Exploring Substances: Acidic, Basic, and Neutral

About this chapter

Exploring Substances: Acidic, Basic, and Neutral is the second chapter of Class 7 Curiosity. The chapter opens at a National Science Day fair where siblings Ashwin and Keerthi see a message appear on a sprayed paper, and that everyday mystery becomes the doorway into acid-base chemistry. Through Activity 2.1 students test lemon juice, soap solution, amla juice, tamarind water, vinegar, baking soda solution, lime water, tap water, washing powder, sugar solution and salt solution with blue and red litmus paper. Substances that turn blue litmus red are acidic, those that turn red litmus blue are basic, and those that change neither colour are neutral. Litmus is obtained from lichens and is the standard acid-base indicator. The chapter then makes three other natural indicators in the classroom — red rose extract (red in acidic, green in basic), turmeric paper (yellow turns red in basic only), and olfactory indicators using onion. mixing an acid with a base in the right quantity gives a neutral solution; this is a neutralisation reaction (Acid + Base to Salt + Water + Heat). carries the idea into daily life — red ant bite treated by baking soda, acidic soil treated by lime, basic soil treated by compost or manure, industrial effluent neutralised before release. P.C. Ray and Acharya Prafulla Chandra Ray are flagged as the Father of Modern Indian Chemistry. CTET Paper 2 Science loves this chapter for indicator colour-change matches, edible-acid identification (citric, ascorbic, tartaric, oxalic), neutralisation-in-daily-life applications, and the careful logic of 'turmeric cannot tell acidic from neutral'. The four tests below — Practice 15 Q, Quiz 15 Q, Hard 15 Q, Mastery 30 Q — cover these ideas at exam depth.