Allen is carrying out reactions in a lab. One of the products in all reactions is found to be either a precipitate, a gas, or water. What is common between all these reactions?

Here are some examples for those type of reactions.

Combustion reaction: CH4 (g) + 2 O2 (g) - - > CO2 (g) + 2 H2O (l)

Decomposition reaction: CaCO3 (s) - - - > CaO (s) + CO2 (g)

Double replacement: AgNO3 (aq) + NaCl (aq) - - - > AgCl (s) + NaNO3 (aq)

One common thing in all is that they are reactions. They have reactants to form new substances called product.