Expt:Indicators and dry ice: demonstration
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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:
- Dilute ammonia solution and/or dilute sodium hydroxide solution (Irritant), 0.1 mol dm–3
- 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.
- 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.
- 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.
- 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.
- 0.1 mol dm–3 ammonia solution should be adequate for this demonstration.
Health & safety
Wear eye protection and use gloves to handle the dry ice since it can cause severe frost burns.
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.
This experiment has been reproduced from Practical Chemistry: