Answer the questions.[Che-3M]

Question 13 Marks

Pratyush took sulphur powder on a spatula and heated it. He collected the gas evolved by inverting a test tube over it.
What will be the action of gas on
i. Dry litmus paper?
ii. Moist litmus paper?
Write a balanced chemical equation for the reaction taking place.

Answer

The gas is sulphur-dioxide $\left(\mathrm{SO}_{2}\right)$
i. It will not react with dry litmus paper.
ii. The gas will bleach moist litmus paper.
The balanced chemical equation is
$\mathrm{S}+\mathrm{O}_{2} \xrightarrow{\text { heat }} \mathrm{SO}_{2}$

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Question 23 Marks

Explain the following in terms of gain or loss of oxygen with two examples each:
a. Oxidation
b. Reduction

Answer

Oxidation- Addition of oxygen or removal of hydrogen in a chemical reaction is called oxidation reaction. For example:
$2 \mathrm{Cu}+\mathrm{O}_{2} \rightarrow 2 \mathrm{CuO}$
$4 \mathrm{Al}+3 \mathrm{O}_{2} \rightarrow 2 \mathrm{Al}_{2} \mathrm{O}_{3}$
Reduction- Addition of hydrogen or removal of oxygen in a chemical reaction is called reduction reaction. For example:
$\mathrm{CuO}+\mathrm{H}_{2} \rightarrow \mathrm{Cu}+\mathrm{H}_{2} \mathrm{O} $
$ \mathrm{H}_{2} \mathrm{~S}+\mathrm{Cl}_{2} \rightarrow 2 \mathrm{HCl}+\mathrm{S}$

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Question 33 Marks

Silver chloride kept in a china dish turns grey in sunlight.
a. Write the colour of silver chloride when it was kept in the china dish.
b. Name the type of chemical reaction taking place and write the chemical equation for the reaction.
c. State one use of the reaction. Name one more chemical which can be used for the same purpose.

Answer

a. White colour
b. The chemical reaction is as:
$2 \mathrm{AgCl}(\mathrm{~s}) \longrightarrow 2 \mathrm{Ag}(\mathrm{~s})+\mathrm{Cl}_{2}(\mathrm{~g})$
This is a decomposition chemical reaction.
c. One major application of decomposition reactions is in the extraction of metals from their ores. For example, zinc can be obtained from calamine by subjecting it to a decomposition reaction. In a similar manner, sodium can be obtained from sodium chloride ( NaCl ).

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Science Answer the questions.[Che-3M]

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