21. Permanent hardness of water can be removed by adding :
(a) chlorine
(b) washing soda
(c) potassium permanganate
(d) bleaching powder
22. Marsh gas is :
(a) nitrogen
(b) ethane
(c) methane
(d) hydrogen
23. Most soluble in water is :
(a) camphor
(b) sulphur
(c) common salt
(d) sugar
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