Although soap is a good cleaning agent, its effectiveness is reduced when used in hard water. Hard water contain a great amount of calcium and magnesium ions. These ions react with the soap to form an insoluble precipitate known as soap scum.
2CH3(CH2)16COO-Na+(aq)(Sodium sterate) + Ca2+(aq)(Calcium ions) -->
[CH3(CH2)16COO]2Ca(s) (Insoluble calcium sterate)(scum)+ 2Na+(aq)(Sodium ions)
2CH3(CH2)16COO-Na+(aq)(Sodium sterate) + Mg2+(aq)(Magnesium ions) -->
[CH3(CH2)16COO]2Mg(s) (Insoluble magnesium sterate)(scum)+ 2Na+(aq)(Sodium ions)
(a) The hydrogen ions in acidic water react with the soap to form long-chain fatty acids.[CH3(CH2)16COO]2Ca(s) (Insoluble calcium sterate)(scum)+ 2Na+(aq)(Sodium ions)
2CH3(CH2)16COO-Na+(aq)(Sodium sterate) + Mg2+(aq)(Magnesium ions) -->
[CH3(CH2)16COO]2Mg(s) (Insoluble magnesium sterate)(scum)+ 2Na+(aq)(Sodium ions)
Formation of soap scum reduces the amount of soap available for cleaning, thus causes wastage of soap. Soaps are only suitable for use in soft water. Soft water is water that contain little or no calcium and magnesium ions. Soaps do not form scum with soft water. The effectiveness of the cleansing action of soap is also reduced when used in acidic water.
(b) Long-chain fatty acids are insoluble in water due to their high relative molecular masses. This reduces the amount of soap available for cleaning. The effectiveness of the cleansing action of soap is thus reduced.
CH3(CH2)16COO-Na+(aq)(Sodium sterate) + H+(aq)(Hydrogen ions) -->
CH3(CH2)16COOH(s) (Insoluble steratic acid) + Na+(aq)(Sodium ions)
CH3(CH2)16COOH(s) (Insoluble steratic acid) + Na+(aq)(Sodium ions)
Detergent do not form scum with hard water. They form soluble substances with calcium and magnesium salts.
2CH3(CH2)11OSO3-Na+(aq)(Sodium dodecyl sulphate) + Ca2+(aq)(Calcium ions) -->
[CH3(CH2)11OSO3-]2Ca2+(aq)( Soleble calcium dodecyl sulphate) + 2Na+(aq)(Sodium ions)
[CH3(CH2)11OSO3-]2Ca2+(aq)( Soleble calcium dodecyl sulphate) + 2Na+(aq)(Sodium ions)
2CH3(CH2)11OSO3-Na+(aq)(Sodium dodecyl sulphate) + Mg2+(aq)(Magnesium ions) -->
[CH3(CH2)11OSO3-]2Mg2+(aq)( Soluble magnesium dodecyl sulphate) + 2Na+(aq)(Sodium ions)
[CH3(CH2)11OSO3-]2Mg2+(aq)( Soluble magnesium dodecyl sulphate) + 2Na+(aq)(Sodium ions)
This means detergent can still perform its cleansing action in hard water. Thus, detergent is more effective than soap in hard water. Detergent do not form precipitate in acidic water as well. Therefore, their cleansing action is not affected.
CH3(CH2)11OSO3-Na+(aq)(Sodium dodecyl sulphate) + H+(aq)(Hydrogen ions) -->
CH3(CH2)11OSO3-H+(aq)( Soluble dodecyl sulphonic acid) + Na+(aq)(Sodium ions)
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