This chapter classifies everyday substances as acidic, basic, or neutral, and shows how indicators reveal their nature. Litmus, a natural dye obtained from lichens, is the central acid-base indicator: substances that turn blue litmus red are acidic, those that turn red litmus blue are basic, and those that change neither are neutral. Through activities, students test common samples such as lemon juice, amla, tamarind, and vinegar (acidic), and soap, baking soda, lime water, and washing powder solutions (basic), while tap water, sugar, and salt solutions stay neutral. Acidic substances tend to taste sour and contain acids such as citric, tartaric, ascorbic, and oxalic acid; basic substances feel slippery and generally taste bitter. The chapter shows how to prepare other natural indicators: a red rose extract turns red in acid and green in base, while turmeric paper turns red in basic solutions but cannot tell acids from neutrals. Some substances, called olfactory indicators (like onion strips), change odour in acidic or basic media. A key idea is neutralisation: when an acid and a base are mixed in sufficient quantity, the result is neither acidic nor basic, producing salt and water along with the release of heat (Acid + Base gives Salt + Water + Heat). Everyday uses of neutralisation include treating ant bites with baking soda, correcting acidic soil with lime and basic soil with organic matter, and treating factory waste before it is released into water bodies.
Acids, bases, and neutral substancesLitmus as an indicatorNatural indicators (rose, turmeric)Olfactory indicatorsNeutralisation in daily life
Key terms
Acid-base indicator
A substance that shows different colours in acidic and basic solutions, such as litmus, red rose extract, or turmeric.
Litmus
A natural dye obtained from lichens, available as blue and red paper; acids turn it red and bases turn it blue.
Acidic substance
A substance that turns blue litmus red and generally tastes sour, such as lemon juice, amla, or vinegar.
Basic substance
A substance that turns red litmus blue, feels slippery, and generally tastes bitter, such as soap or baking soda solution.
Neutral substance
A substance that is neither acidic nor basic and does not change the colour of litmus, such as sugar or salt solution.
Neutralisation
The reaction in which an acid and a base react to form salt and water with the release of heat.
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From where is litmus, a common acid-base indicator, obtained?
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