Changes Around Us: Physical and Chemical
Chemical changes
Investigate chemical changes, starting with blowing air into lime water.
Activity 5.3. Pour a little tap water into glass A and a little lime water into glass B. Blow gently into each through a separate straw. (Blow out only. Never suck the liquid up.)
| Glass | What you see |
|---|---|
| A: tap water | only bubbles; the water looks the same |
| B: lime water | bubbles, and the water turns milky; later a white substance settles at the bottom |
Something new has appeared in glass B. A change in which one or more new substances are formed is a chemical change. New substances are made by a chemical reaction.
Remember from Chapter 2: lime water is made from chuna, not from nimbu. The carbon dioxide in the air you breathe out reacts with it and makes calcium carbonate, a white solid that does not dissolve. That is why it looks milky. A little water forms too. In short:
Calcium hydroxide (lime water) + Carbon dioxide → Calcium carbonate + Water
So lime water turning milky is the test for carbon dioxide.
Activity 5.4. Put a teaspoon of vinegar or lemon juice in a test tube and add a pinch of baking soda. It fizzes and bubbles. Lead the gas through a tube into fresh lime water: it turns milky, so the gas is carbon dioxide. A new substance has formed, so this is a chemical change:
Vinegar + Baking soda → Carbon dioxide + other substances
Try baking soda in plain water. Do you get bubbles this time?
Practise this lesson
Questions that get harder as you improve, a Tutor that helps without giving the answer away, and progress you can see. Free for every learner.
No account, no email address. Opens the real product.