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