Question

Explain the formation of scum when hard water is treated with soap. [3 MARKS]

Answer 1

Each point: 1 Mark

• Soap is a sodium or potassium salt of long chain fatty acids. Hard water contains soluble salts of $\text{Ca}$ and $\text{Mg}$.
  
• When soap is dissolved in hard water, these ${\text{Ca}}^{2+}$ and ${\text{Mg}}^{2+}$ ions replace ${\text{Na}}^{+}$ and ${\text{K}}^{+}$ ions from soap and thus, insoluble salts of ${\text{Ca}}^{2+}$ and ${\text{Mg}}^{2+}$ are formed which are called scum.
  
• ${\text{Ca}}^{2+}+2\text{RCOONa}\to {\text{(RCOO)}}_2\text{Ca}+{\text{Na}}^{+}$
  ${\text{Mg}}^{2+}+2\text{RCOONa}\to {\text{(RCOO)}}_2\text{Mg}+{\text{Na}}^{+}$