4. Acid Bases & Salts
An indicator may be defined as follows. An acid-base indicator is defined as a substance that assumes different colours in acidic, basic and neutral solutions. Litmus, methyl orange and phenolphthalein are some of the most commonly used acid-base indicators that change colour as follows. Litmus :
: It is a natural dye made from small plants called lichens. Blue and red litmus solutions are prepared from two different varieties of lichens. Litmus paper:
Blue or red litmus paper is prepared by dipping a strip of filter paper in blue or red litmus solutions. The paper is then removed from the solution and dried.
Blue litmus paper turns red in an acidic solution and red litmus paper blue in a basic solution. Phenolphthalein:
It is a colourless compound. An alcoholic solution of phenolphthalein is used as an indicator. It is colourless in an acidic solution, but becomes pink (red) in basic solution: Methyl orange :
A very small amount of solid methyl orange is dissolved in hot water and filtered. The filtrate is used as an indicator. It turns red in acid solutions and yellow in basic solutions.