The nature of a substance that weather it is acidic or basic can be determined in terms of concentration of hydrogen ion or hydroxide in it. If an aqueous solution has an equal concentration of hydrogen ions and hydroxide ions, then it is neither acidic nor basic, such a solution is said to be neutral. Now, if the aqueous solution has more concentration of hydrogen ions than hydroxide ions then it will be acidic solution. On the other hand, if an aqueous solution has more concentration of hydroxide ion than hydrogen ion then it will be basic in nature.
In 1909, Sorensen devised a scale known as pH scale on which the acidic nature as well as basic nature of solutions can be expressed only by considering the hydrogen ion concentration in them. The pH of a solution may be defined as:
The ph of a solution is the negative logarithm of hydrogen ion concentration i.e.
pH = - log [H+]
For calculating the pH of the solution, we have to use the concentration of hydrogen ions in moles/litre. We will now find out the pH of pure water.
pH of pure water
The concentration of hydrogen ions in pure water is 10-7 M which means [H+] = 10-7
pH = -log [H+]
= -log [10-7]
= - [-7] [because log [10-7]= -7]
so pH = 7
Thus pH of water is 7. Whenever the pH of a solution is 7, it will be a neutral solution. Such solutions have no effect on litmus paper.
In the similar manner we can also find out the pH of acidic solutions as well as basic solutions by substituting different values of hydrogen ion concentrations [H+] in above relation. It is found that for acidic solution the pH value is always less than 7 and for basic solution the pH value is always more than 7. Thus, we can represent the pH scale in following way:
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