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Carbon and its Compounds — Science STD 10 — Question

CBSE BoardEnglish MediumSTD 10ScienceCarbon and its Compounds5 Marks
Question
  1. What is a soap? Why are soaps not suitable for washing clothes when the water is hard?
  2. Explain the action of soap in removing an oily spot from a piece of cloth.
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Answer

  1. Soap is sodium or potassium salt of higher fatty acids such as oleic acid $\left( C _{17} H _{33} COOH \right)$, stearic acid $\left( C _{17} H _{35} COOH \right)$, palimitic acid $\left( C _{15} H _{31} COOH \right)$, etc. A soap is a sodium or potassium salt of long chain fatty acids
Soaps are not suitable for washing clothes when water is hard: Soaps, when used with hard water, does not lather or produce foam easily because of the formation of a curdy precipitate or scum. This scum is created when soaps react with the calcium and magnesium ions present in hard water.
  1. Cleansing action of soaps: The oily spot present on clothes is organic in nature and insoluble in water. Therefore, it cannot be removed by only washing with water. When soap is dissolved in water, its hydrophobic ends attach themselves to the oily spot and remove it from the cloth. Then, the molecules of soap arrange themselves in the form of micelle and trap the dirt at the centre of the cluster. These micelles remain suspended in the water. Hence, the oily spots are easily rinsed away by water.

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