# What is the reaction of "CO"_2(g) with "Ca"("OH")_2(aq)?

Jun 2, 2017

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

#### Explanation:

The reaction is NOT so obvious. You generally get a fine precipitate of $C a C {O}_{3} \left(s\right)$, the which you might not notice. The solution? Get a saturated $C {O}_{2}$ solution (and this is simply sparkling, i.e. carbonated water that you might otherwise drink), and add this to lime water (which is saturated $C a {\left(O H\right)}_{2} \left(a q\right)$). You should see a dense white precipitate of $C a C {O}_{3}$. This is a control, so that you can appreciate the result of a positive test,

Jun 2, 2017

See below:

#### Explanation:

When $\textsf{C {O}_{2}}$ gas is bubbled through limewater you initially get a milky precipitate of insoluble calcium carbonate: This is an acid - base reaction and is used as a qualitative test for $\textsf{C {O}_{2}}$:

sf(CO_2(g)+Ca(OH)_(2)(aq)rarrCaCO_3(s)+H_2O(l)

If you continue to pass $\textsf{C {O}_{2}}$ through the suspension the acidic nature of the gas causes the precipitate to dissolve rendering the solution clear again due to the formation of soluble calcium hydrogen carbonate:

Carbonic acid is formed first:

$\textsf{C {O}_{2} + {H}_{2} O \left(l\right) r i g h t \le f t h a r p \infty n s {H}_{2} C {O}_{3} \left(a q\right)}$

This then reacts with the calcium carbonate in an acid / base reaction:

$\textsf{{H}_{2} C {O}_{3} \left(a q\right) + C a C {O}_{3} \left(s\right) r i g h t \le f t h a r p \infty n s C a {\left(H C {O}_{3}\right)}_{2} \left(a q\right)}$

This process occurs in nature. Rain water is naturally acidic due to the presence of dissolved $\textsf{C {O}_{2}}$.

Minerals such as limestone, which is a form of calcium carbonate, are subjected to chemical weathering. This results in cave formation in limestone areas.

You will notice that the reactions are reversible. As the solution naturally evaporates this can lead to a deposit of solid calcium carbonate resulting in the formation of stalagtites and stalagmites: Water containing dissolved $\textsf{C {a}^{2 +}}$ and $\textsf{M {g}^{2 +}}$ ions is referred to as "hard water".

A downside of this is that it forms an insoluble "scum" with soap - based detergents which is wasteful.

On the plus side hard water is good for making beer.