Question

A colourless, acidic gas turns lime water milky is:
This question has multiple correct options
A: carbon monoxide
B. hydrogen sulphide
C. carbon dioxide
D. sulphur dioxide

Answer 1

Lime water is known as aqueous solution of calcium hydroxide. It is basic in nature and reacts with acidic gases.Lime Water is clear and colorless, with a slight earthy smell and an astringent/bitter taste.

Complete step by step answer:
Lime water is the common name used for the aqueous solution of $Ca{\left( {OH} \right)_2}$. The gases which are acidic in nature react with the basic hydroxide solution.
Acidic gases combine with water to produce acids. For example gases like carbon dioxide and sulphur dioxide react with water to generate carbonic acid and sulphuric acid.
When such gases are allowed to come in contact with lime water turns it milky due to formation of insoluble carbonates. Actually this method is usually used for testing carbon dioxide and sulphur dioxide as both the gases turn lime water milky.
The gases are colorless but the detection of gases is done by passing into limewater. The insoluble suspension of calcium carbonate formation can be shown as:
$Ca{\left( {OH} \right)_2}\left( {aq} \right) + {\text{ }}C{O_2}\left( g \right) \to {\text{ }}CaC{O_3}\left( s \right) + {\text{ }}{H_2}O\left( l \right)$
When this carbonate is treated with excess $C{O_2}$ , it leads to formation of calcium bicarbonate. The milkiness disappears as calcium bicarbonate formed is water soluble.
$CaC{O_3}\left( s \right) + {\text{ }}{H_2}O\left( l \right) + {\text{ }}C{O_2}\left( g \right) \to Ca{\left( {HC{O_3}} \right)_2}\left( {aq} \right)$
Similarly in case of sulphur dioxide, lime water turns milky due to formation of insoluble calcium sulfite.
$Ca{\left( {OH} \right)_2}\left( {aq} \right) + {\text{ }}S{O_2}\left( g \right) \to {\text{ }}CaS{O_3}\left( s \right) + {\text{ }}{H_2}O\left( l \right)$
In this case the milkiness due to formation of calcium sulphite doesn't disappear instantly with excess $S{O_2}$ rather it disappears on extended passing of gas due to formation of calcium hydrogen sulphite.
$CaS{O_3}\left( s \right) + {\text{ }}{H_2}O\left( l \right) + {\text{ }}S{O_2}\left( g \right) \to Ca{\left( {HS{O_3}} \right)_2}\left( {aq} \right)$

Hence the correct option is C and D.

Note: But lime water cannot differentiate between the two gases. These gases are normally differentiated using following tests:
$C{O_2}$ is odourless gas while $S{O_2}$ is pungent smelling gas.
$S{O_2}$ gas turns potassium dichromate paper green while $C{O_2}$ does not undergo any such change with the paper absorbed with potassium dichromate.