Chemical Reactions and Equations NCERT Exemplar Problems
Long Answer Type
Question 39: On heating blue coloured powder of copper (II) nitrate in a boiling tube, copper oxide (black), oxygen gas and a brown gas X is formed
(a) Write a balanced chemical equation of the reaction.
Answer: 2Cu(NO3 )2 ⇨ 2Cu + 4NO2 + O2
(b) Identify the brown gas X evolved.
Answer: The brown gas 'X' is nitrogen dioxide.
(c) Identify the type of reaction.
Answer: This is a Decomposition reaction
(d) What could be the pH range of aqueous solution of the gas X?
Answer: Oxides of non-metals are acidic and hence, aqueous solution of this gas would be acidic. This means that its pH would be less than 7.
Question 40: Give the characteristic tests for the following gases:
Answer: Carbon dioxide turns lime water milky as it forms calcium carbonate with lime water. This is the characteristics test for carbon dioxide.
Answer: The smell of sulphur dioxide gas is like burning of sulphur, which is the characteristics test for sulphur dioxide. Sulhpur dioxide turns moist litmus paper from blue to red.
Answer: When a burnt candle or match stick is brought near the oxygen gas, it starts burning brightly. This is the characteristic test for oxygen gas.
Answer: When a burnt candle or match stick, it burns with pop sound, which is the characteristic test for hydrogen gas.
Question 41: What happens when a piece of
(a) Zinc metal is added to copper sulphate solution?
Answer: Zinc sulphate and copper is formed when zinc reacts with copper sulphate.
Zn + CuSO4 ⇨ Cu + ZnSO4
(b) Aluminium metal is added to dilute hydrochloric acid?
Answer: Hydrochloric acid forms aluminium chloride and hydrogen gas when reacts with aluminium metal.
2Al + 6HCl ⇨ 2AlCl3 + 3H2
(c) Silver metal is added to copper sulphate solution? Also, write the balanced chemical equation if the reaction occurs
Answer: When silver metal is added to copper sulphate solution, no reaction takes place because silver is less reactive than copper.
Question 42: You are provided with two containers made up of copper and aluminium. You are also provided with solutions of dilute HCl, dilute HNO3, ZnCl2 and H2O. In which of the above containers these solutions can be kept?
Answer: Water does not react with copper or aluminium at room temperature, although aluminium reacts with steam. So, water can be kept in any of the containers. Dilute HCl and ZnCl2 react with aluminium but not with copper. So they can be kept in copper container. Dilute HNO3 reacts with both copper and aluminium. So, it cannot be kept in any of the containers.
Question 43: What happens when zinc granules are treated with dilute solution of H2SO4, HCl, HNO3, NaCl and NaOH, also write the chemical equations if reaction occurs.
Answer: Hydrogen gas and zinc sulphate are formed when zinc granules are treated with sulphuric acid.
Zn + H2SO4 ⇨ ZnSO4 + H2
Zinc chloride and hydrogen gas are formed, when zinc granules are treated with hydrochloric acid.
Zn + 2HCl ⇨ ZnCl2 + H2
Similarly, hydrogen gas and zinc nitrate are formed when zinc granules are treated with nitric acid.
Zn + 2HNO3 ⇨ Zn(NO3 )2 + H2
Since, zinc is less reactive than sodium, thus no reaction takes place when zinc granules are treated with sodium chloride.
Sodium zincate and hydrogen gas are formed when zinc granules are treated with sodium hydroxide and water.
Zn + 2H2O + 2NaOH ⇨ Na2Zn(OH)4 + H2
Question 44: On adding a drop of barium chloride solution to an aqueous solution of sodium sulphite, white precipitate is obtained.
(a) Write a balanced chemical equation of the reaction involved
Answer: A white precipitate of barium sulphite is formed when barium chloride is added to the solution o sodium sulphite.
BaCl2 + Na2SO3 ⇨ BaSO3 + 2NaCl
(b) What other name can be given to this precipitation reaction?
Answer: This precipitation reaction is also a double displacement reaction.
(c) On adding dilute hydrochloric acid to the reaction mixture, white precipitate disappears. Why?
Answer: Barium chloride, suphur dioxide and water are formed hen dilute hydrochloric acid is added to this solution of barium sulphite and sodium chloride. Since barium chloride is a soluble substance, thus white precipitate of barium sulphite disappears.
BaSO3 + HCl ⇨ BaCl3 + SO2 + H2O