A) Ca(OH)2 + CO2 —> CaCO3 + H2O
B) when Ca(OH)2 is reacted with CO2, the CaCO3 produced is a precipitate which turns the solution milky
Explanation:
When the student mixed the solution sodium carbonate with solution of copper(II) sulfate ; Copper Hydroxocarbonate , sodium sulfate and carbon dioxide gas was obtained as a products.
The balanced chemical reaction

Where:
= Copper(II) Hydroxocarbonate
= Sodium carbonate
= Copper(II) sulfate
= Sodium sulfate
= Carbon-dioxide
we can do soo by noticing any colour change, or by the formation of an insoluble precipitate or by the evolution of a gas.