Hard water does not lather readily with soap because the calcium and magnesium salts present in it:
- A.Make the water acidic
- B.Form insoluble salts with soap, seen as scum
- C.Raise the boiling point of the water
- D.Decompose the soap into glycerol
Correct answer
Explanation
The correct answer is B. Soap is the sodium or potassium salt of a long chain fatty acid. The dissolved calcium and magnesium ions of hard water replace that sodium and form insoluble calcium and magnesium stearates, the sticky grey scum seen on clothes and in a basin; soap keeps being used up in this reaction and no lather forms until all the hardness has been removed. Option A is wrong because hardness does not make water acidic; hard water is often slightly alkaline. Option C is wrong because the small rise in boiling point caused by dissolved salts has nothing to do with lathering. Option D is wrong because soap is not broken down into glycerol; glycerol is a by-product of making soap by saponification. Detergents lather in hard water because their calcium salts are soluble.
Read the full article: Chemistry of Water: Hard Water, Heavy Water, Purification
Practice Questions
View allZero hardness of water is achieved by
- A.using lime soda process
- B.excess lime treatment
- C.ion exchange method
- D.using excess alum dosage
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In an ion exchange plant the water passes through a cation resin, which takes up the calcium and magnesium ions, and then through an anion resin, which takes up the sulphate, chloride and bicarbonate ions; because the hardness-causing ions are actually taken out of the water, the hardness can be…
Read the full explanation →Heavy water is chemically known as:
- A.Hydrogen peroxide
- B.Deuterium oxide
- C.Tritium oxide
- D.Distilled water
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Heavy water is D2O, in which both hydrogen atoms are deuterium, the isotope whose nucleus has one proton and one neutron, so its molecular mass is twenty instead of eighteen.
Read the full explanation →The temporary hardness of water is caused by the presence of:
- A.Sulphates of calcium and magnesium
- B.Chlorides of calcium and magnesium
- C.Bicarbonates of calcium and magnesium
- D.Sodium and potassium salts
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These bicarbonates are formed when water containing dissolved carbon dioxide passes over limestone, and they are unstable; on boiling they lose carbon dioxide and settle as insoluble carbonates, which is why this kind of hardness is called temporary and can be removed by boiling alone, or by the Clark method…
Read the full explanation →Water has its maximum density at which temperature?
- A.0 degree Celsius
- B.4 degree Celsius
- C.25 degree Celsius
- D.100 degree Celsius
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Water contracts as it is cooled to four degrees, is densest at that temperature, and then expands again as it cools further to zero, which is called the anomalous expansion of water.
Read the full explanation →Alum is added at a water works mainly to:
- A.Kill bacteria and viruses
- B.Coagulate fine suspended particles so that they settle
- C.Remove the permanent hardness of the water
- D.Add fluoride to the water
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Alum, that is aluminium sulphate or potash alum, is a coagulant: in water it forms a gelatinous hydroxide that gathers the fine clay and colloidal particles into heavier flocs, which then settle in the sedimentation tank and are removed, leaving clear water for filtration.
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