An increased concentration of what ion characterizes a solution of a base in water

Water is primarily composed of H+ and OH- ions. The ionic product of water, at 25 oC is 10^-^1^4. Also, conc. of H+ ions = conc. of OH-

Thus, [H+][OH-]  =  10^-^1^4 mol/dm3

So, when conc. of [H+] = [OH-] = 10^-^7 mol/dm3.

In this event pH of water will be = -log(H+) = 7

Now, for basic solution, pH > 7. For this purpose, conc. of [OH-] ions must be increased in the solution. 

Thus, an increased concentration [OH-] ion characterizes a solution of a base in water

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