1 Add AlCl3 to cola to test carbonic acid. Adding BaCl3 without precipitation proves that weak acid can't put down strong acid.

2.3 is the same as that on the second floor.

Adding water to cola, if there is a lot of gas, proves that the decomposition of carbonic acid into water is a reversible reaction.

The above 4 can also be proved by introducing CO2.

That's all I can think of.