Question

Which of the following is true about $C{O}_2$?

• A. It is a colourless, odourless and tasteless gas
• B. It dissolves in water to form carbonic acid.
• C. It reduces to CO When passed through red hot coke
• D. It can turn lime water milky

Answer 1

Answer: (A), (B), (C), (D)

The correct options are
A

It is a colourless, odourless and tasteless gas

B

It dissolves in water to form carbonic acid.

C

It reduces to CO When passed through red hot coke

D

It can turn lime water milky

To answer this question, Let's look at the properties of $C{O}_2$.

$C{O}_2$ is a colourless, odourless and tasteless gas and is about 1.5 times heavier than air.

Unlike $C{O}_2$ it is not poisonous in nature. However, it does not support life and animals die in it for want of oxygen.

$C{O}_2$ dissolves in water to form carbonic acid (H${}_2$CO${}_3$) in very small amount, and most of the dissolved $C{O}_2$ remains loosely hydrated.
$C{O}_2$,therefore, is also known as carbonic anhydride.

$C{O}_{2\left(g\right)}+H_2$O${}_{\left(l\right)}$$\iff$H${}_2$CO${}_{3\left(l\right)}$

Carbonic acid is a weak dibasic acid, highly unstable and thus cannot be isolated. It forms two series of salts, i.e. hydrogen carbonates and carbonates derived from the anions

HCO${}_3^{\ominus }$ and CO${}_3^{2-}$

H${}_2$CO${}_{3\left(aq\right)}$ + H${}_2$O${}_{\left(l\right)}$ $\rightleftharpoons$ HCO${}_{3\left(aq\right)}^{\ominus }$ + H${}_3$O${}_{\left(aq\right)}^{\ominus }$

HCO${}_{3\left(aq\right)}^{\ominus }$ + H${}_2$O${}_{\left(l\right)}$ $\rightleftharpoons$ CO${}_{3\left(aq\right)}^{2-}$ + H${}_3$O${}_{\left(aq\right)}^{\ominus }$

When passed through red-hot coke, it is reduced to carbon monoxide, CO.

$C{O}_2$, when bubbled through lime water, Ca(OH)${}_2$, initially turns lime water milky, However, when CO${}_2$ is passed in excess, the solution
becomes clear or the milkiness disappears. On heating the clear solution, milkiness reappears. This is explained by the following reactions:

Ca(OH)${}_2$ + CO${}_2$$\to$CaCO${}_3$ $\downarrow$+ H${}_2$O

CaCO${}_3$ + CO${}_2$ + H${}_2$O $\to$ Ca(HCO${}_3$)${}_2$

Ca(HCO${}_3$)${}_{2\left(aq\right)}$$\to$CaCO${}_3$ $\downarrow$ + CO${}_2$ $\uparrow$ + H${}_2$O

Calcium

Carbonate

(Insoluble)

So, all the options (a), (b) (c) and(d) are True.