Complete and balance the following chemical equations :

(i) $\text{NaOH}_{(aq)} + \text{Zn}_{(s)} \rightarrow$

(ii) $\text{CaCO}_{3(s)} + \text{H}_2\text{O}_{(l)} + \text{CO}_{2(g)} \rightarrow$

(iii) $\text{HCl}_{(aq)} + \text{H}_2\text{O}_{(l)} \rightarrow$

OR

During electrolysis of brine, a gas 'G' is liberated at anode. When this gas 'G' is passed through slaked lime, a compound 'C' is formed, which is used for disinfecting drinking water.

(i) Write formula of 'G' and 'C'.

(ii) State the chemical equation involved.

(iii) What is common name of compound 'C'? Give its chemical name.

Q: Complete and balance the following chemical equations : (i) $\text{NaOH}_{(aq)} + \text{Zn}_{(s)} \rightarrow$ (ii) $\text{CaCO}_{3(s)} + \text{H}_2\text{O}_{(l)} + \text{CO}_{2(g)} \rightarrow$ (iii) $\text{HCl}_{(aq)} + \text{H}_2\text{O}_{(l)} \rightarrow$ OR During electrolysis of brine, a gas 'G' is liberated at anode. When this gas 'G' is passed through slaked lime, a compound 'C' is formed, which is used for disinfecting drinking water. (i) Write formula of 'G' and 'C'. (ii) State the chemical equation involved. (iii) What is common name of compound 'C'? Give its chemical name.

### Option 1: (i) $$2\text{NaOH}_{(aq)} + \text{Zn}_{(s)} \rightarrow \text{Na}_2\text{ZnO}_{2(s)} + \text{H}_{2(g)}$$ (ii) $$\text{CaCO}_{3(s)} + \text{H}_2\text{O}_{(l)} + \text{CO}_{2(g)} \rightarrow \text{Ca(HCO}_3)_{2(aq)}$$ (iii) $$\text{HCl}_{(aq)} + \text{H}_2\text{O}_{(l)} \rightarrow \text{H}_3\text{O}^+_{(aq)} + \text{Cl}^-_{(aq)}$$ ### OR Option: (i) Formula of 'G': $$\text{Cl}_2$$ Formula of 'C': $$\text{CaOCl}_2$$ (ii) Chemical Equation: $$\text{Ca(OH)}_2 + \text{Cl}_2 \rightarrow \text{CaOCl}_2 + \text{H}_2\text{O}$$ (iii) Common name: Bleaching powder Chemical name: Calcium oxychloride

Acids, Bases and Salts

Chemical properties of acids and bases

Official Answer

Option 1:

(i) $2\text{NaOH}_{(aq)} + \text{Zn}_{(s)} \rightarrow \text{Na}_2\text{ZnO}_{2(s)} + \text{H}_{2(g)}$

(ii) $\text{CaCO}_{3(s)} + \text{H}_2\text{O}_{(l)} + \text{CO}_{2(g)} \rightarrow \text{Ca(HCO}_3)_{2(aq)}$

(iii) $\text{HCl}_{(aq)} + \text{H}_2\text{O}_{(l)} \rightarrow \text{H}_3\text{O}^+_{(aq)} + \text{Cl}^-_{(aq)}$

OR Option:

(i) Formula of 'G': $\text{Cl}_2$

Formula of 'C': $\text{CaOCl}_2$

(ii) Chemical Equation:

$\text{Ca(OH)}_2 + \text{Cl}_2 \rightarrow \text{CaOCl}_2 + \text{H}_2\text{O}$

(iii) Common name: Bleaching powder

Chemical name: Calcium oxychloride

Sodium zincateCalcium hydrogencarbonateHydronium ionBleaching powderCalcium oxychlorideChlorine gas

Marking Scheme

1Option 1: Correct balanced equation for reaction (i) (1 mark)
2Option 1: Correct balanced equation for reaction (ii) (1 mark)
3Option 1: Correct balanced equation for reaction (iii) (1 mark)
4OR Option: Correct formulas for G ( $Cl_2$ ) and C ( $CaOCl_2$ ) (1 mark)
5OR Option: Correct balanced chemical equation (1 mark)
6OR Option: Correct common name (Bleaching powder) and chemical name (Calcium oxychloride) (1 mark)

Hint

For Option 1, recall the products of metal-base reactions, excess $CO_2$ reaction with carbonate, and acid ionization. For the OR option, think of the chlor-alkali process and bleaching powder.

Quick Oral Answer

The reaction of zinc with sodium hydroxide produces sodium zincate ( $Na_2ZnO_2$ ) and hydrogen gas. Bleaching powder is chemically calcium oxychloride ( $CaOCl_2$ ), prepared by passing chlorine gas over dry slaked lime.

Analysis & Explanation

The first option tests direct chemical equations from the NCERT text:

Activity 2.4 details the reaction of zinc with sodium hydroxide to form sodium zincate and hydrogen gas.
Section 2.1.3 details the reaction of passing excess carbon dioxide through lime water containing calcium carbonate precipitate, yielding soluble calcium hydrogencarbonate.
Section 2.2.1 explains that $HCl$ in water produces hydronium ( $H_3O^+$ ) and chloride ( $Cl^-$ ) ions.

The OR option tests the industrial preparation of bleaching powder:

Chlorine gas ( $Cl_2$ ) is produced during the electrolysis of aqueous sodium chloride (brine).
Passing this chlorine gas over dry slaked lime ( $Ca(OH)_2$ ) yields calcium oxychloride ( $CaOCl_2$ ), commonly known as bleaching powder, which is widely used as a disinfectant for drinking water.

Common Mistakes

1Writing calcium bicarbonate as $CaHCO_3$ instead of $Ca(HCO_3)_2$ .
2Forgetting to balance the sodium hydroxide in the zinc reaction ( $2NaOH$ ).

Interesting Facts

Sodium zincate is a complex salt where zinc is part of the anion ( $ZnO_2^{2-}$ ), showing the amphoteric nature of zinc.

Bleaching powder smells strongly of chlorine because it slowly reacts with carbon dioxide in the air to release chlorine gas.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2020?

This question carries 3 marks in the CBSE Class 10 Science 2020 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Acids, Bases and Salts" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Chemical properties of acids and bases" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2020 paper?

This is a Short Answer question from Section B in the CBSE Class 10 Science 2020 paper.