question_answer

A chemical change may occur by one of the following changes:

(i) Evolution of gas

(ii) Change of colour

(iii) Formation of precipitate

(iv) Absorption or evolution of heat and light

(v) Change of smell

Give an example of each case.

Answer:

(i) Solid calcium carbonate on heating gives carbon dioxide gas. If this gas is passed through lime water, it turns milky.

\[a_{\text{Calcium}\,\,\text{carbonate}}^{CaC{{O}_{2}}}\,\xrightarrow[{}]{\Delta }\,_{\text{Calcium}\,\,\text{oxide}}^{CaO}\,+\,_{\text{Carbon}\,\,\text{dioxide}}^{C{{O}_{2}}}\]

(ii) When white lead nitrate \[Pb{{\left( N{{O}_{3}} \right)}_{2}}\] is heated, reddish coloured nitrogen dioxide \[\left( N{{O}_{2}} \right)\] is evolved and yellow coloured lead monoxide (\[PbO\]) is left behind.

\[2Pb{{\left( N{{O}_{3}} \right)}_{2}}\to 2PbO+4N{{O}_{2}}+{{O}_{2}}\]

(iii) When colourless solutions of silver nitrate \[(AgN{{O}_{3}})\] and sodium bromide \[\left( NaBr \right)\] are mixed, yellow coloured silver bromide \[\left( AgBr \right)\] gets precipitated.

\[AgN{{O}_{3}}(aq)+NaBr(aq)\to AgBr+NaN{{O}_{3}}(aq)\]

(iv) When water is added to quick lime\[(CaO)\], heat energy is evolved with the formation of calcium hydroxide, \[Ca{{(OH)}_{2}}\].

\[CaO+{{H}_{2}}O\to Ca{{(OH)}_{2}}+\text{Heat}\]

(v) When cooked food containing oils and fats is kept in the open for long, pleasant smell changes into foul smell and the food gets spoiled.