19 . Water has maximum density ( 1g / cm 3 ) at 4 o C
20 . Water is amphoteric in nature because it has the ability to act as an acid as well as base
21 . Auto protolysis or self-ionization of water : One water molecule acts as an acid by donating a proton to another water molecule which acts as a base . This is also called auto protolyis of water
H O ( l ) + H O ( l ) H O + ( aq ) + OH - ( aq )
2 2 3
( acid ) ( base ) ( conjugate acid ) ( conjugate base )
22 . Water free from soluble salts of calcium and magnesium is called soft water
23 . Water containing soluble salts of calcium and magnesium in form of hydrogen carbonate , chlorides and sulphates is called hard water
24 . Temporary hardness in water is due to soluble salts of hydrogen carbonates of magnesium and calcium
25 . Permanent hardness in water is due to soluble salts of chlorides and sulphates of calcium and magnesium
26 . Methods to remove temporary hardness
Boiling : During boiling soluble magnesium hydrogen carbonate is
changed to magnesium hydroxide and calcium hydrogen carbonate on heating gets converted into calcium carbonate . Magnesium hydroxide and calcium carbonate being insoluble is filtered off
Mg ( HCO )
Heating
� + 2CO �
����� Mg ( OH ) 3 2 2 2
Ca ( HCO3 ) 2
Heating
����� CaCO3 � + H2O + 2CO2 �
Clark ’ s method : Calculated amount of calcium hydroxide is added to a given amount of water . It precipitates out calcium as calcium carbonate and magnesium as magnesium hydroxide which can be filtered off
Mg ( HCO ) + 2Ca ( OH ) � 2CaCO � + Mg ( OH ) � + 2H O
2 3 2
Ca ( HCO )
3 2 2
+ Ca ( OH ) � 2CaCO
3 � + 2H O
3 2 2 2