Click image to open full size
pH Scale: Acids vs Bases
pH, pOH, Hydrogen Ion Concentration, and Common Examples
Related Tools
Related Labs
The pH scale is a way to measure how acidic or basic a solution is. It helps chemists, biologists, and environmental scientists compare substances such as stomach acid, pure water, and household cleaners. The scale runs from 0 to 14, with lower values showing greater acidity and higher values showing greater basicity. Knowing pH is important because many chemical reactions and living processes only work well within a narrow pH range.
The pH value is connected to the concentration of hydrogen ions in a solution. Acids increase the amount of H+ in water, while bases decrease H+ or increase OH-. Because the pH scale is logarithmic, a change of 1 pH unit means a tenfold change in hydrogen ion concentration. This is why a solution with pH 3 is much more acidic than one with pH 4, not just a little more acidic.
Key Facts
- pH = -log[H+]
- At 25 C, pH + pOH = 14
- Acids have pH < 7, neutral solutions have pH = 7, and bases have pH > 7
- A decrease of 1 pH unit means [H+] becomes 10 times larger
- For pure water at 25 C, [H+] = 1.0 x 10^-7 M and pH = 7
- Strong acids commonly fall near pH 0 to 3, while strong bases commonly fall near pH 11 to 14
Vocabulary
- pH
- pH is a numerical measure of how acidic or basic a solution is based on hydrogen ion concentration.
- Acid
- An acid is a substance that increases the concentration of H+ ions in water.
- Base
- A base is a substance that decreases H+ concentration or increases OH- concentration in water.
- Neutral
- A neutral solution has equal concentrations of H+ and OH- ions, giving a pH of 7 at 25 C.
- Logarithmic scale
- A logarithmic scale changes by powers of ten, so equal steps represent multiplication rather than simple addition.
Common Mistakes to Avoid
- Thinking pH 6 is only slightly more acidic than pH 7, which is wrong because each pH unit represents a tenfold change in hydrogen ion concentration.
- Assuming all acids are equally dangerous, which is wrong because pH shows strength of acidity in solution but does not by itself describe all hazards such as concentration or reactivity.
- Forgetting that pH 7 is neutral only at 25 C, which is wrong because the neutral point depends on temperature.
- Mixing up acids and bases by memorizing the scale backward, which is wrong because low pH means more H+ and high pH means less H+.
Practice Questions
- 1 A solution has [H+] = 1.0 x 10^-3 M. What is its pH, and is it acidic, basic, or neutral?
- 2 If one solution has pH 2 and another has pH 5, how many times greater is the hydrogen ion concentration in the pH 2 solution?
- 3 A student says that a liquid with pH 8 is strongly basic because it is above 7. Explain why this statement is not fully correct.