You are comparing a reaction that produces a chemical change and one that produces a physical change. What evidence could you use to determine which type of change is occurring?

Matter undergoes two types of changes viz: physical and chemical changes. A physical change, as the name suggests, involves a change in the physical nature of matter. A physical change does not change the chemical property of the substance involved. Since there is no chemical modification to the substance, no new substance is formed from a physical change. Hence, the substance only undergoes a change of state. For example, water (liquid) changing to ice (solid) is a physical change. Physical changes are usually reversibleb i.e. can be changed back to their previous form.

Chemical change is another type of change in which substances undergo a change in their chemical properties to form new substances. Changes like formation of gas bubbles, change in temperature, color change etc will be noticed during chemical changes. Examples of chemical changes are rusting, burning because they irreversibly change the chemical composition of the substances involved etc.

In a nutshell, in order to determine the type of change occuring in a reaction, one should look out for whether or not there is a change in nature of the substance leading to the formation or not of a new substance.