Baking Soda When Heated Lab Report

During this experiment, I learned what happens to baking soda when heated. In the beginning, I hypothesized that a physical change would occur, and that the substance would remain the same, but change in form. I thought this because I thought the heat would change the baking soda into a new form. In reality, the heated baking soda hardened and was unable to dissolve unlike its unheated counterpart. I believe that it created a chemical reaction because the results of the tea test were different. In one tube, the tea mixed with the cool baking soda to create a clear light brown mixture, in the heated tube, the baking soda was hardened and created a murky brown-green mixture. Also, while heating the tube, a gas filled the collecting bottle.
Over all i conclude that the lab created a chemical change in the baking soda once it was heated.