A white crystalline solid A dissolves in water and effervesces with dilute HCl to give a gas that turns lime water milky. On heating to constant mass A loses $31.0%$ of its mass, leaving a white residue B that does not decompose further. The identity of A and its flame colour are:

Answer: A

For

2 MHCO X_{3} M X_{2} CO X_{3} + CO X_{2} + H X_{2} O

the fractional loss is

62/ (2 M_{r})

. For

KHCO X_{3}

(

M_{r} = 100

) it is

62/200 = 31.0%

, whereas

NaHCO X_{3}

would lose

62/168 = 36.9%

and

NH X_{4} HCO X_{3}

would vanish entirely. Potassium gives a lilac flame, so A is

KHCO X_{3}

(B is

K X_{2} CO X_{3}