Question

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?

Answer 1

All decomposition reactions. 
Hope this helps!(: 

Answer 2

Answer:

Decomposition reaction.

Explanation:

There is formation of a precipitate or evolution of gas or formation of water generally during decomposition reactions.

They involve breaking of a compound into two or more new compounds or one or more element.

Example:

Decomposition of calcium carbonate.

$CaCO_{3}--->CaO(s)+CO_{2}(g)$

$H_{2}SO_{3}--->H_{2}O+SO_{2}(g)$