Tag: hard and soft water

Questions Related to hard and soft water

In a sample of temporary hard water which of the following water softening process does NOT produced any precipitate of cation responsible for hardness ? 

evs experiments with water water and its types hard and soft water what floats - what sinks

  1. Boiling water

  2. Addition of lime water

  3. Addition of sodium hexameta phosphate

  4. Addition of sodium carbonate

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Correct Option: B

One litre hard water contains $12 mg\, Mg^{2+}$ milli-equivalent of washing soda required to remove its hardness is: 

evs experiments with water water and its types hard and soft water what floats - what sinks

  1. $1$

  2. $12.16$

  3. $1\times 10^{-3}$

  4. $12.16\times 10^{-3}$

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Correct Option: A

Which of the following pair of ions makes the water hard?

evs experiments with water water and its types hard and soft water what floats - what sinks

  1. $Na^+,SO _4^{+2}$

  2. $Ca^{2+},HCO _3^-$

  3. $Ca^{2+},NO _3$

  4. $NH _4^-,Cr$

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Correct Option: B
Explanation:

Causes

1) High mineral (calcium and magnesium) content.
2) It can be found in groundwater as it passes through lime.
Types 
1) Permanent: It is caused due to the presence of sulphate and chloride salts.  Heating does not remove the permanent hardness of water so it is treated with chemicals.  Resins or water softeners are used to remove the permanent hardness.
2) Temporary: It is caused due to the presence of bicarbonate salts.  It can easily be removed by boiling.  Boiling decomposes the salts into insoluble carbonates.  Water can also be treated with calcium hydroxide to remove the hardness.Water Hardeners are Ca2+ and HCO3.   Nitrates and sulfates are used as water softener.  

Hardness of water is estimated by simple titration with 

evs experiments with water water and its types hard and soft water what floats - what sinks

  1. $EDTA$

  2. $H _{4}EDTA$

  3. $Na _{2}H _{2}EDTA$

  4. $Na _{3}HEDTA$

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Correct Option: A

In water:

evs experiments with water water and its types hard and soft water what floats - what sinks

  1. temporary hardness is due to the bicarbonates of $Ca^{2+}$ and $Mg^{2+}$.

  2. permanent hardness is due to chlorides and sulphates of $Ca^{2+}$ and $Mg^{2+}$.

  3. hardness can be removed by adding phosphates.

  4. all of the above.

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Correct Option: D

$Na _2CO _3$ is widely used in softening of hard water. If 1 L of hard water required $0.0106 g$ of $Na _2CO _3$, The hardness in ppm (parts per million i.e., $10^{6}$ ml) of $CaCO _3$ is:

evs experiments with water water and its types hard and soft water what floats - what sinks

  1. $0.01\,$ ppm $CaCO _3$

  2. $0.10\,$ ppm $CaCO _3$

  3. $1.00\,$ ppm $CaCO _3$

  4. $10.00\,$ ppm $CaCO _3$

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Correct Option: D
Explanation:

Hardness (in ppm)=$\cfrac {Weight\quad of\quad Na _2CO _3\quad required\quad (in\quad mg)}{Volume\quad of\quad Hard\quad water\quad (in\quad L)}$

=$\cfrac {0.0106\times 10^{3}}{1}$
=$10.6$
$\approx 10 ppm$
$\therefore$ Hardness (in ppm)= $10$ ppm $CaCO _3$

A volume of 100 L of hard water requires 5.6 g of lime for removing temporary hardness. The temporary hardness in ppm of CaCO3CaCO3 is:

evs experiments with water water and its types hard and soft water what floats - what sinks

  1. 56

  2. 100

  3. 200

  4. 112

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Correct Option: C
Explanation:

Temporary hardness is amount of $ CaCO _{3} $ in grams present in $ 10^{6}\,ml/grams $ of water


$ Ca(HCO _{3}) _{2}+CaO\rightarrow 2CaCO _{3}+H _{2}O $

$ 56\,g  $                      $ 2\times 100 = 200\,g $

$ 5.6\,g $                      $ 20\,g $ 

20 g $ CACO _{3} $ present in 100 L $ H _{2}O $

$ = 100\times 10^{3} = 10^{5}\,ml $

Then $ 10^{6} $ ml water contains 200g $ CaCO _{3} $

$ \therefore $ 200 ppm is temporary hardness 

Hence, the correct option is $\text{C}$.

Which element has variable covalency?

evs experiments with water water and its types hard and soft water what floats - what sinks

  1. F

  2. H

  3. I

  4. O

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Correct Option: C

$ RH _{2} $ (ion exchange resin ) can replace $ Ca^{2+} $ in hard water as : 
$ RH _{2}+Ca^{2+}\rightarrow RCa+2H^{+} $.


One litre of hard water after passing through $ RH _{2} $ has pH = 2. Hence, hardness in ppm of $ Ca^{2+} $ is:

evs experiments with water water and its types hard and soft water what floats - what sinks

  1. 200

  2. 100

  3. 50

  4. 125

Show answer
Correct Option: A
Explanation:

According to the given reaction 


$ RH _{2}+Ca^{2+}\rightarrow RCa+2H^{+} $

Each mole $ Ca^{2+} $ ion replaced by 2 moles $ H^{+}$

1 mole $ H^{+} $ replaced $ \Rightarrow \dfrac{1}{2} = 0.5\,mole \,Ca^{2+} $

Given,
$ pH = 2 $

$ H^{+} = 10^{-2} = 0.01 $

0.01 mole $ H^{+} $ replaced $ = 0.01\times 05 = 0.005\,moles\,Ca^{2+} $

Mass $ Ca^{2+}$ replaced $ = 0.005\times 40 = 0.2\,g = 200\,mg $

Concentration or Hardness of $ Ca^{2+} = 200\,mg/L $

$ = 200\,ppm $   

Hence, the correct option is $\text{A}$

In a laboratory, hard water, required for an experiment, is not available. However, the following salts are available in the school laboratory :
1. Sodium sulphate
2. Calcium sulphate
3. Magnesium chloride
4. Sodium chloride
5. Calcium chloride
6. Potassium sulphate
Which of the above salts may be dissolved in water to obtain hard water for the experiment?

evs experiments with water water and its types hard and soft water what floats - what sinks

  1. 1,3 and 5

  2. 1,2 and 5

  3. 1,2,4 and 6

  4. 3 and 5 only

  5. 2,3 and 5

Show answer
Correct Option: E
Explanation:

The following salts may be dissolved in water to obtain hard water for the experiment
2. Calcium sulphate
3. Magnesium chloride
5. Calcium chloride