Tag: hard and soft water

Questions Related to hard and soft water

A water sample is said to contain permanent hardness if water contains:

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

  1. sulfates and chlorides of calcium and magnesium.

  2. carbonates of calcium and magnesium.

  3. bicarbonates of calcium and magnesium.

  4. sulfates and chlorides of sodium and potassium.

Show answer
Correct Option: A
Explanation:
Permanent hardness of water is due to sulfates and chlorides of calcium and magnesium. Temporary hardness of water is due to bicarbonates of calcium and magnesium.

Which of the following compounds is used for water softening?

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

  1. $Ca _3 (PO _4) _2$

  2. $Na _3PO _4$

  3. $Na _6 P _6O _18$

  4. $Na _2HPO _4$

Show answer
Correct Option: C
Explanation:

Sodium hexametaphosphate $ (Na _6P _6O _{18}),$ commercially called 'calgon' is used for water softening. Calgon reacts with salts of $Ca/Mg$ (present in hard water) and forms inactive complex anions that do not precipitate in soap solution.

The brownish-white deposit on kettles, heating elements and utensils formed by boiling water is due to:

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

  1. moisture in the air

  2. temporary hardness of water

  3. permanent hardness of water

  4. reaction of sulphur dioxide in the air with hot water

Show answer
Correct Option: B
Explanation:

The temporary hardness of water is due to the presence of calcium or magnesium bicarbonates. 

The brownish-white deposit on kettles, heating elements, and utensils formed by boiling water are due to the temporary hardness of water. 
It can be removed from water by heating due to which the hydrogen carbonate decomposes to original insoluble carbonate.

What is meant by demineralised water?

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

  1. Water free from cations and anions

  2. Watrer free from minerals dissolved in it

  3. Wtaer free from impurities

  4. Water free from $Na^{+}$ and $K^{+}$ ions

Show answer
Correct Option: A
Explanation:

Water free from cations $(Ca^{2+}, Mg^{2+}, etc.)$ and anions $(Cl^{-}, SO^{2-} _{4}, HCO^{-} _{3}, etc)  $ is called deionised or demineralised water.

Permanent hardness is due to presence of soluble salts of Mg and Ca in the form of chlorides and sulphates in ${H} _{2}O$. It can be removed by:

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

  1. boiling

  2. Clark's method

  3. treatment with ${Na} _{2}{CO} _{3}$

  4. all of these

Show answer
Correct Option: C
Explanation:

Answer:-

Sodium carbonate can remove temporary and permanent hardness from water. Sodium carbonate is soluble but calcium carbonate and magnesium carbonate are insoluble.
The carbonate ions from sodium carbonate react with the calcium and magnesium ions in the water to produce insoluble precipitates.

${{Ca}^{+2}} _{\left( aq. \right)} + {{Na} _{2}C{O} _{3}} _{\left( aq. \right)} \longrightarrow {CaC{O} _{3}} _{\left( s \right)}  + 2{{Na}^{+}} _{\left( aq. \right)}$
${{Mg}^{+2}} _{\left( aq. \right)} + {{Na} _{2}C{O} _{3}} _{\left( aq. \right)} \longrightarrow {MgC{O} _{3}} _{\left( s \right)}  + 2{{Na}^{+}} _{\left( aq. \right)}$

Which of the following ions will cause hardness in water sample?

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

  1. $Ca^{2+}$

  2. $Na^+$

  3. $Cl^-$

  4. $K^+$

Show answer
Correct Option: A
Explanation:

Presence of calcium and magnesium salts in the form of hydrogencarbonate, chloride and sulphate in water makes water 'hard'.

Permanent hardness is due to the presence of soluble salts of Mg and Ca in the form of chlorides and sulphates in ${ H } _{ 2 }O$. It can be removed by:

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

  1. boiling

  2. the Clark's method

  3. treatment with ${ Na } _{ 2 }{ CO } _{ 3 }$

  4. all of these.

Show answer
Correct Option: C
Explanation:

Boiling and Clark's method are used for removing temporary hardness. They cannot be used for removing permanent hardness. Permanent hardness can be removed by treatment with $Na _2CO _3$ which takes place as follows:-

$Ca^{+2}(aq)+Na _2CO _3(aq)\longrightarrow CaCO _3(s)+2Na^{+}(aq)$
Due to the removal of dissolved $Ca^{+2}$ and $Mg^{+2}$ ions, there is no longer existence of salts which makes the water soft.

