The blue substance you noticed forming on one terminal is copper hydroxycarbonate. It forms from the copper in the battery and the carbonate in the baking soda that was left over. It is the same stuff that turns copper statues (like the Statue of Liberty) a blue-green color.

The chemical reaction that occurs to make this compound is

4Cu (s) + 6H2O (l) + 2CO3 2- (aq) –> 2Cu2(OH)2CO3 (aq) + O2 (g) +
4H2 (g)

Half of that chemical reaction occurs on the positive wire, and the other half occurs on the negative wire. You will find that the positive wire is the one with the copper hydroxycarbonate. When you get to Module 16, you will learn how half of the reaction can occur at one wire and the other half at the other.