Key Points

Substances are classified as acidic, basic, or neutral based on their chemical properties. Acids generally taste sour, while bases are bitter and feel soapy.

Indicators are special substances that change their color when added to an acidic or basic solution. They are used to test the nature of other substances.

Litmus is a natural indicator extracted from lichens. It is available as a solution or in the form of blue and red litmus paper strips.

Acidic substances turn blue litmus paper to red. Examples include lemon juice, vinegar, and tamarind water.

Basic substances turn red litmus paper to blue. Examples include soap solution, baking soda solution, and lime water.

Substances that do not change the color of either blue or red litmus paper are called neutral substances. Tap water, sugar solution, and salt solution are examples.

Turmeric is a natural indicator that turns red in basic solutions. It shows no color change in acidic or neutral solutions.

The extract from red rose petals can be used as an indicator. It turns to a shade of red in acidic solutions and green in basic solutions.

Some substances are called olfactory indicators because their odor changes in acidic or basic media. Finely chopped onion is a common example.

The reaction between an acid and a base is known as a neutralisation reaction. In this process, salt and water are produced with the evolution of heat.

The general word equation for the reaction is: Acid + Base $\rightarrow$ Salt + Water + Heat. The resulting products are often neutral.

An ant bite injects formic acid into the skin. The stinging effect can be neutralised by rubbing a mild base like moist baking soda on the affected area.

Excessively acidic soil is treated with bases like lime to neutralise it for better plant growth. Basic soil is treated with organic matter which releases acids.

The waste from many factories contains acids. It is neutralised by adding basic substances before being released into water bodies to protect aquatic life.