Common Mistakes in Chemistry

1. Common Mistakes in Chemistry

2. Read the question (5 - 10 s) • Think (10 s) • Pair-up and discuss your answer (10 – 15 s) • One group will be invited to share the answer with whole class (10 – 15 s)

3. What’s wrong with the following statement? Oxygen relights a growing splint.

4. Correct statement Oxygen relights a glowing splint.

5. What’s wrong with the following statement? Nitrogen dioxide is an acidic oxide. It dissolves in water to form nitric acid.

6. Correct statement Nitrogen dioxide is an acidic oxide. It dissolves in water to form nitric acid and nitrous acid. 2NO2 + H2O  HNO3 + HNO2

7. What’s wrong with the following statement? Ethanol turns green acidified potassium dichromate solution to orange.

8. Correct statement Ethanol turns orange acidified potassium dichromate solution to green.

9. What’s wrong with the following statement? Ammonia gas turns red litmus paper blue.

10. Correct statement Ammonia gas turns moist red litmus paper blue.

11. What’s wrong with the following statement? Water vapour turns cobalt chloride to pink.

12. Correct statement Water vapour turns dry/ anhydrous/ blue cobalt chloride to pink.

13. What’s wrong with the following statement? Nitrogen is an inert gas.

14. Correct statement Nitrogen is an inactive gas. N2(g) + 3H2(g) 2NH3(g) Nitrogen acts as oxidising agent. N2(g) + O2(g)  2NO(g) Nitrogen acts as reducing agent.

15. What’s wrong with the following statement? Chlorine gas turns moist blue litmus paper red.

16. Correct statement Chlorine gas turns moist blue litmus paper red and then white.

17. What’s wrong with the following statement? Sulphur dioxide turns potassium dichromate from orange to green.

18. Correct statement Sulphur dioxide turns acidified potassium dichromate from orange to green.

19. What’s wrong with the following statement? Ethene turns acidified potassium dichromate from orange to green.

20. Correct statement Ethene turns acidified potassium permanganate from purple to colourless.

21. What’s wrong with the following statement? Carbon dioxide turns limewater milky. The white precipitate is not soluble in excess carbon dioxide.

22. Correct statement Carbon dioxide turns limewater milky. Ca(OH)2(aq) + CO2(aq)  CaCO3(s) + H2O(l) Excess carbon dioxide turns the milky solution colourless. CaCO3(s) + CO2(aq) + H2O(l)  Ca(HCO3)2(aq)

23. What’s wrong with the following statement? Hydrogen burns with a ‘pop’ sound.

24. Correct statement Pure hydrogen burns with a blue flame. A mixture of hydrogen and air/ oxygen burns with a ‘pop’ sound. We can use a burning splint to test for hydrogen gas collected in a test tube because hydrogen mixes up with air when we remove the stopper of the test tube.

25. What’s wrong with the following statement? Seperating funnel is used to seperate two immiscible liquids.

26. Correct statement Separating funnel is used to separate two immiscible liquids.

27. What’s wrong with the following statement? We use mortar to grind the calcium carbonate into powder.

28. What’s wrong with the following statement? We use mortar andpestle to grind the calcium carbonate into powder.

29. What’s wrong with the following statement? We use spoon to transfer chemicals from reagent bottle to the test-tube.

30. What’s wrong with the following statement? We use spatula to transfer chemicals from reagent bottle to the test-tube.

31. What’s wrong with the following statement? Tripod stand, wire gauze, Bunsen burner, crucible & heat-proof mat are used to convert magnesium to magnesium oxide.

32. What’s wrong with the following statement? Tripod stand, pipe-clay triangle, Bunsen burner, crucible & heat-proof mat are used to convert magnesium to magnesium oxide.

33. What’s wrong with the following statement? pH of 1M hydrochloric acid is higher than that of 1M ethanoic acid.

34. Correct statement pH of 1M hydrochloric acid is lower than that of 1M ethanoic acid.

35. What’s wrong with the following statement? The oxidation number of silver in Ag2O is 1.

36. Correct statement The oxidation number of silver in Ag2O is +1.

37. What’s wrong with the following statement? Acid is a hydrogen-containing compound which dissolves in water to give H+(aq).

38. Correct statement Acid is a hydrogen-containing compound which dissolves in water to give H+(aq) as the only cation.

39. The following answer is imprecise, amend it. Base is a substance which reacts with acid to give salt and water only. Base comprises metal oxide and metal hydroxide.

40. Precise answer Base is a substance which reacts with acid to give salt and water only. Base comprises metal oxide, metal hydroxide and ammonia.

41. The following answer is imprecise, amend it. Salt is a compound derived from acid. The hydrogen ion in acid is replaced by metal ion.

42. Precise answer Salt is a compound derived from acid. The hydrogen ion in acid is replaced by metal ion or ammonium ion.

43. What’s wrong with the following statement? In the electrolysis of dilute sodium chloride solution, hydrogen ion discharges instead of sodium ion because hydrogen ion is above sodium ion in the electrochemical series.

44. Correct statement In the electrolysis of dilute sodium chloride solution, hydrogen ion discharges instead of sodium ion because hydrogen ion is below sodium ion in the electrochemical series.

45. What’s wrong with the following statement? In the electrolysis of dilute sodium chloride solution, hydroxide ion discharges instead of chloride ion because hydroxide ion is below chloride ion in the electrochemical series.

46. Correct statement In the electrolysis of dilute sodium chloride solution, hydroxide ion discharges instead of chloride ion because hydroxide ion is above chloride ion in the electrochemical series.

47. What’s wrong with the following structure?

48. Correct structure

49. What’s wrong with the following statement? The functional group of ethanol is –HO.

50. Correct statement The functional group of ethanol is –OH.