NCERT Solutions for Class 7th Science Chapter 5 .2 A Substance May Change in Appearance and Not Remain the Same! — Activity 5.3: Let us explore
Book page 605 Updated on2026-09-19
Q1.
Now, blow air (exhale) into each glass tumbler, one at a time, using separate straws (Fig. 5.2) and observe them. Do you notice any changes?
Fig. 5.2, page 60 — blowing air (exhaling) through a straw into (a) tap water and (b) lime water.
Answer
Yes — but only in tumbler B.
Tumbler
Contains
What is seen on blowing air
Kind of change
A
Tap water
Only bubbles rise. The water looks exactly as it did before.
No chemical change
B
Lime water
Bubbles rise and the clear lime water turns milky (cloudy). Left standing, a white substance settles at the bottom.
Chemical change
The white solid that settles is a new substance — calcium carbonate — which is insoluble in water. Because one or more new substances are formed, this is a chemical change, brought about by a chemical reaction:
Why it happens: The air we breathe out is rich in carbon dioxide. In tumbler A it simply escapes as bubbles, because it does not react with plain water in any visible way. In tumbler B the carbon dioxide reacts with the calcium hydroxide dissolved in the lime water and makes tiny white grains of calcium carbonate. These grains do not dissolve, so they hang in the liquid and make it look milky.
Caution: Blow gently through the straw and never suck. Lime water is harmful if swallowed.