Question

Soap forms lather in distilled water. When a small amount of magnesium chloride is added to distilled water no lather is formed with soap. Write the reaction responsible for the second observation.

Answer 1

Step 1: Introduction

Soap is used to remove the dirt from the clothes. The cleansing action of soap depends on the hydrophilic and hydrophobic parts.

The ionic part of the soap is hydrophilic in nature and easily soluble in water. The non-ionic part is hydrophobic in nature and dissolves in oil or grease.

Soaps are biodegradable in nature.

Step 2: Observation for soap added to hard water:

• When a small amount of Magnesium chloride $\left({\mathrm{MgCl}}_2\right)$ is added to distilled water no lather is formed with soap.
• When a small amount of Magnesium chloride is added to distilled water then Carboxylate ions $\left({\mathrm{C}}_{17}{\mathrm{H}}_{35}{\mathrm{COO}}^{-}\right)$ combine with Magnesium ions $\left({\mathrm{Mg}}^{2+}\right)$ and precipitate out. It sticks onto the clothes, utensils and skin.

The chemical reaction between carboxylate ions combined with Magnesium ions is given as:

$$\begin{gathered} 2{\mathrm{C}}_{17}{\mathrm{H}}_{35}\mathrm{COONa}+{\mathrm{MgCl}}_2\to {\left({\mathrm{C}}_{17}{\mathrm{H}}_{35}\mathrm{COO}\right)}_2\mathrm{Mg}+2\mathrm{NaCl} \\ \mathrm{Sodium}\mathrm{stearate}\mathrm{Magnesium}\mathrm{stearate} \\ \mathrm{soap}\mathrm{precipitate} \end{gathered}$$