Question

(a) A student dropped a few pieces of marble in dilute hydrochloric acid contained in
a tast 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:
(i) as an antacid (ii) as a constitunt of baking powder.

Answer 1

When a piece of marble `(CaCO_(3))` is dropped in dil `HCl CO_(2)` gas will be evolved
which turns lime water milky . When carbon dioxide gas is passed through lime
water, it turns milky due to the formation of calcium carbonate which is insoluble in
water.
`underset("Limestone")(CaCO_(3)(s)) + underset("dil")(3HCl(l))to CaCl_(2)(s) + H_(2)O + CO_(2)`
`underset("Lime water ")(Ca(OH)_(2)) + CO_(2) to underset("White ppt")(CaCO_(3) darr)+ H_(2)O`
(b) (i) Being alkaline, sodium hydrogen carbonate (baking sode) neutralises the excess
acid present in the stomach and relieves indigestion.
(ii) Baking power is a mixture of baking sode and mild edible acid such as
tartaric acid . when baking powder mixes with water (present in dough made for
baking cake or bread), then sodium hydrogen carbonate reacts with tartaric acid
to evolve `CO_(2)` gas (on heating).
`NaHCO_(3) (aq) + underset("from tartaric acid")underset("Hydrogen ions")(H^(+)(aq)) to Na^(+) (aq) + CO_(2) (g) + H_(2)O(l)`.