Kalkwasser sample

The lime water sample is understood to be a detection reaction with which carbon dioxide (CO ₂ ) or calcium hydroxide (Ca (OH) ₂ ) can be detected.

Dissolving calcium oxide in water produces lime water (Ca (OH) _{2 ( aq )} ):

${\displaystyle \mathrm {CaO_{(s)}+H_{2}O_{(l)}\rightarrow Ca(OH)_{2(aq)}} }$

When carbon dioxide is introduced, the solution becomes cloudy due to the formation of calcium carbonate . Water is formed as a by-product of the reaction.

${\displaystyle \mathrm {Ca(OH)_{2(aq)}+CO_{2(g)}\rightarrow CaCO_{3(s)}+H_{2}O_{(l)}} }$

The reaction can also be observed when lime water is kept in an unsealed vessel for a long time. The liquid then withdraws CO ₂ from the air and becomes cloudy. If CO ₂ is passed through for a long time , the calcium carbonate can also dissolve again with the formation of soluble calcium hydrogen carbonate :

${\displaystyle \mathrm {CaCO_{3(s)}+H_{2}O+CO_{2(g)}\rightleftharpoons Ca^{2+}+2HCO_{3}^{-}} }$

When the solution is heated, the reaction is reversed and calcium carbonate precipitates again.

Sulfur dioxide also forms a white precipitate of calcium sulfite with lime water :

${\displaystyle \mathrm {Ca(OH)_{2(aq)}+SO_{2(g)}\rightarrow CaSO_{3(s)}+H_{2}O_{(l)}} }$

This precipitate is slowly oxidized to calcium sulfate in air .

Individual evidence

1. ↑ Entry on calcium hydroxide. In: Römpp Online . Georg Thieme Verlag, accessed on December 29, 2014.
2. ^ E. Schweda: Jander / Blasius: Inorganic Chemistry I - Introduction & Qualitative Analysis . 17th edition. Hirzel, 2012, ISBN 978-3-7776-2134-0 , pp. 253 f .