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NCERT Solutions for Class 7th Science Chapter 2 Activity 2.2: Let us relate and explore — Taste, touch and the acids in food

Book page 11 Updated on2026-09-19

Q1.
Are all the substances in Group A of Table 2.2 edible? Have you ever tasted these edible substances? Can you recall their taste?
Group A
(turn blue litmus red)		Group B
(turn red litmus blue)		Group C
(affect neither paper)
Lemon juiceSoap solutionTap water
Amla juiceBaking soda solutionSugar solution
Tamarind waterLime waterSalt solution
VinegarWashing powder solution 
Table 2.2: Grouping of samples tested in Table 2.1 — page 9. Column headings as printed; the entries are the ones recorded in Activity 2.1.
Answer

Yes — every substance in Group A is edible: lemon juice, amla juice, tamarind water and vinegar are all used in Indian kitchens. And every one of them tastes sour.

What this tells you: the four samples that turned blue litmus red are exactly the four that taste sour. So substances that taste sour tend to contain acids and are acidic in nature — the tongue and the litmus paper give the same verdict.
Caution: the book's warning matters. Do not taste anything until asked to do so, and never taste an unknown substance. Group B substances such as washing powder solution are harmful. Taste is a clue only for foods you already eat.
Q2.
Find out and write the names of the most common acids present in the following substances — Lemon________, Curd________, Tamarind________, Vinegar________.
Answer
SubstanceMost common acid present
LemonCitric acid
CurdLactic acid
TamarindTartaric acid
VinegarAcetic acid

Fig. 2.4 of the book gives four more: orange — citric acid; amla — ascorbic acid (Vitamin C) and citric acid; tamarind — tartaric acid; tomato — citric acid and oxalic acid.

Why the names sound like the food: many of these acids were first taken out of that very food and named after it — citric from citrus fruits, lactic from milk (lac = milk), tartaric from the tartar left in wine barrels, acetic from acetum, the Latin for vinegar.
Did you know? Curd is sour only because milk has been fermented. Fresh milk is almost neutral; the bacteria in the culture make lactic acid from milk sugar, and that acid both sets the curd and gives it the sour taste.
Q3.
Now, let us take one of the substances — baking soda solution — from Group B. Rub the baking soda solution between your fingers. What do you observe?
Answer

It feels soapy or slippery between the fingers.

Why it happens: that slipperiness is a standard test for a base. A basic solution reacts with the thin layer of oil and protein on your skin and turns some of it into a soap-like substance — so you are literally making a trace of soap on your fingertips. Basic substances are generally slippery to touch.

Bases also generally taste bitter — but the book adds an important warning: not everything bitter contains a base. Bitter gourd (karela) tastes bitter yet is not basic in nature.

Tip: rinse your fingers well afterwards. Strong bases such as washing powder solution are slippery for the same reason and can harm the skin, which is why the safe test is always litmus, not touch.
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