The pH scale is a quick way to describe how acidic or basic an aqueous solution is. It runs from 0 to 14, with pH 7 considered neutral at 25°C. Values below 7 are acidic, and values above 7 are basic. pH matters because acidity affects reactions, living systems, water quality, foods, medicines, and industrial processes.
The scale is logarithmic, so each one-unit change in pH represents a tenfold change in hydrogen ion concentration. Acids increase H3O+ in water, while bases reduce H3O+ or increase OH-. A pH meter measures voltage related to ion activity, while indicators change color over certain pH ranges.
Understanding pH helps students compare solutions such as lemon juice, pure water, soap, and bleach using both numbers and chemical meaning.
Key Facts
- pH = -log[H3O+]
- At 25°C, neutral water has pH = 7 and [H3O+] = 1.0 x 10^-7 M.
- Acidic solutions have pH < 7 and [H3O+] > 1.0 x 10^-7 M.
- Basic solutions have pH > 7 and [H3O+] < 1.0 x 10^-7 M.
- pH + pOH = 14 at 25°C for aqueous solutions.
- A change of 1 pH unit means a 10 times change in [H3O+].
Vocabulary
- pH
- A logarithmic measure of hydrogen ion concentration in an aqueous solution.
- Acid
- A substance that increases the concentration of H3O+ ions in water.
- Base
- A substance that decreases H3O+ concentration or increases OH- concentration in water.
- Indicator
- A chemical dye that changes color over a specific pH range.
- Neutral solution
- A solution in which [H3O+] equals [OH-], giving pH 7 at 25°C.
Common Mistakes to Avoid
- Treating pH as a linear scale: this is wrong because pH is logarithmic, so pH 3 is 10 times more acidic than pH 4 and 100 times more acidic than pH 5.
- Thinking pH 7 is always neutral: this is incomplete because pH 7 is neutral only at 25°C, and the neutral pH changes slightly with temperature.
- Confusing strong acid with concentrated acid: this is wrong because strength describes how completely an acid ionizes, while concentration describes how much solute is present.
- Assuming indicators give exact pH values: this is wrong because indicators show an approximate pH range based on color, while a calibrated pH meter gives a more precise measurement.
Practice Questions
- 1 A solution has [H3O+] = 1.0 x 10^-3 M. Calculate its pH and state whether it is acidic, neutral, or basic.
- 2 A solution has pH = 10. What is its pOH at 25°C, and what is [OH-]?
- 3 Two unlabeled solutions have pH values of 4 and 6. Explain which solution is more acidic and by what factor, using the logarithmic nature of the pH scale.