Answers Assignment – 1

Chemical Reactions and Equations

All the subjective answers of the previous post are provided in the post Answers Assignment 1 – Chemical Reactions and Equations

Ans 1. (i) Decomposition reaction

            (ii) Double displacement reaction

Ans 2. Silver does not evolve hydrogen on reacting with dil. H2SO4 as silver is less reactive metal than hydrogen.

Ans 3. Diamond and graphite are the two allotropes of carbon but they do not evolve the same amount of heat on combustion because they differ in the arrangement of carbon atoms and also their shapes one different.

Ans 4. The oxidizing agent supplies the oxygen in a reaction or it removes the hydrogen.

Ans 5. When quick lime is added to water calcium hydroxide (Slaked lime) is formed with a hissing sound and a lot of heat is evolved during the reaction. The reaction involved is

            CaO (s) + H2O(Quick Lime) → Ca (OH) 2 (s) + Heat (Slaked Lime)

Ans 6. Combustion reaction because it is always carried out in the presence of air or oxygen for e.g.
CH4 (g) + 2O2 (g) → CO2 (g) +2H2O (l)

Ans 7. Aluminium foil does not corrode in the atmosphere even if kept for a long time because a protective coating of aluminium oxide (Al2O3) is formed on the surface of the foil and stops any further reaction of the metal with air and water thus food particles do not get spoiled.

Ans 8. When CO2 gas is bubbled through lime water in excess then initially it becomes milky but then its milkiness disappears.

Ans 9. Combustion is an oxidation reaction as it is always carried out in the presence of oxygen.  For Example:

              CH4  +  2 O2   →   CO2  +  2 H2O

Ans 10. (i) Combination reaction

               (ii) Displacement reaction

Ans 11. A chemical change cannot be reversed back because products so formed in a chemical reaction are totally different from the reactants.

Ans 12. The substance oxidized is Zinc oxide and the copper oxide gets reduced to copper.

Ans 13. X = Calcium oxide (Quick lime), Combination reaction.

Ans 14. X= Fe, Y = Cu, Displacement reaction.

Ans 15. X = Mg, Y = MgO,

             2  Mg  +  O2   →  2MgO

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