Question

A student dropped a few pieces of marble in dilute hydrochloric acid contained in a test tube. The evolved gas was passed through lime water. What change would be
observed in lime water ? Write balanced chemical equations for both the changes observed.

(b) State the chemical property in each case on which the following uses of baking
soda are based :
1.as an antacid
2.as a constituent of baking powder.
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Answer 1

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(a) Marble used here is calcium carbonate - $\rm{CaCO_3}$. When a few pieces of marble are dropped in HCl, $\rm{CO_2}$ gas is evolved which when passed through lime water turns it milky due to the formation of insoluble calcium carbonate.

Chemical equations :-

• $\rm{CaCO_{3(s)}+HCl_{(aq)}\longrightarrow{}CaCl_{2(aq)}+H_2O_{(l)}+CO_{2(g)}}$

• $\rm{Ca(OH)_{2(aq)}+CO_{2(g)}\longrightarrow{}CaCO_{3(s)}+H_2O_{(l)}}$

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(b) (i) Use of baking soda as an antacid : Baking soda is a mild non-corossive basic salt. Hence, it is basic in nature and neutralizes the excess acid formed in the stomach during indigestion.

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(ii) Baking powder contains baking soda and a mild edible acid. On heating, $\rm{NaHCO_3}$ decomposes to give $\rm{CO_2}$ gas and sodium carbonate. The bitter taste of $\rm{Na_2CO_3}$ is neutralized by tartaric or citric acid and $\rm{CO_2}$ gas produced causes bread and cake to rise.