A white crystalline solid A dissolves in water to give a solution that turns red litmus blue. Dilute HCl added to A gives a colourless, odourless gas that turns lime water milky. Adding aqueous $CaCl X_{2}$ to a cold solution of A gives an immediate white precipitate. Which is the correct identification of A and the precipitate?

Answer: B

An odourless gas turning lime water milky is

CO X_{2}

(sulfite would give pungent

SO X_{2}

), so A is a carbonate or hydrogencarbonate. Cold

NaHCO X_{3}

solution has too little free

CO X_{3} X^{2 -}

and gives no immediate precipitate with

Ca X^{2 +}

, and

Ca (HCO X_{3}) X_{2}

is soluble. The immediate precipitate

Na X_{2} CO X_{3} + CaCl X_{2} CaCO X_{3} + 2 NaCl

identifies A as sodium carbonate.