P is a white solid (sodium hydrogencarbonate). Gentle heating decomposes it,
releasing carbon dioxide (limewater → milky) and water vapour (condensation near the mouth) —
with very little change to the solid itself. No effervescence is seen when a DRY solid is heated;
effervescence only appears in (b), where the acidic solution Q reacts with P.
(c)–(f) Solution Q
Q = copper(II) chloride (blue).
Nitric acid + barium nitrate → no visible change → no sulfate.
Ammonia added slowly → light blue precipitate, then both precipitates dissolve in excess
to a dark blue solution → Cu²⁺. (Add it slowly, or you skip the light blue stage!)
NaOH → light blue precipitate, insoluble in excess → Cu²⁺ confirmed.
P made CO₂ with heat and with Q → P is a carbonate (contains CO₃²⁻).
(g) Comparing R₁ and R₂
R₁ and R₂ are solutions of the same basic compound. Indicator I (screened methyl orange) is
green in alkali, purple in acid. Counting drops of dilute nitric acid to the colour change:
R₁ = 13 drops, R₂ = 7 → R₁ is about twice as concentrated.
Examiner tips
Write "no visible change" when nothing happens — never leave a cell blank.
The copper precipitates are light blue; only the final ammonia solution is dark blue.
(f) justifications must quote your own recorded observations, and don't swap the
cation/anion evidence.