$(a)$ Differentiate between soap and detergent.
$(b)$ Explain why soaps form scum with hard water,whereas detergents do not.

(N/A)

$Soaps$	$Detergents$
$(i)$ $Soaps$ are sodium or potassium salts of long-chain carboxylic acids.	$(i)$ $Detergents$ are ammonium or sulphonate salts of long-chain carboxylic acids.
$(ii)$ $Soaps$ have lesser cleansing action compared to detergents.	$(ii)$ $Detergents$ have better cleansing action compared to soaps.
$(iii)$ $Soaps$ are made from animal or plant fats.	$(iii)$ $Detergents$ are made from petrochemicals.
$(iv)$ $Soaps$ are biodegradable.	$(iv)$ $Detergents$ are generally non-biodegradable.

$(b)$ Hard water contains $Ca^{2+}$ and $Mg^{2+}$ ions. These ions react with the long-chain fatty acid part of the soap to form insoluble precipitates known as scum,which hinders the cleaning process. Detergents,however,form soluble salts with these ions,so they do not form scum and work effectively in hard water.
Chemical reaction: $2C_{17}H_{35}COONa + Ca^{2+} (Mg^{2+}) \longrightarrow (C_{17}H_{35}COO)_{2}Ca (Mg) + 2Na^{+}$