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?
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:
Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
(Lime water) (from exhaled air) (insoluble, white)
(Lime water) (from exhaled air) (insoluble, white)
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.