CBSE Class 10 Science 2020 · Paper 31/2/1 · Question 25(b)

Metals and Non-metals 5 marks Medium

What happens when (Write the balanced equation involved) –

Answer

Copper forms black $CuO$ ; $Al_{2} O_{3}$ reacts with both $HCl$ and $NaOH$ ; potassium reacts violently with water; cinnabar gives mercury.

(i) Copper combines with oxygen and a black coating of copper(II) oxide forms:

2 Cu + O_{2} heat 2 CuO

(ii) Aluminium oxide reacts, as a basic oxide, to form aluminium chloride and water:

Al_{2} O_{3} + 6 HCl ⟶ 2 AlCl_{3} + 3 H_{2} O

(iii) Potassium reacts violently with cold water, forming potassium hydroxide and hydrogen; the heat released sets the hydrogen on fire:

2 K + 2 H_{2} O ⟶ 2 KOH + H_{2} + heat energy

(iv) Cinnabar (mercury(II) sulphide) is converted into mercury(II) oxide and sulphur dioxide is given off:

2 HgS + 3 O_{2} heat 2 HgO + 2 SO_{2}

; on further heating the oxide gives mercury,

2 HgO heat 2 Hg + O_{2}

(v) Aluminium oxide reacts, as an acidic oxide, to form sodium aluminate and water:

Al_{2} O_{3} + 2 NaOH ⟶ 2 NaAlO_{2} + H_{2} O

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