Indicators and dry ice demonstration

In this short but spectacular experiment, students observe as dry ice is added to a range of indicator solutions mixed with dilute ammonia or sodium hydroxide solution. The reactions that take place produce bubbles and a ‘fog’, along with a gradual colour change.

The demonstration is a great way to illustrate neutralisation reactions, pH changes and the fact that that carbon dioxide forms weakly acidic solutions. It will be most memorable if it is done on a fairly large scale. Several different indicators could be used at the same time.

This demonstration can be used when discussing acids, alkalis, indicators, or the properties of carbon dioxide. With the appropriate audience, it could also be used to introduce a discussion of the pH changes that take place during the titrations of weak acids with strong and weak alkalis – and hence buffers.

It is also a good fun demonstration for more general audiences.

Equipment

Apparatus

• Eye protection
• Measuring cylinders, 1 dm³, as many as the number of indicators to be used (see note 7 below)
• Expanded polystyrene cool-box to store the dry ice (see note 3)
• Tongs or large spoon/scoop for transferring dry ice
• Long stirring rod
• Gloves (leather or insulated) for handling dry ice

Chemicals

• Dry ice, allow 100 g for each indicator
• 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, 0.1 M (IRRITANT)

Health, safety and technical notes

• Read our standard health and safety guidance.
• Wear eye protection throughout and use heavily insulated gloves to handle the dry ice since it can cause severe frost burns.  
• Dry ice (solid carbon dioxide), CO₂(s) – see CLEAPSS Hazcard HC020a. 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. 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. Boxes containing dry ice should not be kept in confined areas (eg small store rooms) because of the build up of carbon dioxide gas could lead to asphyxiation.
• Ammonia solution, NH₃(aq) – see CLEAPSS Hazcard HC006 and CLEAPSS Recipe Book RB006. 0.1 M ammonia solution should be adequate for this demonstration.
• Sodium hydroxide solution, NaOH(aq), (IRRITANT) – see CLEAPSS Hazcard HC091a. 
• Indicators (HIGHLY FLAMMABLE) – see CLEAPSS Hazcard HC032. 
• 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.

Procedure

1. 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.
2. Add a few drops of ammonia solution or sodium hydroxide solution to give an alkaline solution. Stir to mix the solution thoroughly.
3. 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.

Teaching notes

Carbon dioxide is a weakly acidic oxide which reacts with sodium hydroxide to produce sodium carbonate:

2NaOH + CO₂ → Na₂CO₃ + H₂O

However, in this experiment the solution of sodium hydroxide is very dilute, and the reactions involved are more complex.

Carbon dioxide dissolves in and reacts with water to produce hydrogen ions (H⁺). The acidic solution produced then reacts with and neutralises the alkali present.

Carbon dioxide dissolves reversibly in water:

CO₂(g) ⇔ CO₂(aq)

(This is the basis of the fizz you get when taking the top off a bottle of carbonated water – the CO₂ comes out of solution when the pressure is released.)

Some of the dissolved CO₂ reacts reversibly with water to form an acidic solution:

CO₂(aq) + H₂O(l) ⇔ HCO₃^–(aq) + H⁺(aq)

This acidic solution then reacts with the alkali present.

If the alkali is sodium hydroxide, the equation for the neutralisation reaction is:

HCO₃^–(aq) + H⁺(aq) + Na+(aq) + OH^–(aq) → Na⁺(aq) + HCO₃^–(aq) + H₂O(l)

If the alkali is ammonia solution, the colour change takes place more slowly because ammonia, unlike sodium hydroxide, is a weak alkali. Ammonia itself reacts reversibly with water:

NH₃(g) + H₂O(l) ⇔ NH₄⁺(aq) + OH^–(aq)

The equation for the neutralisation reaction involving ammonia is:

HCO₃^–(aq) + H⁺(aq) + NH₄⁺(aq) + OH^–(aq) → NH₄⁺(aq) + HCO₃^–(aq) + H₂O (l)

The final pH reached is about 4.5. It is best to use indicators which change colour at pH values above this, or use universal indicator.

The expected colour changes (alkali → acid) for the suggested indicators are:

• Phenolphthalein: pink → colourless (pH range: 8.2–10.0)
• Thymolphthalein: blue → colourless (pH range: 8.3–10.6)
• Thymol blue: blue → yellow (pH range: 8.0–9.6)
• Phenol red: red → yellow (pH range: 6.8–8.4)
• Bromothymol blue: blue → yellow (pH range: 6.0–7.6)

You may want to demonstrate colour changes at lower pH values. If so add a few drops of concentrated hydrochloric acid at the end.