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1. 1.

   Balance the following equations:

   1. HgO → Hg + O₂     _________________ [balanced equation]

             i] Balance the oxygen atoms  ii] Balance the mercury atoms:

   2. H₂O₂  → H₂O + O₂      _________________ [balanced equation]

             i] Balance the oxygen atoms  ii] Balance the hydrogen atoms:

   3. P₂O₅ + H₂O → H₃PO₄    _________________ [balanced equation]

            i]  Balance the phosphorus atoms  ii] Balance the oxygen atoms:

2. 2.

   Explain the term 'symbol'. State a reason why the symbol of calcium is 'Ca' & of copper is 'Cu'.

3. 3.

   Define the term 'valency'. With reference to water & ammonia as compounds respectively, state the valency of oxygen & nitrogen. Magnesium [2, 8, 2] has valency 2⁺. Give reasons.

4. 4.

   Explain the term 'variable valency'. Copper having electronic configuration 2, 8, 18, 1 exhibits variable valency. Give a reason for the same & name the compound CuCl & CuCl₂.

5. 5.

   State the valencies of the following metallic elements - a] Potassium b] Sodium c] Calcium d] Magnesium e] Zinc f] Aluminum g] Chromium [write each symbol with the valency]

6. 6.

   Certain metals exhibit variable valencies which include valencies: 1⁺, 2⁺, 3⁺ & 4⁺. State the variable valency of the following metals a] Copper b] Silver c] Mercury d] iron e] Tin f] Lead - [write each symbol with the variable valency].