i. When copper powder is heated in air, it combines with oxygen to form copper (II) oxide. Here, the reddish brown coloured copper changes to black copper (II) oxide. Thus, change in colour indicates that a chemical reaction has taken place.
ii. Sodium reacts with water vigorously to form sodium hydroxide and hydrogen gas releasing a large amount of heat. As a result, the temperature of the reaction mixture increases. Thus, change in temperature indicates that a chemical reaction has taken place.
