Which one of the following salts is responsible for temporary hardness of water?
- A.Sulphates of calcium
- B.Chlorides of magnesium
- C.Bicarbonates of magnesium
- D.Sulphates of magnesium
Correct answer
Explanation
The correct answer is C, Bicarbonates of magnesium. Temporary hardness is caused by the bicarbonates (hydrogencarbonates) of calcium and magnesium dissolved in water. It is called temporary because it can be removed simply by boiling: on heating, the soluble bicarbonates break down into insoluble carbonates, or magnesium hydroxide, which settle out, and carbon dioxide escapes. It can also be removed by Clark's method, adding a calculated amount of lime. Hard water does not lather well with soap and forms scale in kettles and boilers. Permanent hardness, on the other hand, is caused by the chlorides and sulphates of calcium and magnesium, which are not removed by boiling and need washing soda, ion-exchange resins or other treatment. Option A is wrong because calcium sulphate causes permanent hardness. Option B is wrong because magnesium chloride also causes permanent hardness. Option D is wrong because magnesium sulphate likewise causes permanent hardness. Exam tip: bicarbonates mean temporary hardness, removed by boiling; chlorides and sulphates mean permanent hardness.
Practice Questions
View allConsider the following statement : "Atomic number of an element is a more fundamental property than its atomic mass." Who among the following scientists has made the above statement ?
- A.Dmitri Mendeleev
- B.Henry Moseley
- C.J.J. Thomson
- D.Ernest Rutherford
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In 1913 the English physicist Henry Moseley showed that atomic number, not atomic mass, is the fundamental property of an element.
Read the full explanation →How many Carbon atoms are present in a Pyruvate molecule ?
- A.2
- B.3
- C.4
- D.5
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Read the full explanation →The energy of a photon, whose momentum is 10 MeV/c, where c is the speed of light, is given by
- A.10 MeV
- B.100 MeV
- C.1 MeV
- D.0.1 MeV
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For a photon, energy equals momentum multiplied by the speed of light, E = pc, so a momentum of 10 MeV/c gives an energy of exactly 10 MeV.
Read the full explanation →People prefer to wear cotton clothes in summer season. This is due to the fact that cotton clothes are
- A.good absorbers of water
- B.good conveyors of heat
- C.good radiators of heat
- D.good absorbers of heat
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Cotton soaks up sweat well, and as that sweat evaporates from the cloth it takes heat away from the body and keeps us cool.
Read the full explanation →Cell wall is not present in cells of
- A.Bacteria
- B.Plants
- C.Fungi
- D.Humans
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Human cells, like all animal cells, have only a flexible plasma membrane on the outside and no rigid cell wall.
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