Chapter 2 · Acids, Bases and Salts

NCERT Exemplar Class 10 Science Chapter 2 Long Answer Questions

All 6 questions of NCERT Exemplar Class 10 Science Chapter 2 Long Answer Questions (Acids, Bases and Salts), in the book's order, each with its answer and a step-by-step solution.

1. Q43
   In the following schematic diagram for the preparation of hydrogen gas as shown in Figure 2.3, what would happen if following changes are made?

   1. (a)In place of zinc ​granules, ​same amount ​of zinc dust ​is taken in ​the test tube
   2. (b)Instead of dilute sulphuric acid, dilute hydrochloric acid is taken
   3. (c)In place of zinc, copper turnings are ​taken
   4. (d)Sodium hydroxide is taken in place of dilute sulphuric acid and the tube is ​heated.

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   Zinc dust reacts faster; dilute HCl changes nothing; copper gives no reaction; heated NaOH still gives hydrogen, as zinc is amphoteric.

   (a) The reaction becomes much faster, because zinc dust exposes a far larger surface area than granules of the same mass; hydrogen is evolved more rapidly, but the total volume finally collected is unchanged. (b) No change in the observation — hydrogen is still evolved and still gives a pop sound; only the salt differs, zinc chloride instead of zinc sulphate: $\text{Zn}+2\text{HCl}\to {\text{ZnCl}}_2+{\text{H}}_2$. (c) No reaction occurs. Copper lies below hydrogen in the reactivity series and cannot displace hydrogen from a dilute acid, so no gas is evolved. (d) Hydrogen is still evolved, because zinc is amphoteric and reacts with alkalis as well as with acids: $\text{Zn}+2\text{NaOH}\stackrel{\Delta }{\to }{\text{Na}}_2{\text{ZnO}}_2+{\text{H}}_2$.

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2. Q44
   For making cake, ​baking powder is taken. If at home your mother uses baking soda instead of baking powder ​in cake,

   1. (a)how will ​it affect the ​taste ​of the cake and why?
   2. (b)how can baking soda be converted into baking ​powder?
   3. (c)what is the role of tartaric acid added to baking ​soda?
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   Baking soda leaves bitter sodium carbonate; adding tartaric acid makes baking powder, and the acid neutralises that carbonate.

   (a) The cake will taste bitter. On heating, baking soda decomposes to sodium carbonate, water and carbon dioxide, $2{\text{NaHCO}}_3\stackrel{\Delta }{\to }{\text{Na}}_2{\text{CO}}_3+{\text{H}}_2\text{O}+{\text{CO}}_2$; the carbon dioxide escapes and raises the cake, but the sodium carbonate left behind is strongly alkaline and bitter. (b) Baking soda is converted into baking powder by mixing a suitable amount of a mild edible acid, tartaric acid, with it. (c) The tartaric acid neutralises the sodium carbonate formed on heating, so no bitter residue remains and the cake does not taste bitter.

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3. Q45
   A metal carbonate ​X ​on reacting with an acid gives a gas ​which when passed through ​a solution Y gives the carbonate back. On the other hand, a gas G that is obtained at anode during ​electrolysis of brine is passed on dry ​Y, it gives a compound Z, used for ​disinfecting ​drinking water. Identity X, Y, G and Z.
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   X is ${\text{CaCO}}_3$, Y is ${\text{Ca(OH)}}_2$, G is ${\text{Cl}}_2$ and Z is bleaching powder, ${\text{CaOCl}}_2$.

   X is calcium carbonate, ${\text{CaCO}}_3$. Y is calcium hydroxide, ${\text{Ca(OH)}}_2$ (slaked lime, used here as lime water). G is chlorine, ${\text{Cl}}_2$, liberated at the anode. Z is bleaching powder, calcium oxychloride, ${\text{CaOCl}}_2$. The reactions are ${\text{CaCO}}_3+2\text{HCl}\to {\text{CaCl}}_2+{\text{H}}_2\text{O}+{\text{CO}}_2$; ${\text{CO}}_2+{\text{Ca(OH)}}_2\to {\text{CaCO}}_3\downarrow +{\text{H}}_2\text{O}$; $2\text{NaCl}(aq)+2{\text{H}}_2\text{O}(l)\to 2\text{NaOH}(aq)+{\text{Cl}}_2(g)+{\text{H}}_2(g)$; and ${\text{Ca(OH)}}_2+{\text{Cl}}_2\to {\text{CaOCl}}_2+{\text{H}}_2\text{O}$.

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4. Q46
   A ​dry pellet ​of a common base B, when kept ​in open absorbs moisture and turns sticky. The compound is also a ​by–product of ​chloralkali process. Identify B. What type ​of ​reaction occurs when B is treated with an acidic oxide? Write a balanced chemical ​equation for one such solution.
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   B is sodium hydroxide, $\text{NaOH}$; its reaction with an acidic oxide such as ${\text{CO}}_2$ is neutralisation, giving a salt and water.

   B is sodium hydroxide, $\text{NaOH}$ — its dry pellets are deliquescent, absorbing moisture from the air and turning sticky, and it is obtained from the chlor-alkali process. With an acidic oxide it undergoes a neutralisation reaction, giving a salt and water. For example: $2\text{NaOH}(aq)+{\text{CO}}_2(g)\to {\text{Na}}_2{\text{CO}}_3(aq)+{\text{H}}_2\text{O}(l)$.

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5. Q47
   A sulphate salt of Group 2 element ​of the Periodic Table is a white, soft substance, which can ​be moulded into different shapes by making ​its ​dough. When this compound is ​left in open for some time, it becomes a solid ​mass and cannot be used for moulding purposes. Identify the sulphate salt and why ​does it show such a behaviour? Give the reaction involved.
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   Plaster of Paris, ${\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}$, absorbs moisture and sets into hard gypsum, ${\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}$.

   The salt is plaster of Paris, calcium sulphate hemihydrate, ${\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}$ — calcium is a Group 2 element. Left in the open it absorbs moisture from the air and takes up water of crystallisation, changing into gypsum, ${\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}$, which is a hard solid mass and cannot be moulded. The reaction is ${\text{CaSO}}_4\cdot \frac{1}{2}{\text{H}}_2\text{O}+1\frac{1}{2}{\text{H}}_2\text{O}\to {\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}$. It should therefore be stored in a moisture-proof container.

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6. Q48
   Identify the compound ​X on the basis of ​the reactions ​given ​below. Also, write the name and chemical ​formulae of ​A, ​B and C.

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   X is sodium hydroxide; A is sodium zincate (${\text{Na}}_2{\text{ZnO}}_2$), B is sodium chloride ($\text{NaCl}$), C is sodium acetate (${\text{CH}}_3\text{COONa}$).

   X is sodium hydroxide, $\text{NaOH}$. A is sodium zincate, ${\text{Na}}_2{\text{ZnO}}_2$, from $\text{Zn}+2\text{NaOH}\to {\text{Na}}_2{\text{ZnO}}_2+{\text{H}}_2$. B is sodium chloride, $\text{NaCl}$, from $\text{NaOH}+\text{HCl}\to \text{NaCl}+{\text{H}}_2\text{O}$. C is sodium acetate, ${\text{CH}}_3\text{COONa}$, from $\text{NaOH}+{\text{CH}}_3\text{COOH}\to {\text{CH}}_3\text{COONa}+{\text{H}}_2\text{O}$.

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