P is a white solid. Gentle heating gives off a colourless gas,
carbon dioxide (a white precipitate forms in the limewater), and water vapour (droplets may form on the cooler part of the tube) —
with very little change to the white solid itself. No effervescence is seen when a DRY solid is heated;
effervescence only appears in (b), where solution Q is added to P (carbon dioxide again, and a light blue precipitate settles).
(c)–(f) Solution Q
Q = copper(II) chloride (blue).
Nitric acid + barium nitrate → no observable change → no sulfate.
Ammonia added slowly → a light blue precipitate, which dissolves 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.
Carbon dioxide was obtained from P, 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. Count the drops of dilute nitric acid to the colour change, using the same
volume of each and the same dropper (sample readings: R₁ = 13 drops, R₂ = 7). The solution that needs
more drops contains more of the base in each cm³, so it is the more concentrated — here R₁.
Examiner tips
Write "no observable 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.