Question

What happens when ${\mathrm{CO}}_2$ gas is passed through lime water and why does it disappear on passing excess ${\mathrm{CO}}_2$?

Answer 1

Reaction of Carbon dioxide gas with lime water

• Formation of Calcium carbonate $\left({\mathrm{CaCO}}_3\right)$ is observed when carbon dioxide $\left({\mathrm{CO}}_2\right)$ gas is passed through lime water $\left(\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\right)$.
• Calcium carbonate is responsible for the milkiness of the solution.
• The reaction is as follows:

$\underset{\left(\mathrm{Carbon}\mathrm{dioxide}\right)}{{\mathrm{CO}}_2\left(\mathrm{g}\right)}+\underset{\left(\mathrm{Calcium}\mathrm{hydroxide}\right)}{\mathrm{Ca}{\left(\mathrm{OH}\right)}_2\left(\mathrm{aq}\right)}\to \underset{\left(\mathrm{Calcium}\mathrm{carbonate}\right)}{{\mathrm{CaCO}}_3\downarrow }+\underset{\left(\mathrm{Water}\right)}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}$

Lime water milkiness

• The formed milkiness vanishes when an excess of carbon dioxide is passed through lime water.
• This is because calcium bicarbonate $\left(\mathrm{Ca}{\left({\mathrm{HCO}}_3\right)}_2\right)$, which is colorless and soluble in water, forms.
• The reaction is as follows:

$\underset{\left(\mathrm{Calcium}\mathrm{carbonate}\right)}{{\mathrm{CaCO}}_3\downarrow }+\underset{\left(\mathrm{Water}\right)}{{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)}+\underset{\left(\mathrm{Excess}\right)}{\underset{\left(\mathrm{Carbon}\mathrm{dioxide}\right)}{{\mathrm{CO}}_2\left(\mathrm{g}\right)}}\to \underset{\left(\mathrm{Calcium}\mathrm{bicarbonate}\right)}{\mathrm{Ca}{\left({\mathrm{HCO}}_3\right)}_2\left(\mathrm{aq}\right)}$