Most of ionic compound are polar in nature and when it added to the water they broke down in ions, polarity of the water molecules attracts this ions and so it is soluble in water.
 benzen is non polar organic compound, boken down ions are not attracts to benzen, so it is insoluble in it. 
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Water is a ploar solvent and has  high di-electric constant .
Due to the high di-electric constant(80) , the force of attraction b/w the oppositely charged ions of the ionic compounds is reduced to 1/80 times it's original strength and hence weakens the bond b/w them and after dissociation it reacts with the oppositely charged ions of water.

Nothing such happens in case of benzene as it is covalently bonded and mostly composed of Molecules rather than ions.
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