Exploring Substances: Acidic, Basic, and Neutral
Class 07 ScienceSubstances around us may be classified as acidic, basic, and neutral in nature.
Litmus as an Indicator
Litmus is a natural substance obtained from lichens. It is available both as a solution and in the form of paper strips, known as litmus paper. The litmus paper is available in two colours - blue and red.
Substances that turn blue litmus paper to red are acidic in nature, while those that turn red litmus paper to blue are basic in nature. Since litmus shows different colours in acidic and basic solutions, it is called an acid-base indicator.
Some other substances, both natural and synthetic, can also be used as indicators. Synthetic indicators are made in laboratories.
The substances such as lemon juice, amla juice, tamarind water, and vinegar turn the blue litmus paper to red, implying that these substances are acidic in nature. The substances such as soap solution, baking soda solution, lime water, and washing powder solution turn the red litmus paper to blue. Hence, these substances are basic in nature. The substances such as tap water, sugar solution, and salt solution, did not change the colour of either litmus paper. These substances are said to be neutral because they are neither acidic nor basic.
Substances that taste sour tend to contain acids and are acidic in nature. Bases generally taste bitter, but everything that tastes bitter may not contain a base.
Red rose as an Indicator
The red rose extract can also be used to test the nature of the substances. Hence, it is another example of an acid-base indicator.
The red rose extract seems to give red colour in an acidic solution and green colour in a basic solution.
Turmeric as an Indicator
Turmeric paper can be used to test basic substances. However, it cannot differentiate between acidic and neutral substances.
Mixing Acidic with Basic Substances
When a drop of blue litmus solution is added to the lemon juice solution, the colour of the solution turns red. When lime water is added to this test tube, the colour of the solution eventually changes from red to blue. This shows that the solution in the test tube is no longer acidic. Lime water has neutralised the effect of the acid.
When the solution of an acid is mixed with the solution of a base in sufficient quantity, the resulting solution is neither acidic nor basic. Such reactions are called neutralisation reactions. In a neutralisation reaction, salt and water are formed with the evolution (release) of heat.
Acid + Base → Salt + Water + Heat
Neutralisation in Daily Life
When an ant bites, it injects an acidic liquid (formic acid) into the skin. The effect of the acid can be neutralised by rubbing moist baking soda, which is a base.
When the soil is too acidic, the plants do not grow well. It can be treated with lime, which is a base. If the soil is basic, organic matter like manure and composted leaves are added to it. Organic matter releases acids that neutralise the basic nature of the soil. Sometimes, the soil may be neutral, but the plants growing in it may still show poor health, which can be due to the deficiency of nutrients in the soil.