Question

give a reason:
a) reactivity of al decrease if it is dipped in HNO3.
b) zinc is used in galvanising iron and not copper.
c) brass and copper vessel aren't used to store curd and sour substance.

3mrks question

Answer 1

(i) $HNO_{3}$ is an oxidising agent. When aluminium is dipped in $HNO_{3}$ , an oxide layer (of aluminium) is formed on the surface of the metal, which prevents it from further reaction.
(ii) Na, Mg etc., metals are quite and present at the top of the reactivity series. Hence, their affinity with oxygen is high and their oxides cannot be reduced by carbon.
(iii) Ions of $NaCl$ cannot move to carry the charge in dry state due to electrostatic forces of attraction. But they are free in molten state and in aqueous solution to carry the charge and hence, $NaCl$ conducts electricity in molten state as well as in aqueous solution.
(iv) In galvanisation, a thin layer of zinc is formed over the iron articles by dipping them in molten zinc, which prevents iron from corrosion.
(v) These metals are quite reactive so, they cannot exist in free state and hence, they are found in nature in the form of their compounds like $NaCl$, $KCl$ , $CaCO_{3}$ , $MgO$, $MgCO_{3}$ etc.



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