6. Support with an example that compound formation takes place with energy changes, using water formation and iron sulphide formation.

• The compound formation involves the absorption or release of energy in the form of heat, light, or electricity.
• The chemical reaction for water formation with the absorption of energy in the form of heat is given as:

$\underset{\mathrm{Metahne}}{{\mathrm{CH}}_4}+\underset{\mathrm{Oxygen}}{{\mathrm{O}}_2}\to \underset{\mathrm{Crabon}\mathrm{dioxide}}{{\mathrm{CO}}_2}+\underset{\mathrm{Water}}{{\mathrm{H}}_2\mathrm{O}}$

• Methane on burning (heat) in the presence of air gives carbon dioxide and water.

The chemical reaction for sulphide formation with the absorption of energy in the form of heat is given as:

$\underset{\mathrm{Sodium}\mathrm{hydrosulphide}}{\underbrace{\mathrm{NaHS}}}+\underset{\mathrm{Sodium}\mathrm{hydroxide}}{\mathrm{NaOH}}\to \underset{\mathrm{Sodium}\mathrm{sulphide}}{{\mathrm{Na}}_2\mathrm{S}}+\underset{\mathrm{Water}}{{\mathrm{H}}_2\mathrm{O}}$

The reaction of sodium hydrosulphide and sodium hydroxide or caustic soda gives sodium sulphide and water.