Change in temperature: Some chemical reactions are characterised by change of temperature such as addition of water to quicklime produces slaked lime and a lot of heat. As a result, the temperature of the reaction mixture increases. Thus, change in temperature indicates that a chemical reaction has taken place.$\mathrm{CaO} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{Ca}(\mathrm{OH})_2 + \text{ Heat }$(b) Evolution of a gas: The chemical reaction between zinc and dilute sulphuric acid is characterised by the evolution of hydrogen gas.$\mathrm{Zn}(\text{s}) + \mathrm{H}_2\mathrm{SO}_4(\text{aq}) \rightarrow \mathrm{ZnSO}_4(\text{s}) + \mathrm{H}_2(\text{g})$(c) Change in colour: The reaction between lead nitrate solution and potassium iodide solution.In this reaction colour changes from colourless to yellow.