Indicators Class X

You will able to learn different types of indicators to know whether the substance is acid or base.
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1. ACIDS , BASES AND SALTS Chapter 2 Sumesh Parihar

2. One hundred and eighteen different chemical elements are known to us at present. These elements combine to form a large number of compounds. On the basis of their chemical properties, all the compounds can be classified into three groups : 1 . Acids, 2. Bases, and 3. Salts In order to know whether a substance is an acid or a base, we should first know the meaning of the term ‘acid-base indicator ’ or just ‘indicator’.

3. Indicator An indicator is a ‘dye’ that changes colour when it is put into an acid or a base. An indicator gives different colours in acid and base. Thus, an indicator tells us whether the substance we are testing is an acid or a base by change in its colour . Natural Synthetic Olfactory The three most common indicators to test for acids and bases are : Litmus, Methyl orange and Phenolphthalein.

4. Natural Olfactory Synthetic Types Of Indicators:

5. Natural Indicators Natural ( i ) An acid turns blue litmus to red . ( ii ) A base (or alkali) turns red litmus to blue . Litmus is a natural indicator. Litmus solution is a purple dye which is extracted from a type of plant called ‘lichen’. Turmeric is also a natural indicator. Turmeric ( haldi ) contains a yellow dye. It turns red in basic solutions. Many times we have noticed that a yellow stain of curry on a white cloth (which is due to the presence of turmeric in curry ) turns reddish-brown when soap is scrubbed on it . The red cabbage extract (obtained from red cabbage leaves ) is also a natural indicator. It is red in colour . The red cabbage extract remains red in acidic solutions but turns green on adding to basic solutions.

6. Synthetic Indicators Synthetic Methyl orange and phenolphthalein are synthetic indicators. The neutral colour of methyl orange is ‘ orange’. ( i ) Methyl orange indicator gives red colour in acid solution. ( ii ) Methyl orange indicator gives yellow colour in basic solution. The neutral colour of phenolphthalein is ‘ colourless ’. ( i ) Phenolphthalein indicator is colourless in acid solution. ( ii ) Phenolphthalein indicator gives pink colour in basic solution.

7. Olfactory Indicators Olfactory ( i ) Onion has a characteristic smell. When a basic solution like sodium hydroxide solution is added to a cloth strip treated with onions (or onion extract), then the onion smell cannot be detected. An acidic solution like hydrochloric acid, however, does not destroy the smell of onions. This can be used as a test for acids and bases. Those substances whose smell (or odour ) changes in acidic or basic solutions are called olfactory indicators. ( ii ) Vanilla extract has a characteristic pleasant smell. If a basic solution like sodium hydroxide solution is added to vanilla extract, then we cannot detect the characteristic smell of vanilla extract. An acidic solution like hydrochloric acid, however, does not destroy the smell of vanilla extract. This can be used as a test for acids and bases.

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