix them so that there is about 3 cm of electrode in each side of the U-tube – see diagram. This is most easily done using electrode holders. e Attach leads and connect to a power pack set to 10 V. f Turn on the power pack and observe closely what happens. A piece of white paper held behind the U-tube can help. Make sure the U-tube is kept very still during the experiment. g Turn off the power as soon as you notice any change at the positive electrode, or when you smell a ‘bleachy, swimming pool’ smell. This will probably take less than 5 minutes.
