Question

Which of the following turns lime water milky?
A. CO
B. Carbon dioxide
C. Sulphur trioxide
D. None of these

Answer 1

The formula of lime water is given by ${\text{Ca}}{\left( {OH} \right)_2}$ i.e. calcium hydroxide. When carbon dioxide reacts with lime water, limestone $\left[ {{\text{CaC}}{{\text{O}}_3}} \right]$ is formed which is an insoluble ppt. Due to this insoluble ppt., the solution appears milky.

Complete step by step answer:
Let us first recall the chemical formula of lime water.
Calcium hydroxide is commonly known as lime water and its formula is given by ${\text{Ca}}{\left( {OH} \right)_2}$ .
Carbon monoxide and sulphur trioxide usually don't react with lime water.
When carbon dioxide reacts with lime water, limestone $\left[ {{\text{CaC}}{{\text{O}}_3}} \right]$ is formed which is an insoluble ppt. Due to this insoluble ppt., the solution appears milky.
The reaction involved in this is given below:
${\text{Ca}}{\left( {OH} \right)_2}\left( {aq} \right) + {\text{C}}{{\text{O}}_2}\left( {\text{g}} \right) \to {\text{CaC}}{{\text{O}}_3}\left( s \right) + {{\text{H}}_2}{\text{O}}\left( l \right)$
When we add more carbon dioxide, the precipitate starts dissolving to form a colourless solution of calcium hydrogencarbonate i.e. ${\text{Ca}}{\left( {{\text{HC}}{{\text{O}}_3}} \right)_2}$ . The reaction involved in this process is given below:
${\text{CaC}}{{\text{O}}_3}\left( s \right) + {\text{C}}{{\text{O}}_2}\left( g \right) + {{\text{H}}_2}{\text{O}}\left( l \right) \to {\text{Ca}}{\left( {{\text{HC}}{{\text{O}}_3}} \right)_2}\left( {aq} \right)$
Therefore to get a milky white solution the amount of carbon dioxide added to lime water should be limited.
Hence option B is correct.

Note:
The reaction of lime water with carbon dioxide is used as a test for carbon dioxide. When it is bubbled through lime water i.e. if we create bubbles of carbon dioxide in lime water, the lime water changes its colour from colourless to milky.
Limestone formed is this reaction is a type of sedimentary rock which has numerous applications. It is often used as a building material or used as a component of concrete which is further used to build the base of roads. Due to its white colour, it is also used as white pigment in toothpastes and paints.