Curd, lemon juice, orange juice and vinegar taste sour. These substances taste sour because they contain ACIDS. The chemical nature of such substances is acidic. The word acid comes from the Latin word acere which means sour. The acids in these substances are natural acids. What about BAKING soda? Does it also taste sour? If not, what is its taste? Since, it does not taste sour it means, that it has no acids in it. It is bitter in taste. If you rub its solution between fingers, it feels soapy. Generally, substances like these which are bitter in taste and feel soapy on touching are known as bases. The nature of such substances is said to be BASIC. If we cannot taste every substance, how do we find its nature? 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. Explanation: