chemistry experiments

Acids and Bases

Topic : Disappearing ink

Timing : 30 min.

Apparatus and equipment (per group)

1. 100 cm3 Beaker
2. 10 cm3 Measuring cylinder
3. Small paint brush to test the ink.

Chemicals (per group)

1. Ethanol (Highly flammable)
2. Sodium hydroxide 0.4 mol dm–3 (Irritant)
3. Thymolphthalein solution (50 per cent water, 50 per cent ethanol) (Highly flammable)

Background theory :

1. The amount of indicator can be adjusted to give a deep blue colour. The compound produced, Na2CO3, is commonly called washing soda.
2. Sodium hydroxide reacts with carbon dioxide in the air to form sodium carbonate. 2NaOH(aq) + CO2(g) -- Na2CO3(aq) +H2O(l)
3. Sodium carbonate is less basic than sodium hydroxide and causes the indicator to change from blue to colourless. The colourless range for thymolphthalein is below pH 9.3. The blue range is above pH 10.5 and the colour change takes place between these two. The alcohol evaporates and leaves a clear and colourless residue.

Procedure:

1. Place 10 cm3 of ethanol (Highly flammable) in a small beaker.

2. Add a few drops of thymolphthalein (Highly flammable) indicator solution.

3. Add just enough NaOH solution (Irritant), dropwise, to produce a deep blue colour in the solution.

4. Using a small paint brush test the ‘disappearing ink’ on a white page.

Safety : Wear eye protection.

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EESHA MOHANTY

Roll no. 3