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Study the reaction scheme below and then answer (a), (b) and (c) which follow :

Give the chemical name and formula of (i) marble (ii) gas A (iii) solid B. ICSE Questions Set in Previous Years, Concise Chemistry Solutions ICSE Class 9.

(a) Give the chemical name and formula of (i) marble (ii) gas 'A' (iii) solid 'B'

(b) What would you expect to see (or observe) when drops of water are added to solid calcium oxide?

(c) (i) Rewrite the following sentences filling in the blanks :

Solution C is normally called ……………

When gas 'A' is bubbled through 'C'., a white precipitate of …………… is formed.

(ii) On bubbling excess of gas A through the resulting suspension, the white precipitate dissolves and then reappears on boiling. Suggest an explanation for these observations.

Practical Chemistry

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Answer

(a) CaCO3 ⟶ CaO + CO2 [Gas A]

CaO + H2O ⟶ Ca(OH)2

Ca(OH)2 + H2O [excess] ⟶ CaO + H2O

(i) marble — Calcium carbonate [CaCO3]

(ii) gas 'A' — Carbon dioxide [CO2]

(iii) solid 'B' — Calcium hydroxide [Ca(OH)2]

(b) When drops of water are added to solid calcium oxide, a large amount of heat is liberated and a hissing sound is heard along with the formation of calcium hydroxide [Ca(OH)2]. It is an exothermic reaction.

(c) (i) Solution C is normally called lime water

When gas 'A' is bubbled through 'C'., a white precipitate of calcium hydroxide is formed.

(ii) When excess of gas A [Carbon dioxide] is passed through the resulting suspension, the white ppt. of calcium carbonate dissolves due to the formation of calcium hydrogen carbonate which is soluble in water.

CaCO3 + H2O + CO2 [excess] ⟶ Ca(HCO3)2 [soluble]

On boiling, calcium hydrogen carbonate decomposes to form calcium carbonate due to which, the white ppt. reappears.

Ca(HCO3)2 → Δ \xrightarrow{\space \Delta \space} H2O + CO2 + CaCO3↓

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