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

Which of the following is not a disadvantage of using hard water?

  1. In production of steam in boilers

  2. Formation of scales in cooking utensils

  3. In cooking,bathing and washing

  4. In ion exchanges
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

In an ion exchange using hard water is not a disadvantage since it becomes soft by exchanging anions or cations. $Na^+$, $Ca^{2+}$ and $Mg^{2+}$ ions (from water) are exchanged with $H^+$ ions (from cation exchange resins). $Cl^-$, $SO _4^{2-}$ and $NO _3^{-}$ ions (from water) are exchanged with $OH^-$ ions (from anion exchange resins).

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

Polyphosphates like sodium hexametaphosphate (calgon) are used as water softening agents because they:

  1. form soluble complexes with anionic species

  2. precipitate anionic species

  3. form soluble complexes with cationic species

  4. precipitate catatonic species.
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

 Polyphosphates like sodium hexametaphosphate (calgon) form souble complex with cations like $Ca^{2+}$ and  $Mg^{2+}  $present in hard water and hence are used as water softening agents.
$Na _{6}P _{6}O _{18} \rightarrow 2Na^{+}+ Na _{4}P _{6}O^{2-} _{18}$
$M^{2+} + Na _{4}P _{6}O^{2-} _{18}\rightarrow [Na _{2}MP _{6}O _{18}]^{2-} + 2Na^{+}$

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

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

  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.
Reveal answer Fill a bubble to check yourself
A Correct answer
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.
Multiple choice evs experiments with water water and its types hard and soft water what floats - what sinks

Which of the following compounds is used for water softening?

  1. $Ca _3 (PO _4) _2$
  2. $Na _3PO _4$
  3. $Na _6 P _6O _18$
  4. $Na _2HPO _4$
Reveal answer Fill a bubble to check yourself
C Correct answer
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.

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

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

  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
Reveal answer Fill a bubble to check yourself
B Correct answer
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.

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

What is meant by demineralised water?

  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
Reveal answer Fill a bubble to check yourself
A Correct answer
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.

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

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:

  1. boiling

  2. Clark's method
  3. treatment with ${Na} _{2}{CO} _{3}$

  4. all of these
Reveal answer Fill a bubble to check yourself
C Correct answer
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.

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

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

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:

  1. boiling

  2. the Clark's method
  3. treatment with ${ Na } _{ 2 }{ CO } _{ 3 }$

  4. all of these.
Reveal answer Fill a bubble to check yourself
C Correct answer
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.