Chapter 1 · Chemical Reactions and Equations

NCERT Class 10 Science Chapter 1 Intext Questions (Page 10)

All 2 questions of NCERT Class 10 Science Chapter 1 Intext Questions (Page 10) — Chemical Reactions and Equations — in the book's order, each with its answer and a step-by-step solution.

1. Q1
   A solution of a ​substance ‘X’ is used for whitewashing.

   1. (i)​Name ​the ​substance ‘X’ ​and write its ​formula.
   2. (ii)Write the reaction of ​the substance ‘X’ ​named in (i) above with water.
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   $X$ is calcium oxide, $\text{CaO}$, which reacts with water to give calcium hydroxide and a large amount of heat.

   (i) $X$ is calcium oxide (quick lime), formula $\text{CaO}$; its solution in water (slaked lime solution) is used for whitewashing. (ii) Calcium oxide reacts vigorously with water to form calcium hydroxide (slaked lime) with the release of a large amount of heat: $\text{CaO}(s)+{\text{H}}_2\text{O}(l)\to {\text{Ca(OH)}}_2(aq)+\text{heat}$. This is an exothermic combination reaction.

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2. Q2
   Why ​is the ​amount ​of gas collected in one ​of the ​test tubes in ​Activity 1.7 double of the ​amount collected in the other? Name this gas.
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   Electrolysis of water gives hydrogen and oxygen in a 2 : 1 volume ratio, so the gas in double amount is hydrogen.

   Activity 1.7 is the electrolysis of water: $2{\text{H}}_2\text{O}(l)\stackrel{\text{electric current}}{\to }2{\text{H}}_2(g)+{\text{O}}_2(g)$. Water contains hydrogen and oxygen atoms in the ratio $2:1$, so on decomposition two molecules — and hence two volumes — of hydrogen are produced for every one of oxygen. The gas collected in double the amount is therefore hydrogen (${\text{H}}_2$), which forms at the cathode; oxygen, collected at the anode, has half its volume.

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