Expt:Indicators and dry ice: demonstration

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Introduction

Dry ice is added to indicator solutions. Bubbles and a ‘fog’ are produced along with a gradual colour change. The experiment is a great way to demonstrate neutralisation reactions and pH changes, as well as to highlight that carbon dioxide forms weakly acidic solutions.

Apparatus and chemicals

The teacher requires:

• Eye protection
• Measuring cylinders (1 dm³) - as many as the number of indicators to be used (see note 3)
• Expanded polystyrene cool-box to store the dry ice (see note 1)
• Tongs or large spoon/scoop for transferring dry ice
• Long stirring rod
• Gloves (leather or insulated) for handling dry ice
• Dry ice – allow 100 g for each indicator (see note 2)
• Access to a range of indicator solutions. Suitable ones include:
  • Universal Indicator
  • Phenolphthalein
  • Thymolphthalein
  • Thymol blue
  • Phenol red
  • Bromothymol blue
• Dilute ammonia solution and/or dilute sodium hydroxide solution (Irritant), 0.1 mol dm^–3

Technical notes

• Dry ice (solid carbon dioxide). Refer to CLEAPSS® Hazcard 20.
• Ammonia solution. Refer to CLEAPSS® Hazcard 6 and Recipe Card 4.
• Sodium Hydroxide solution (Irritant). Refer to CLEAPSS® Hazcard 91.
• Indicators (various hazards including Highly flammable). Refer to CLEAPSS® Hazcard 32.

1. For storing the dry ice, the expanded polystyrene box in which Winchester bottles are often supplied is ideal. Never put dry ice in a sealed container.
2. The dry ice should be bought, since dry ice made from a carbon dioxide cylinder will float and be much less effective at saturating the solutions. Dry ice can be obtained from universities or other higher education institutions, hospitals, industrial firms - and from some undertakers.
3. If 1 dm³ measuring cylinders are not available, 1 dm³ ‘tall form’ beakers are suitable substitutes. The measuring cylinders or beakers should be glass rather than plastic – the colour change is much easier to see.
4. 0.1 mol dm^–3 ammonia solution should be adequate for this demonstration.

Procedure

Health & safety

Wear eye protection and use gloves to handle the dry ice since it can cause severe frost burns.

Procedure

A. For each indicator, fill a large measuring cylinder with water to the 1 dm³ mark, or a large beaker to within 5 cm of the top. Add enough indicator to give an easily visible colour.

B. Add a few drops of ammonia solution or sodium hydroxide solution to give an alkaline solution. Stir to mix the solution thoroughly.

C. Add a few lumps of dry ice. These will sink to the bottom and bubble as gaseous carbon dioxide is produced. A spectacular fog is produced at the top of the cylinder. After several minutes, the colour of the indicator will change.

Reference

This experiment has been reproduced from Practical Chemistry:

• http://www.practicalchemistry.org/experiments/indicators-and-dry-ice-demonstration,53,EX.html