Write the characteristics of chemical reactions.

1. Evolution of gas:

• The chemical reaction between zinc and dilute sulphuric acid is characterized by the evolution of hydrogen gas.
• $\mathrm{Zn}\left(\mathrm{s}\right)+{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)\to {\mathrm{ZnSO}}_4\left(\mathrm{aq}\right)+{\mathrm{H}}_2\left(\mathrm{g}\right)$

2. Change in colour:

• The reaction between the lead nitrate solution and potassium iodide solution.
• $\mathrm{Pb}{\left({\mathrm{NO}}_3\right)}_2\left(\mathrm{aq}\right)+2\mathrm{KI}\to {\mathrm{PbI}}_2\left(\mathrm{s}\right)+2{\mathrm{KNO}}_3\left(\mathrm{aq}\right)$
• In this reaction colour changes from colourless to yellow.

3. Formation of precipitate:

• The action of Barium Chloride on sodium sulphate.
• ${\mathrm{BaCl}}_2\left(\mathrm{aq}\right)+{\mathrm{Na}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)\to {\mathrm{BaSO}}_4\left(\mathrm{s}\right)+2\mathrm{KNaCl}\left(\mathrm{aq}\right)$

4. Change in temperature:

• The action of dilute sulphuric acid on zinc:
• $\mathrm{Zn}\left(\mathrm{s}\right)+{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right)\to {\mathrm{ZnSO}}_4\left(\mathrm{aq}\right)+{\mathrm{H}}_2\left(\mathrm{g}\right)$
• In this reaction, heat is evolved.

5. Change in the state:

• When the wax is burnt in the form of the wax candle then water and carbon dioxide are formed.
• Wax is solid, water is a liquid whereas carbon dioxide is a gas.