What evidence can be observed during a chemical reaction? When a chemical reaction occurs, an observable change can often be seen during the progress of the reaction. Once the reaction is complete, a new substance or substances will be formed. The new substances will have different properties than the original substances. In this virtual investigation you will combine two substances and make observations before, during, and after any reaction. Not all substances will react, but there will be some observable evidence for any reaction that takes place. Of the chemical reactions that occur, you will see the different types of reactions: synthesis, decomposition, single replacement, and double replacement.