[esthermay]

In my chemistry class we recently did an experiment on decomposition reactions.

Each student heated Copper Carbonate in a testube with a stopper and tube into a testube with limewater.
As the copper carbonate heated CO₂ was given off and this was seen when the limewater turned milky.

However everyones limewater immediantly went clear again. Do you know why this was? Our chemistry teacher couldn't explain it.
When we poured some universal indicator into the limewater it went greeny/yellow indicating it was neutral to slightly acidic.
e.

[Borek]

In the presence of excess CO₂, bicarbonate is produced, much more soluble than calcium carbonate. This process is responsible for karst landscapes sculpting.

IMHO if your chemistry teacher doesn't know it, you should look for another one.