Which is a clue that a chemical change has taken place?

Well, there are four clues to a chemical change. The first clue is a transfer of energy. For example, energy is transferred from methane in natural gas to cook food. The second clue is a change in color. Food browning while being cooked is an example of this. The third clue is the production of a gas. Some cleaning chemicals create bubbles upon contact with soap scum. This is evidence of a chemical change. The last clue is the formation of a precipitate. The ring of soap scum in a bathtub is a precipitate, and evidence of a chemical change.