(a) What is meant by a chemical reaction? Explain with the help of an example.

(b) Give one example each of a chemical reaction characterised by:

(i) evolution of a gas

(ii) change in color

(iii) formation of a precipitate

(iv) change in temperature

(v) change in state

(a) A chemical reaction is the process in which new substances with new properties are formed.

For example: When magnesium ribbon is heated, it burns in air to form a white powder called magnesium oxide.
$2Mg\ ( s) \ +\ O_{2}( g) \ \rightarrow \ 2MgO\ ( s)$

(b) (i) Evolution of gas: The chemical reaction between zinc and dilute hydrochloric acid gives off hydrogen gas.
$Zn\ ( s) \ +\ 2HCl\ ( aq) \ \rightarrow \ ZnCl_{2} \ ( aq) \ +\ H_{2} \ ( g)$

(ii) Change in colour: The chemical reaction between citric acid and purple coloured potassium permanganate solution is characterised by a change in colour (from purple to colourless).

(iii)  formation of a precipitate: the chemical reaction between potassium iodide and lead nitrate is characterised by the formation of a yellow precipitate of lead iodide.
$2KI\ ( aq) \ +\ Pb( NO_{3})_{2\ }( aq) \ \rightarrow \ PbI_{2} \ ( s) \ +\ 2KNO_{3} \ ( aq)$

(iv)  Change in temperature: the reaction between quick lime and water to form slaked lime is characterised by a change in temperature.
$CaO\ ( s) \ +\ H_{2} O\ ( l) \ \rightarrow \ Ca( OH)_{2} \ ( aq) \ +\ Heat$
   Calcium oxide +  Water  → Calcium hydroxide

(v) Change in the state: when the wax is burned, then water and carbon dioxide are formed. Wax is a solid; water is a liquid whereas carbon dioxide is a gas.
$Wax( s) \ +\ O_{2}( g) \ \rightarrow CO_{2}( g) \ +\ H_{2} O( l) \ $

 

Updated on: 10-Oct-2022

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