Question 13 Marks
Give a reason why $-$
$(a)$ Heating a platinum wire is a reversible change but, heating a magnesium wire is an irreversible change.
$(b)$ Addition of zinc nitrate to water is a physical change but, addition of zinc to dilute nitric acid is a chemical change.
$(c)$ Rusting of iron is a chemical change but, magnetization of iron is a physical change.
$(a)$ Heating a platinum wire is a reversible change but, heating a magnesium wire is an irreversible change.
$(b)$ Addition of zinc nitrate to water is a physical change but, addition of zinc to dilute nitric acid is a chemical change.
$(c)$ Rusting of iron is a chemical change but, magnetization of iron is a physical change.
Answer
View full question & answer→$(a)$ Heating a platinum wire, final product is again platinum wire $($as platinum is noble metal and does not react with air$)$, i.e. no change in composition and is physical change. Heating magnesium wire forms $\ce{MgO}$ i.e. composition changes a new substance white powder of magnesium oxide is formed is chemical change.
$(b)$ Zinc nitrate $($soluble in water$)$ disolves in water and is reversible. No change in composition takes place is a physical change where as zinc reacts with dil. nitric acid to produce $H_2$ gas which gets oxidized to water by nitric acid $($oxidizing agent$)$ and hence is a chemical change.
$(c)$ Rusting of iron is a chemical change as composition changes, reddish brown flaky powder is formed with new properties and is irreversible, permanent change but magnetization of iron is temporary and reversible change with no change in composition is a physical change.
$(b)$ Zinc nitrate $($soluble in water$)$ disolves in water and is reversible. No change in composition takes place is a physical change where as zinc reacts with dil. nitric acid to produce $H_2$ gas which gets oxidized to water by nitric acid $($oxidizing agent$)$ and hence is a chemical change.
$(c)$ Rusting of iron is a chemical change as composition changes, reddish brown flaky powder is formed with new properties and is irreversible, permanent change but magnetization of iron is temporary and reversible change with no change in composition is a physical change.