One big indication that a chemical change has occurred is a color change. Many times a chemical reaction will start as relatively colorless or only lightly colored (often times yellow). During the course of a chemical reaction, reactions will often turn a darker color (dark yellow, brown, even black), thus indicating a chemical change. Another indicator of a chemical change can be smell. Some chemical reactions produce a gas either as a main product or as a byproduct. It this gas has a particular odor like ammonia or a sulfur compound, this is also an indicator of a chemical change (chemical bonds are broken and reformed to produce the gas). Finally, change in pH can often indicate a chemical change. pH is the measure of the acidity of a chemical or a mixture, so if an acid is produced or consumed as part of a reaction, this will change the pH of the chemical reaction mixture.