Thiosulfate + acid make solid sulfur: Na₂S₂O₃(aq) + 2HCl(aq) → 2NaCl(aq) + H₂O(l) + SO₂(g) + S(s). The fine sulfur particles scatter light, so the mixture clouds over and the marker dot behind the tube vanishes. The MORE concentrated the thiosulfate, the sooner enough sulfur builds up — so rate = 1000/t rises with the volume of B.
C behaves as copper(II) carbonate with a chloride impurity: heating gives black CuO (green → black) + CO₂ (limewater milky) + condensation; acid gives effervescence and a blue solution; ammonia gives a light blue ppt that dissolves in excess to the dark blue tetraamminecopper(II) ion; Ba(NO₃)₂ shows no sulfate; AgNO₃ gives white AgCl → chloride.
The gravimetric part uses the mass lost as CO₂: (175.83 + 0.70) − 176.46 = 0.07 g → 0.07/44 = 0.00159 mol CO₂ ≡ mol CuCO₃ → × 124 = 0.197 g → 28.1 % of the 0.70 g sample.
Propanal boils at 49 °C, far below propanoic acid (141 °C) and water (100 °C) — perfect for fractional distillation: collect the distillate while the thermometer at the top of the column reads a constant 49 °C. To test for leftover propanoic acid use a reactive metal or a carbonate — effervescence means acid is present (an indicator is banned by the question; KMnO₄ would miss the acid entirely).