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.

(b) Evolution of a gas: The chemical reaction between zinc and dilute sulphuric acid is characterised by the evolution of hydrogen gas.
$\mathrm{Zn}\left(\mathrm{}\mathit{s}\right)+{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathit{a}\mathit{q}\right)\to {\mathrm{ZnSO}}_4\left(\mathrm{}\mathit{s}\right)+{\mathrm{H}}_2\left(\mathit{g}\right)$
(c) Change in colour: The reaction between lead nitrate solution and potassium iodide solution.

In this reaction colour changes from colourless to yellow.