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

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)`.

Step-by-step 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)`.