The pH Scale Explained: Everything About Acids and Bases
Whether you're testing swimming pool water, making soap, or studying for AP Chemistry, the concepts of acids, bases, and pH are everywhere. But what exactly makes something acidic, and how does the pH scale mathematically function?
What is pH?
The term "pH" stands for the "potential of Hydrogen." It is a logarithmic scale used to specify the acidity or basicity of an aqueous liquid. Because it's logarithmic, a change of one whole number on the pH scale means a tenfold change in concentration.
- pH < 7: Acidic (more hydrogen ions, H⁺)
- pH = 7: Neutral (pure water)
- pH > 7: Basic / Alkaline (more hydroxide ions, OH⁻)
Calculate pH Instantly
You calculate pH using the formula: pH = −log[H⁺].
Don't want to use a scientific
calculator? Try our Interactive pH Calculator.
Strong vs. Weak Acids
The strength of an acid isn't about its pH; it's about how thoroughly it breaks apart (dissociates) in water.
Strong Acids completely dissociate. If you put 1 mole of HCl in water, you get nearly 1 mole of H⁺ ions. Because of this, calculating the pH of a strong acid is very straightforward.
Weak Acids only partially dissociate. Think of vinegar (acetic acid). Most of the acid molecules stay intact. To calculate the pH of a weak acid, you need to use the Acid Dissociation Constant, known as Ka (or its logarithmic form, pKa).
Managing Dissociation Constants
Working with weak acids? Convert between Ka and pKa effortlessly using our pKa / Ka Calculator.
Buffer Solutions and the Henderson-Hasselbalch Equation
A buffer is a special solution that resists changes in pH when small amounts of acid or base are added. Our blood is a perfect example of a complex buffer system keeping us alive!
To calculate the pH of a buffer solution, chemists rely on the famous Henderson-Hasselbalch equation:
pH = pKa + log([A⁻]/[HA])
You can quickly model buffer scenarios using our Henderson-Hasselbalch Calculator.
Summary
Understanding acid-base chemistry requires managing very large and very small numbers—which is why the logarithmic pH curve was invented! Remember to check whether your acid is strong or weak before beginning calculations, and always double-check your logarithms.