Tag: evs

Questions Related to evs

The process used for the removal of hardness of water is:

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

  1. Baeyer

  2. Calgon

  3. Hoope

  4. Serpeck

Show answer
Correct Option: B
Explanation:

Hard water is  percolated over Calgon (sodium hexameta phosphate $Na _6P _6O _{18}$). Calgon reacts calcium ( or magnesium) ions present in hard water. Inactive complex ions are formed that do not precipitate with soap.

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

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

  1. In production of steam in boilers

  2. Formation of scales in cooking utensils

  3. In cooking,bathing and washing

  4. In ion exchanges

Show answer
Correct Option: D
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).

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

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

  1. form soluble complexes with anionic species

  2. precipitate anionic species

  3. form soluble complexes with cationic species

  4. precipitate catatonic species.

Show answer
Correct Option: C
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.
$Na _{6}P _{6}O _{18} \rightarrow 2Na^{+}+ Na _{4}P _{6}O^{2-} _{18}$
$M^{2+} + Na _{4}P _{6}O^{2-} _{18}\rightarrow [Na _{2}MP _{6}O _{18}]^{2-} + 2Na^{+}$

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.