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Q6 (Medium): Is curdling of milk a physical or chemical change? Justify your statement.
Answer:
We have studied that
chemical changes
form
new substances
with
different properties
. This is why curdling is a chemical change.
- New substance curd is formed from milk
- Different taste - sour instead of sweet
- Different texture - thick instead of liquid
- Cannot be reversed back to original milk
Q10 (Difficult): The experiments shown in Fig. 5.11a, b, c, and d were performed. Find out in which case(s) did lime water turn milky and why?
Answer:
Lime water turns milky when
carbon dioxide (CO₂)
gas is passed through it. CO₂ reacts with calcium hydroxide (lime water) to form
calcium carbonate
, which is a white insoluble precipitate giving the milky appearance.
Looking at each experiment in Fig. 5.11:
- (a) Vinegar + baking soda (sodium hydrogen carbonate) — acid reacts with baking soda → CO₂ produced → lime water turns milky ✓
- (b) Vinegar + common salt — this reaction does NOT produce CO₂ → lime water does not turn milky ✗
- (c) Lemon juice + vinegar — both are acids; no baking soda present → no CO₂ produced → lime water does not turn milky ✗
- (d) Lemon juice + baking soda (sodium hydrogen carbonate) — acid reacts with baking soda → CO₂ produced → lime water turns milky ✓
Conclusion: Lime water turns milky in experiments (a) and (d) . In both cases, an acid (vinegar or lemon juice) reacts with baking soda (sodium hydrogen carbonate) to release carbon dioxide gas . Common salt does not react with acids to produce CO₂, so experiment (b) gives no milky result.
