Class 7 Science Chapter 2: Exploring Substances: Acidic, Basic, and Neutral Notes

What is this chapter about?

The chapter opens at a school science fair on National Science Day, 28 February. Visitors are handed blank white sheets, and the moment a volunteer sprays them, a message welcoming them to the wonderful world of science appears. How? The answer lies in a simple idea: many substances are acidic, basic or neutral, and some substances, called indicators, change colour to show which.

Testing with litmus

Litmus is a natural dye obtained from lichens, and it comes as blue and red papers. Put a drop of each sample on both. Vinegar, lemon juice, tamarind water and amla juice turn blue litmus red: they are acidic. Baking soda solution, soap solution, washing powder solution and lime water turn red litmus blue: they are basic. Tap water, sugar solution and salt solution leave both papers unchanged: they are neutral. Because litmus takes different colours in acids and bases, it is an acid-base indicator.

Sour and bitter

The acidic samples that we can eat all taste sour, because they contain acids: citric acid in oranges, ascorbic acid (vitamin C) and citric acid in amla, tartaric acid in raw mango and tamarind, and citric and oxalic acids in tomatoes. Bases usually taste bitter and feel slippery, like baking soda solution between the fingers. But bitter does not always mean basic: karela is bitter, yet it is not a base. And never taste an unknown substance. Read more in acids, bases and indicators.

Indicators from nature

Litmus is not the only indicator. Red rose extract, made by soaking crushed petals in hot water and filtering, turns red in acids and green in bases. Turmeric paper turns red in bases but stays yellow in acids and neutral substances, so it can detect bases but cannot tell an acid from a neutral substance. That is why a yellow curry stain turns red when soap is rubbed on it. Beetroot, purple cabbage, jamun and red hibiscus also make indicators, and hydrangea plants even grow blue flowers in acidic soil and pink or red ones in basic soil. Some indicators work by smell: strips kept with chopped onions lose their onion smell in a base. See natural indicators.

Neutralisation

In Activity 2.7, a drop of lemon juice in water turns red with blue litmus solution. When lime water is added drop by drop, the colour changes from red to blue: the solution is no longer acidic, because the base has cancelled the acid. This is neutralisation. When an acid and a base react in the right amounts, the result is neither acidic nor basic, and salt, water and heat are produced:

Acid + Base → Salt + Water + Heat

Neutralisation in daily life

The same reaction solves everyday problems. A red ant bite injects formic acid, so rubbing moist baking soda, a base, eases the sting. Too much chemical fertiliser can make soil acidic; farmers add lime, a base. Basic soil is treated with manure or compost, whose organic matter releases acids. And acidic waste from factories should be neutralised with a base before it reaches a lake, to protect the fish. Read neutralisation in daily life.

The mystery solved, and a great chemist

The welcome message at the fair could have been written in soap solution, which is basic, and then sprayed with turmeric solution: the paper turns yellow and the hidden letters turn red. The chapter also introduces Acharya Prafulla Chandra Ray, the Father of Modern Indian Chemistry, who set up India’s first pharmaceutical company in 1901 and wrote about chemistry in ancient India.

Answers to every activity question and all twelve exercise questions, with the Dive Deeper puzzle, are in the NCERT solutions for Chapter 2.