biological chemistry

Why the pH of water is 7

Biochemical Principle Behind pH Scale and pH of Water
(The Relationship Between Equilibrium Constant, Ion Product of Water and pH Scale)


pH is defined as the negative logarithm of hydrogen ion concentration in a medium. For example, if you are saying a solution with pH 6.5, it means that the concentration of H+ ions in that solution expressed in negative logarithmic term is 6.5. pH is a scale, a scale to measure the H+ ion concentration in a medium. The pH scale starts from 0 to 14. The middle value in the scale (7) is considered as a neutral pH and any solution with pH below 7 is considered as acidic pH. Similarly any solution with pH above 7 is considered as a basic solution. Like the Celsius temperature scale, the pH scale is also constructed on the basis of water, the universal solvent and also the solvent of life.

pH of water is 7 and it is considered as a neutral pH. The term neutral does not mean that water does not contain H+ ions. In fact the term neutral means water contains equal number of H+ and OH- ions. Everyone knows that the pH of water is 7 and it is a neutral pH. But people often forgot to think why the pH of water is 7; from where the value 7 comes… is it a randomly assigned value to water? No, not at all.., the value 7 is an absolute value derived from the ion product of water. The ion product (the product of ions in water i.e., product of H+ and OH-) of water thus forms the basis of pH scale. The video tutorial explains how water got the pH value 7 and how the ion product of water is related to pH scale.


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