Question

Solution X mixed with magnesium chloride solution to form a white precipitate. The white precipitate obtained and reacted with dil HCL to give colourless gas bubbles turn lime water milky
What is solution X?

Answer 1

Answer:

Solution X might be a solution of carbonate ions $\text{CO}_3^{2-}\;(aq)$.

The question doesn't tell much about the positive ion in solution X. The ion itself shall not react with carbonate ions $\text{CO}_3^{2-}\;(aq)$ to form a precipitate. For that, cations from group 1 metals will work. For example, X can be a solution of sodium carbonate, which contain a large number of sodium ions $\text{Na}^{+}\;(aq)$.

Explanation:

Start with the first observation:

"Solution X mixed with magnesium chloride solution to form a white precipitate."

A solution of magnesium chloride $\text{MgCl}_2\;(aq)$ contains both

• $\text{Mg}^{2+}\;(aq)$ ions and
• $\text{Cl}^{-}\;(aq)$ ions.

Both may react to form a white precipitate.

$\text{Mg}^{2+}\;(aq)$ ions:

• $\text{Mg}^{2+}\;(aq)$ might react with hydroxide ions $\text{OH}^{-}\;(aq)$ to form the white-colored, weakly-soluble base magnesium hydroxide $\text{Mg}(\text{OH})_2\;(s)$.
• $\text{Mg}^{2+}\;(aq)$ might also react with carbonate ions $\text{CO}_3^{2-}$ to form the white-colored, insoluble salt magnesium carbonate, $\text{MgCO}_3\;(s)$.

$\text{Cl}^{-}\;(aq)$ ions:

• $\text{Cl}^{-}\;(aq)$ ions might react with silver ions $\text{Ag}^{+}\;(aq)$ to form the white-yellowish insoluble salt silver chloride $\text{AgCl}\;(s)$.

Second observation:

"The white precipitate obtained and reacted with [dilute] HCl to give [colorless] gas bubbles."

• Indeed $\text{Mg}(\text{OH})_2\;(s)$ will react with dilute HCl, but no gas will be formed: $\text{Mg}(\text{OH})_2\;(s) +2\;\text{HCl}\;(aq)\to\text{MgCl}_2\;(aq) + 2\;\text{H}_2\text{O}\;(l)$.
• $\text{MgCO}_3\;(s)$ reacts with dilute HCl to produce the colorless gas carbon dioxide $\text{CO}_2\;(g)$: $\text{MgCO}_3\;(s) + 2\;\text{HCl}\;(aq) \to\text{MgCl}_2\;(aq)+\text{H}_2\text{O}\;(l) +\text{CO}_2\;(g)$.
• Silver chloride $\text{AgCl}\;(s)$ does not react with dilute HCl.

The carbon dioxide gas from the reaction between $\text{MgCO}_3\;(s)$ and dilute HCl reacts with lime water (saturated calcium hydroxide $\text{Ca}(\text{OH})_2\;(aq)$ solution in water) to form the white-colored, insoluble salt calcium carbonate $\text{CaCO}_3\;(s)$. The $\text{CaCO}_3\;(s)$ precipitate will turn the lime water milky.

In summary,

$\rm \underbrace{\text{Na}_2\text{CO}_3\;(\rm aq)}_{\text{White Precipitate}} + \text{MgCl}_2 \;(aq) \to 2\;\text{NaCl}\;(aq) + \underbrace{\text{MgCO}_3\;(\rm s)}_{\text{White Precipitate}$.

$\rm \underbrace{\text{MgCO}_3\;(\rm s)}_{\text{From Solution X}} + \text{HCl} \;(aq) \to \text{MgCl}_2\;(aq) + \rm H_2O\;( aq) + \underbrace{{\rm CO_2}\;(g)}_{\rm Colorless\;Gas}$.

$\rm Ca(OH)_2\;(aq)+CO_2\;(g)\to \underbrace{\rm CaCO_3\;(s)}_{\begin{aligned}&\small\text{Turns Lime}&\\[-0.5em]&\small\text{Water Milky}&\end{aligned}} + H_2O\;(l)$.