Hoshi squeezed a lemon into his tea and it tasted sharp and sour. Then he added baking soda to neutralise it — and it fizzed. Two everyday substances, completely opposite in nature. One is an acid, one is a base. Understanding acids, bases, and salts is one of the most practical chapters in Class 7 Science — everything from your food to your stomach to your soap connects to it.
Acids are substances that taste sour and turn blue litmus red.
The word "acid" comes from the Latin word acidus, meaning sour. You've tasted acids without knowing it — the sourness of a lemon, tamarind, or curd comes from the acids present in them.
Bases are substances that taste bitter, feel soapy or slippery, and turn red litmus blue.
Bases are the opposite of acids. The bitterness of bitter gourd (karela), the slippery feel of soap, the feel of baking soda — all of these are characteristics of bases.
An indicator is a substance that changes colour in the presence of an acid or a base, helping us identify whether a substance is acidic or basic. Indicators are tools scientists use to test substances without tasting them.
Litmus is a natural dye extracted from lichens. It is the most commonly used indicator in Class 7 Science.
Acid + Base → Salt + Water
When an acid and a base are mixed together, they react and cancel each other out. The acid becomes less acidic, the base becomes less basic, and the resulting solution is neutral (or close to neutral). This reaction is called neutralisation.
The products formed are: Salt and Water.
Example: Hydrochloric acid (HCl) + Sodium hydroxide (NaOH) → Sodium chloride (NaCl) + Water (H₂O)
Salts are the products of a neutralisation reaction between an acid and a base. They are ionic compounds that are generally neutral — though some can be slightly acidic or basic.
Acids are sour substances that turn blue litmus red. Bases are bitter, soapy substances that turn red litmus blue. Salts are formed when acids react with bases in a neutralisation reaction (e.g. common salt — NaCl).
An indicator is a substance that changes colour in the presence of an acid or a base. It is used to test whether a substance is acidic or basic.
Neutralisation is the reaction between an acid and a base that produces salt and water. The acid and base cancel each other out, making the solution neutral.
Toothpaste contains mild bases that neutralise the acids produced by bacteria in your mouth. Bases taste bitter — which is why toothpaste has that characteristic flavour.
When an ant stings, it injects formic acid (methanoic acid) into the skin, which causes the burning sensation. To relieve it, apply a mild base like baking soda solution to neutralise the acid.