Identify that chemical reactions and phase changes involve energy changes, commonly observable as changes in the temperature of the surroundings and/or the emission of light.

When a student adds zinc powder to copper(II) sulphate solution, the temperature of the mixture increases significantly and the solution changes colour from blue to colourless. (a) Identify what type of energy change occurs in this reaction. (b) Explain how the temperature change indicates this energy change.

When potassium metal is added to water, the reaction vessel becomes very hot and a flame appears above the surface. Which of the following statements best explains these observations?

When magnesium metal reacts with oxygen gas to form magnesium oxide, the temperature of the surrounding air increases significantly and light is emitted. Which statement correctly identifies the type of energy change occurring?