$6$ ml of a standard soap solution $(1$ ml $= 0.001$ g$)$ of  $CaCO _{3}$ were required in titrating $50$ ml of water to produce a good lather. Its degree of hardness is:

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

  1. $120$ ppm

  2. $100$ ppm

  3. $50$ ppm

  4. $60$ ppm

Show answer
Correct Option: A
Explanation:

Degree of hardness $=\dfrac {6\times 0.001}{0.05} =120$ ppm.

On treatment with cold water, an element (A) reacts readily liberating a colorless, odorless gas (B) and a solution (C). Lithium is reacted with (B) yielding a solid product (D) which effervescence with water to give a strongly basic solution (E). When $CO _2$ gas is bubbled through the solution (C), a white ppt (F) is formed but this redissolved forming solution (G) when more $CO _2$ is passed. The precipitate (F) effervesced when moistened with conc.$HCl$ and give deep red coloration to the Bunsen burner flame. (F) on heating with an excess of carbon at $2000^oC$ give (H).

Solution (G) contains a salt which:

(i) causes permanent hardness of water
(ii) can not be obtained in solid-state
(iii) causes temporary hardness of water
(iv) can be obtained in solid-state


Identify the salt with correct options.

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

  1. (i) and (ii)

  2. (i) and (iv)

  3. (ii) and (iii)

  4. (ii) and (iv)

Show answer
Correct Option: C
Explanation:

The metal A is calcium. It reacts with cold water to form calcium hydroxide (C) and hydrogen gas (B) which is colourless and odourless.
$Ca+2H _2O \rightarrow Ca(OH) _2 + H _2$

Lithium reacts with hydrogen gas (B) to form a solid product LiH (D).
$2Li+H _2 \rightarrow 2LiH$

olid product LiH (D) effervescence with water to give a strongly basic solution (E). 
$LiH + H _2O \rightarrow LiOH+ H _2 $

When $CO _2$ gas is bubbled through calcium hydroxide solution (C) , a white ppt of calcium carbonate (F) is formed but this redissolved forming calcium bicarbonate (G) when more $CO _2$ is passed. 

$Ca(OH) _2 + CO _2 \rightarrow CaCO _3 \rightarrow+H _2O$

$CaCO _3+H _2O+CO _2 \rightarrow Ca(HCO _3) _2$

The precipitate (F) (calcium carbonate) effervesced when moistened with concentrated HCl and give deep red colouration to the Bunsen burner flame
$CaCO _3+2HCl \rightarrow CaCl _2 + H _2O +CO _2$

The deep red colour is the characteristic colour of calcium.
Calcium carbonate (F) on heating with excess of carbon at $2000^oC$ give calcium carbide (H).
$CaCO _3 + 3C \xrightarrow {2000^0C} CaC _2+CO+CO _2$

It is $CaHCO _3$ which cannot be obtained in solid-state and causes temporary hardness. Temporary hardness is due to the presence of calcium hydrogen carbonate.   

Hence Option-C is correct

Calculate the temporary and permanent hardness of water sample having the following the following constituents per litre:
$ Ca(HCO _{3}) _{2} = 162\, mg, MgCl _{2} = 95 =\, mg, $
$ NaCl = 585\, mg, Mg(HCO _{3}) _{2} = 73\, mg, $
$ CaSO _{4} = 136\, mg $ 

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

  1. 200 ppm, 150 ppm

  2. 100 ppm, 150 ppm

  3. 150 ppm, 200 ppm

  4. 150 ppm, 150 ppm

Show answer
Correct Option: C
Explanation:

mole of $ Ca(HCO _{3}) _{2} = \dfrac{162\times 10^{-3}mg}{162\,g/mole} = 1\times 10^{-3}\,moles $


Mole of $ Ca(SO _{4}) = \dfrac{136\times 10^{-3}g}{136\,g/mole} = 1\times 10^{3}\,mole $

Total mole of $ Ca = 2\times 10^{-3}\,mole $

mass of $ CaCO _{3} = 2\times 10^{-3}\times 100 = 0.2\,g $

$ \therefore $ ppm (permanent hardness) $ = \dfrac{6.2}{1000}\times 10^{6} = 200\,ppm $

Mole of $ MgCl _{2} = \dfrac{95\times 10^{-3}}{95} = 1\times 10^{-3}\,mole $ 

Mole Mg $ (HCg) _{2} = \dfrac{73\times 10^{-3}}{146} = 5\times 10^{-4}\,mole $
mole Mg $ = 1.5\times 10^{-4}\,mole $

Mole g $ CaCO _{3} $ (In terms of mg) $ = 1.5\times 10^{-3} $

mass $ = 1.5\times 10^{-3} = 0.150\,g $

ppm (temporary hardness) $ = \dfrac{0.150}{100}\times 106 = 150\,ppm $

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