點解calcium chloride 會react with sodium carbonate?

此乃復分解反應(double decomposition reaction):-
例如離子化合物的水溶液反應:
AB + CD -------> AD + CB
如果AD 或 BC 任何一個不是水、沉澱或氣體，會在水溶液中失去離子性的話，那AD 和BC再會變回成 AB 和 CD

CaCl2(aq) + Na2CO3 (aq)-------> CaCO3 (s)+ 2NaCl (aq)
A B C D A D C B
因為CaCl2 (AB)、Na2CO3(CD)、NaCl (CB)都是水溶，但CaCO3(AD)是不溶，生成沉澱。這會使AD 和BC再不能變回成 AB 和 CD，這樣會使更多的AB和CD反應。

The answer is very very simple. In aqueous solution, calcium chloride exists as Ca^2+(aq) and Cl^-(aq) ions, while sodium carbonate exists as Na^+(aq) and CO3^2-(aq) ions. Ca^2+(aq) ions combine with CO3^-(aq) ions to form CaCO3(s) precipitate.
Ca^2+(aq) + CO3^2-(aq) -> CaCO3(s)

The first replier stated that precipitation is metathesis and "Metathesis is the phenomenon whereby two sounds that .......". Only ??? understand.

I also wonder about the ages of the two repliers who both use the term "double decomposition". When I was a secondary school student 35 years ago, my teacher told me that the term "double decomposition" was out-dated at that time.