CBSE Class 10 Science 2019 · Paper 31/4/2 · Question 9

Acids, Bases and Salts 3 marks Medium

Salt ‘P’, commonly used in bakery products, on heating gets converted into another salt ‘Q’ which itself is used for the removal of hardness of water and a gas ‘R’ is evolved. The gas ‘R’ when passed through freshly prepared lime water turns milky. Identify ‘P’, ‘Q’ and ‘R’, giving chemical equation for the justification of your answer.

Answer

P is sodium hydrogencarbonate, Q is sodium carbonate and R is carbon dioxide, which turns lime water milky.

P: Sodium hydrogencarbonate (baking soda),

NaHCO_{3}

. Q: Sodium carbonate,

Na_{2} CO_{3}

(as washing soda,

Na_{2} CO_{3} \cdot 10 H_{2} O

, it removes the permanent hardness of water). R: Carbon dioxide,

CO_{2}

. On heating P:

2 NaHCO_{3} heat Na_{2} CO_{3} + H_{2} O + CO_{2}

R turns lime water milky:

Ca(OH)_{2} + CO_{2} \to CaCO_{3} + H_{2} O

(calcium carbonate is a white, insoluble solid).

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CBSE Class 10 Science 2019 · Paper 31/4/2 · Question 9

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