10th Class Science Solved Paper - Science-2017 Delhi Set-I

  • question_answer Soaps and detergents are both types of salts. State the difference between the two. Write the mechanism of the cleansing action of soaps. Why do soaps not form lather (foam) with hard water? Mention any two problems that arise due to the use of detergents instead of soaps.


      Soap Detergents
    (i) Soaps are sodium salts of long chain Detergents are sodium salt of long chain benzene sulphonic acids.
    (ii) The ionic group in soap is \[COON{{a}^{+}}\] The ionic groups in detergents is \[SO_{3}^{-},\,N{{a}^{+}}\]
    (iii) Soaps are not useful when water is hard. Detergent can be used for washing purpose even when water is hard
    (iv) Soaps are biodegradable. Some of the detergents are non-biodegradable.
    (v) Soaps have relatively weak cleansing action. Detergents have strong cleansing action.
    Mechanism of the cleansing action of soaps:
    The dirt present on clothes is organic matter 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 dirt and removes it from the cloth. Then, the molecules of soap arranges themselves in micelles form and traps the dirt at the centre of the cluster. These micelles remain suspended in the water, Hence, the dust particles are easily rinsed away by water.
    Soaps do not form lather with hard water:
    Soap is a sodium or potassium salt of long chain fatty acids. Hard water contains salts of calcium and magnesium. When soap is added to hard water, calcium and magnesium ions present in water displaces sodium or potassium ion present in soap forming insoluble substance called scum. A lot of soap is wasted in this process.
    Problems that arise due to the use of detergents instead of soap:
    (a) Detergents being non-biodegradable, they accumulate in the environment causing pollution. In soil, the presence of detergents leads to pH changes making soil infertile.
    (b) The entry of detergents into food chain leads to bio-accumulation in living beings and tends towards serious health issues.

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