Acid or base? The colour tells it all

Indrani Das Sen and Swapna Narvekar

In chemistry, demonstrations can help trigger discussions on topics to be taught or on topics being taught. Students, particularly beginners, enjoy watching demonstrations and therefore such activities help develop their interest in chemistry. The demonstrations need to be planned appropriately and it is important they go beyond entertainment. Often, the demonstration should help students ask questions and thus learn the basic principles or concepts. In this article, we are presenting demonstrations related to acid-base chemistry which is one of the first topics introduced in chemistry textbooks. All these demonstrations have been tested and performed in front of different age groups and have been appreciated by the viewers. Our observations indicate that these demonstrations have succeeded as they are simple, colourful, technically less demanding and make use of easily available materials. Thus, we feel that these demonstrations are doable in any classroom and without difficulties.


1. Which substances are more acidic or basic?
Principle:
Like litmus paper, the universal indicator is another indicator that exhibits different colours at different pH levels. Thus, one is able to categorize the substances from highly acidic to highly basic. The colour strip exhibiting the colour as function of pH is given on the bottle of the indicator, serving as a reference.

Substances: Household ammonia (NH3), baking soda (sodium bicarbonate, NaHCO3), washing soda (sodium carbonate, Na2CO3), lemon juice (citric acid, C6H8O7), vinegar (acetic acid, CH3COOH), cream of tartar (Potassium bitartrate, KHC4H4O6), antacids (e.g. containing calcium carbonate, calcium hydroxide, magnesium hydroxide), muriatic acid or masonry’s cleaner (hydrochloric acid, HCl), soap solution (containing potassium hydroxide, KOH or sodium hydroxide, NaOH), shampoos (you can add to the list), universal indicator solution.

Apparatus: Test tubes

Procedure: You need to take small amounts of the substance (1-2 mL) solutions in different test tubes and add a few drops of the universal indicator. Observe the colour and arrange the solution from highly acidic to basic.

The authors work in the chemistry cell of Homi Bhabha Centre for Science Education (HBCSE). Their area of work includes the selection and training of students for the International Chemistry Olympiad held each year in different countries. They also conduct the NIUS (National Initiative on Undergraduate Sciences) in chemistry and Resource Generation Camps for chemistry teachers and also exposure camps in chemistry olympiads. They can be reached at indrani21105@gmail.com and narvekar0372@gmail.com.

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