TeacherExpt:Colourful electrolysis

From Learn Chemistry Wiki

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 cm³)
• Spatula
• Stirring rod
• Access to:

• Sodium chloride (salt)
• Universal indicator (Highly flammable)
• Distilled water

Procedure

Health & Safety

Wear eye protection.

Procedure

A. Before the demonstration, assemble the apparatus as shown in first diagram. Ensure that the connections to the boiling tubes are the correct way round.

B. Place a piece of blue cobalt chloride paper into the first boiling tube and half-fill the second boiling-tube with limewater.

C. At the start of the demonstration, turn on the pump so that a gentle stream of air is drawn through the apparatus.

D. Light the candle and leave for a few minutes until the cobalt chloride paper turns pink (from blue) and the limewater goes milky. This indicates the presence of water and carbon dioxide respectively.

Teaching notes

Some students will know that air contains both water vapour and carbon dioxide. To show that the changes observed are not due to these alone, repeat the experiment without the candle and note how much longer it takes for any changes to be observed.

Understanding the process of burning is important at all levels of chemistry. Emphasise that burning in air is a reaction with oxygen. The elements hydrogen and carbon are present in hydrocarbons, such as candle wax. Students will quite readily appreciate that carbon reacts with oxygen to form carbon dioxide, but often need help to grasp that hydrogen combines with oxygen to form water.

The production of carbon dioxide could lead to discussion of the role of this gas in the greenhouse effect.

The experiment could be extended to burning alcohols with a spirit burner.

Reference

This experiment has been reproduced from Practical Chemistry: http://www.nuffieldfoundation.org/practical-chemistry/colourful-electrolysis

Standard procedures

• ¹ Filter pumps (http://www.practicalchemistry.org/standard-techniques/filtering,51,AR.html)
• ² Make your own cobalt chloride paper (http://www.practicalchemistry.org/standardtechniques/preparing-and-using-cobalt-chloride-indicator-papers,43,AR.html)