Q.(a) What happens when CO_2 gas is passed through lime water ?

Carbon monoxide and carbon dioxide are carbon's two important oxides, differing sharply in both bonding and behaviour. CO has a formal triple bond (one $\sigma$, two $\pi$) with a lone pair remaining on carbon ($:C \equiv O:$); this lone pair makes CO both an electron-pair donor (forming metal carbonyls) and a powerful reducing agent, capable of reducing nearly all metal oxides except those of the most electropositive metals — the basis of its industrial use in metal extraction. CO's severe toxicity arises because it binds haemoglobin roughly 300 times more strongly than oxygen does, blocking the blood's oxygen-carrying capacity. $CO_2$, in contrast, has a linear, $sp$-hybridised structure with two equal C–O bonds and zero net dipole moment (best described by resonance structures), and is not itself poisonous — but as an acidic, weakly-soluble gas it forms carbonic acid, $H_2CO_3$, in water, which ionises in two steps to give the bicarbonate/carbonate ions ($HCO_3^-$, $CO_3^{2-}$) whose own resonance-stabilised structures reflect delocalised C–O bonding ac …