A chemical change can be confirmed by any of the following observations:
Change in temperature
Evolution of gas
For example: Calcium oxide reacts vigorously with water to produce calcium hydroxide. During this process, a large amount of heat is also evolved, which increases the temperature of the system. This confirms that a chemical reaction has taken place. Also, when calcium carbonate is heated, it decomposes to form calcium oxide and carbon dioxide. In this reaction, calcium carbonate breaks down to form calcium oxide and carbon dioxide. Here, evolution of the gas (carbon dioxide) confirms that a chemical reaction has taken place.