# Ionic Product, K_{w} and lonisation Constant, K of water

Though water is regarded as a non-conductor, yet even the purest water has a small conductivity as it ionises to a very slight extent.

Since it is a reversible process, the law of mass action can be applied to the equilibrium, therefore,

where K is called the **Dissociation constant or ionisation constant of water**.

Since the degree of ionisation of *water* is very low (nearly one molecule ionises out of 550 molecules), the concentration of ionised water may be assumed as constant.

is a constant at a given temperature and is known as **Ionic product of water** and may be defined as: *“The product of I-1+ and OH” ions concentration is constant at a constant temperature and is called the ionic product of water”.* The value of ionic product of water is approximately equal to.

And = no. of *water* moles per liter

Now substituting both the values in above equation, we get

Hence the value of ionisation constant of water is times that of , the ionic product of water.

It is evident from the above discussion that, if

the solution is neutral

the solution is acidic

the solution is basic or alkaline.

At 298 K

When temperature is increased the value of also increases. For e.g., at the value of

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