Understanding pH Changes When Acids Are Added to Water

Many basics of chemistry often get misunderstood or missquoted, leading to confusion among students and enthusiasts. A common misconception is that adding an acid to water increases the pH. This article aims to clarify what happens when an acid is added to water and why water is neither an alkali (base) nor an acid by examining the concept of pH and hydrogen-ion concentration.

Introduction to pH and Hydrogen-Ion Concentration

The phenomenom of pH is a measurement that describes the concentration of hydrogen ions ((^{ })((_{3})H_{o})) in a solution. The pH scale ranges from 0 to 14, where a pH of 7 is considered neutral.

The Relationship Between Hydrogen-Ion Concentration and pH

The mathematical relationship between hydrogen-ion concentration (([H^{ }])) and pH is given by the formula:

pH -log_{10}([H^{ }]

This equation reveals that the higher the concentration of hydrogen ions, the lower the pH. Conversely, the lower the concentration of hydrogen ions, the higher the pH.

Effect of Acids on pH

Adding an acid to water will decrease the pH because acids increase the concentration of hydrogen ions in the solution. Let's take an example using 0.1 M HCl to illustrate this:

For 0.1 M HCl, the pH is 1, as the hydrogen-ion concentration is 10-1 M. For 0.01 M HCl, the pH is 2, as the hydrogen-ion concentration is 10-2 M. For 0.001 M HCl, the pH is 3, as the hydrogen-ion concentration is 10-3 M.

These calculations demonstrate that the addition of an acid results in an increase in the concentration of hydrogen ions, which in turn leads to a decrease in pH.

The Nature of Pure Water

It's important to clarify that water, by itself, is neither an acid nor a base. Under normal conditions, water (H2O) acts as a solvent that can ionize (or partly dissociate) into the hydronium ion ((^{ })((_{3})Ho)) and the hydroxide ion (OH(^{-})). The degree of this ionization is very small in pure water, leading to a pH of 7, which is neutral.

Conclusion

In summary, the addition of an acid to water will decrease the pH because the concentration of hydrogen ions (([H^{ }])) increases. This is a fundamental principle in chemistry and a key concept in understanding the behavior of solutions. Water is neutral, neither acid nor base, but acid addition can make water solutions acidic.