Indicators

Defination of Indicators

Special type of substances are used to test whether a substance is acidic or basic. These substances are known as indicators. The indicators change their colour when added to a solution containing an acidic or a basic substance. Turmeric, litmus, china rose petals (Gudhal), etc., are some of the naturally occurring indicators.

Common acid base indicators are

1. Litmus: Litmus is a purple dye that is extracted from ' lichen' a plant belonging to the variety Thallophyta. It can also be applied on paper in the form of strips and is available as blue and red stripes. A blue litmus strip, when dipped in an acid solution acquires red color. Similarly, a red strip when dipped in a base solution becomes blue.
2. Phenolphthalein: It is also an organic dye and acidic. In a neutral or acidic solution; it remains colorless while in the basic solution, the color of the indicator changes to pink.
3. Methyl Orange: Methyl orange is an orange-colored dye(yellow) and is basic. In the acidic medium, the color of the indicator becomes red.
4. Red cabbage juice:It is yellow in colour and remains as such in the neutral and acidic medium. In the basic medium its colour becomes reddish or deep brown.

Activity of Indicators

Mix some water with lemon juice in a plastic cup/tumbler/test tube. Then Put a drop of the above solution on a strip of the red litmus paper with the help of a dropper. Is there any change in color? Repeat the same exercise with the blue litmus paper. Note down if there is any change in color. Perform the same activity with the following substances: Tap water, detergent solution, aerated drink, soap solution, shampoo, common salt solution, sugar solution, vinegar, baking soda solution, milk of magnesia, washing soda solution, lime water. If possible make solutions in distilled water. Record your observations and make a Table of your result, than check are there any substances on which litmus had no effect? Name those substances. The solutions which do not change the color of either red or blue litmus are known as neutral solutions. These substances are neither acidic nor basic.

Questions on Indicators

1. Question The substances are used to test whether a substance is acidic or basic are known as ……….
2. Question........... is yellow in colour and remains as such in the neutral and acidic medium.
3. Question........ is an orange coloured dye(yellow) and basic in nature.

Answers

1. 1.Indicators
2. Red cabbage juice
3. Methyl Orange