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Practice each question, then open the answer or worked solution.
Give two examples for each of the following cases:
(a) Physical changes which are reversible.
(b) Physical changes which are not reversible.
(c) Chemical changes.
Ans: Physical changes which are reversible:
i) Melting of ice.
ii) Folding of clothes.
Ans: Physical changes which are not reversible:
i) Breaking a bottle.
ii) Cutting of trees.
Ans: Chemical changes:
i) Burning wood.
ii) Digesting food
Give an example of a chemical reaction for each of the following situations:
(a) A change in colour is observed.
(b) A gas has evolved.
(c) Sound is produced.
Ans: The chemical reaction between 'copper sulphate' solution and iron metal results in a colour shift. Because of the creation of iron sulphate, the blue colour of 'copper sulphate' changes to vivid green in this chemical reaction. The response is,
$\mathop {Copper\;sulphate}\limits_{\left( {blue} \right)} + \mathop {Iron}\limits_{\left( {grey} \right)} \to \;\mathop {Iron\;sulphate}\limits_{\left( {bright/light\;green} \right)} \; + \;\mathop {Copper}\limits_{\left( {brown} \right)} $
Ans: When 'baking soda' and 'vinegar' are mixed, a chemical reaction occurs. Following this, bubbles of 'carbon dioxide' gas, combined with another material, are generated. The response is,
$\mathop {Baking\;soda}\limits_{\left( {Sodium\;hydrogen\;carbonate} \right)} + \mathop {Vinegar}\limits_{\left( {acetic\;acid} \right)} \to Sodium\;acetate\; + \;Carbon\;dioxide\; + \;water\;\;\;$
Ans: Burning crackers also generate a loud sound.
If you leave a piece of iron in the open for a few days, it acquires a film of brownish substance, called rust.
(a) Do you think rust is different from iron?
(b) Can you change rust back into iron by some simple method?
(c) Do you think the formation of rust from iron is a chemical change?
(d) Give two other examples of a similar type of change.
Ans: Rust is brown dust made up of 'iron oxide.' Rust, on the other hand, is not the same as iron.
Ans: No, rusting is a chemical reaction that is irreversible.
Ans: In the presence of moisture, iron rusts and combines with oxygen. This results in the formation of a new chemical known as 'iron oxide.' Rust is another name for iron oxide.
Ans: The following are two examples:
(i) Production of 'magnesium oxide' by burning' magnesium ribbon'.
(ii) The formation of curd from milk.
A student took a solution of copper sulphate in a beaker and put a clean iron nail into it and left it for about an hour.
(a) What changes do you expect?
(b) Are these changes chemical in nature?
(c) Write a word equation for the chemical change, if any.
Ans: The following are the expected changes based on the supplied statement:
(i) The colour changes from blue to green.
(ii) A brown colour deposit is discovered on an iron nail (surface).
Ans: Yes, these alterations are chemical in nature. The reason for this is that the reaction produces 'iron sulphate' and copper.
Ans: The reaction is:
$\mathop {Copper\;sulphate}\limits_{\left( {Blue} \right)} + \;\mathop {Iron}\limits_{\left( {Shining\;white} \right)} \to \;\mathop {Iron\;sulphate}\limits_{\left( {Green} \right)} + \;\mathop {Copper}\limits_{\left( {Brown} \right)} $
$\underset{\left( Blue \right)}{\mathop{Copper~sulphate}}\,~+~\underset{\left( Shining~white \right)}{\mathop{Iron}}\,~\to ~\underset{\left( Green \right)}{\mathop{Iron~sulphate}}\,~+~\underset{\left( Brown \right)}{\mathop{Copper}}\,~$