Q.If water contains dissolved calcium hydrogencarbonate, out of soaps and synthetic detergents which one will you use for cleaning clothes ?

Water containing dissolved calcium hydrogencarbonate, $Ca(HCO_3)_2$, is a form of (temporarily) hard water, since it supplies $Ca^{2+}$ ions.

If soap is used in this water, the soap anion reacts with the $Ca^{2+}$ ions to form an insoluble calcium salt of the fatty acid, e.g.:

$2C_{17}H_{35}COO^-Na^+ + Ca(HCO_3)_2 → (C_{17}H_{35}COO)_2Ca↓ + 2NaHCO_3$

This insoluble precipitate (scum) wastes the soap and gives poor lather/cleaning until all the $Ca^{2+}$ is used up. …

Recognise that dissolved calcium hydrogencarbonate makes the water hard. Recall that soap precipitates with $Ca^{2+}$ (wasteful, poor lather) while synthetic detergent' …

• Choosing soap because it is 'natural'/traditional, ignoring the hard-water precipitation problem. …