Question

What would you observe in the following cases? (i) Sodium and potassium are placed in cold water. (ii) Burning magnesium is introduced into steam. (iii) Copper hydroxide is heated. (iv) Potassium nitrate is heated strongly. (v) Copper nitrate is heated strongly. (vi) Mercuric nitrate is heated strongly.

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Solution

(a) A violent reaction occurs when sodium and potassium are placed in cold water. They form the respective alkalis and liberate hydrogen gas. $2\mathrm{Na}+2{\mathrm{H}}_{2}\mathrm{O}\stackrel{}{\to }2\mathrm{NaOH}+{\mathrm{H}}_{2}\phantom{\rule{0ex}{0ex}}\mathrm{Sodium}\mathrm{Water}\mathrm{Sodium}\mathrm{Hydrogen}\mathrm{g}\mathrm{as}\phantom{\rule{0ex}{0ex}}\mathrm{hydroxide}\phantom{\rule{0ex}{0ex}}2\mathrm{K}+2{\mathrm{H}}_{2}\mathrm{O}\stackrel{}{\to }2\mathrm{KOH}+{\mathrm{H}}_{2}\phantom{\rule{0ex}{0ex}}\mathrm{Potassium}\mathrm{Water}\mathrm{Potassium}\mathrm{Hydrogen}\mathrm{g}\mathrm{as}\phantom{\rule{0ex}{0ex}}\mathrm{hydroxide}$ (b) Burning magnesium reacts with steam to form magnesium oxide and liberates hydrogen gas. $2\mathrm{Mg}+{\mathrm{H}}_{2}\mathrm{O}\stackrel{}{\to }2\mathrm{MgO}+{\mathrm{H}}_{2}\phantom{\rule{0ex}{0ex}}\mathrm{Magnesium}\mathrm{Steam}\mathrm{Magnesium}\mathrm{oxide}\mathrm{Hydrogen}\mathrm{g}\mathrm{as}$ (c) On heating, copper hydroxide decomposes to form cupric oxide and water. $\mathrm{Cu}\left(\mathrm{OH}{\right)}_{2}\underset{\mathrm{decomposes}}{\overset{\mathrm{heated}}{\to }}\mathrm{CuO}+{\mathrm{H}}_{2}\mathrm{O}\phantom{\rule{0ex}{0ex}}\mathrm{Copper}\mathrm{hydroxide}\mathrm{Cupric}\mathrm{oxide}\mathrm{W}\mathrm{ater}$ (d) On heating, potassium nitrate decomposes to form potassium nitrite and oxygen. $2{\mathrm{KNO}}_{3}\underset{\mathrm{decompose}}{\overset{\mathrm{heat}}{\to }}2{\mathrm{KNO}}_{2}+{\mathrm{O}}_{2}\phantom{\rule{0ex}{0ex}}\mathrm{Potassium}\mathrm{nitrate}\mathrm{Potassium}\mathrm{nitrite}\mathrm{Oxygen}\mathrm{gas}$ (e)When copper nitrate is heated strongly, it decomposes to form cupric oxide and liberates nitrogen dioxide and oxygen. $2\mathrm{Cu}\left({\mathrm{NO}}_{3}{\right)}_{2}\left(\mathrm{s}\right)\underset{\mathrm{decompose}}{\overset{\mathrm{heat}}{\to }}2\mathrm{CuO}\left(\mathrm{s}\right)+4{\mathrm{NO}}_{2}\left(\mathrm{g}\right)+{\mathrm{O}}_{2}\left(\mathrm{g}\right)\phantom{\rule{0ex}{0ex}}\mathrm{Copper}\mathrm{nitrate}\mathrm{Cupric}\mathrm{oxide}\mathrm{Nitrogen}\mathrm{dioxide}\mathrm{Oxygen}\mathrm{gas}$ (f) On heating, mercuric nitrate decomposes to form a metal, nitrogen dioxide and oxygen. $\mathrm{Hg}\left({\mathrm{NO}}_{3}{\right)}_{2}\left(\mathrm{s}\right)\underset{\mathrm{decompose}}{\overset{\mathrm{heat}}{\to }}\mathrm{Hg}\left(\mathrm{l}\right)+2{\mathrm{NO}}_{2}\left(\mathrm{g}\right)+{\mathrm{O}}_{2}\left(\mathrm{g}\right)\phantom{\rule{0ex}{0ex}}\mathrm{Mercuric}\mathrm{nitrate}\mathrm{Mercury}\mathrm{Nitrogen}\mathrm{dioxide}\mathrm{Oxygen}\mathrm{gas}$

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