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CBSE 10th Standard Science Subject Chemical Reactions and Equations HOT Questions 2 Mark Questions 2021

Published on: 29/05/2021

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Questions:

Generic 2

5 x 2 = 10

Why are certain reagents like silver bromide stored in dark bottles in the labs?

Give two examples in which reactants react to show combination reaction and a new product is formed. This new product decomposes to give the initial reactants.
\( \text { e.g., } X+Y \stackrel{\text { combination }}{\longrightarrow} Z \ \ \ \ \ Z \stackrel{\text { decomposition }}{\longrightarrow} X+Y \)

Give one example of a combination reaction in which an element combines with a compound to give you a new compound

List four changes which help us to determine whether a chemical reaction has taken place

A metal X available in the form of ribbon burns in air with a dazzling white light once heated to its ignition temperature. It leaves a white powder Y which dissolves in water. The solution so formed turns red litmus blue. Identify X and Y and write a balanced chemical equation of the reaction.
(a) Identify 'X' and 'Y'.
(b) Write a balanced chemical equation for this reaction.

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Answers Key:

Generic 2

5 x 2 = 10

Reagents/chemicals like silver bromide decompose when exposed to light. Hence, they are kept in dark bottles in labs, to prevent exposure to light and their decomposition.

The reactions in which reactants combine to form products and then again decomposes to give initial reactants are called reversible reactions.
(i) N₂ + 3H₂ \(\longrightarrow\) 2NH₃ (ii) CaO + CO₂\(\longrightarrow\) CaCO₃
NH₃\(\longrightarrow\)N₂ + 3H₂ CaCO₃ \(\longrightarrow\) CaO + CO₂

\(\begin{equation} \mathrm{O}_{2}+2 \mathrm{SO}_{2} \ 3 / 4 ® \ 2 \mathrm{SO}_{3} \ \text { or } 2 \ \mathrm{Mg}+\ \mathrm{O}_{2} 3 / 4 \text { ® } 2 \mathrm{MgO} \end{equation}\)

(a) Change in state,
(b) Change in colour,
(c) Change in temperature,
(d) Evolution of a gas

(a) When magnesium is heated in oxygen of air, it leaves white powder of magnesium oxide which dissolves in water. The magnesium oxide being alkaline in nature turns red litmus to blue. Also, the metal in ribbon form is Magnesium [Mg]. Thus 'X' is identified ad magnesium and 'Y' is identified as magnesium oxide.
(b)The chemical reaction is:
2Mg(s)+O₂(g)\(\rightarrow\)2MgO