# When carbon dioxide is passed through lime water, it turns milky. Why?

Jun 17, 2017

Because carbon dioxide forms an insoluble carbonate with lime water........

#### Explanation:

$\text{Limewater}$ is SATURATED $C a {\left(O H\right)}_{2} \left(a q\right)$ (and in fact calcium hydroxide is reasonably insoluble in aqueous solution). It undergoes the following reaction with carbon dioxide.....

$C a {\left(O H\right)}_{2} \left(a q\right) + C {O}_{2} \left(a q\right) \rightarrow C a C {O}_{3} \left(s\right) \downarrow + {H}_{2} O \left(l\right)$

Calcium carbonate precipitates as a milky white precipitate. And in fact this is a qualitative test for the presence of hydroxide salts.

When you do these reactions, you should pre-do a test with limewater, and a source of $C {O}_{2} \left(a q\right)$, so that you know the results of a positive test. And in fact the best source of $C {O}_{2} \left(a q\right)$ is sparkling mineral water, which is certainly saturated (probably supersaturated) with respect to $C {O}_{2}$.