Question
What is the pH scale? Explain what pH values of 0, 7, and 14 mean. Give real-life examples of substances at different pH values.
Solution — Step by Step
pH stands for “potential of hydrogen” (from the German Potenz Hydrogen). It is a measure of the concentration of hydrogen ions (H⁺, or more accurately hydronium ions H₃O⁺) in an aqueous solution.
The mathematical definition:
where [H⁺] is the hydrogen ion concentration in mol/L (M).
The pH scale conventionally runs from 0 to 14 (though values outside this range are possible for very concentrated strong acids/bases).
Pure water at 25°C has:
A pH of 7 means the solution is neutral — equal concentrations of H⁺ and OH⁻. Not acidic, not basic.
Real-life example: Pure water, blood (pH ≈ 7.4, very slightly basic).
Acidic solutions have MORE H⁺ ions than OH⁻ ions. [H⁺] > mol/L.
Since , a HIGHER concentration gives a LOWER pH number.
pH 0: [H⁺] = mol/L. Extremely acidic — corresponds to a very concentrated strong acid (like 1M HCl).
pH 1: [H⁺] = mol/L. Strongly acidic.
pH 3–4: Moderately acidic. Examples: vinegar (pH ≈ 2.4–3.4), lemon juice (pH ≈ 2.2), orange juice (pH ≈ 3.5), coffee (pH ≈ 5), rainwater (pH ≈ 5.6 due to dissolved CO₂).
Lower pH = More acidic = More H⁺ ions.
Basic (alkaline) solutions have MORE OH⁻ ions than H⁺ ions. [H⁺] < mol/L.
pH 14: [H⁺] = mol/L. Extremely basic — corresponds to a concentrated strong base (like 1M NaOH).
pH 8–10: Mildly basic. Examples: seawater (pH ≈ 8.1), baking soda (NaHCO₃, pH ≈ 8.3), soap (pH ≈ 9–10).
pH 11–13: Strongly basic. Examples: household ammonia (pH ≈ 11), bleach (pH ≈ 12.5), caustic soda/drain cleaner (pH ≈ 13).
Higher pH = More basic = More OH⁻ ions.
This is what makes the question “hard” — understanding the logarithmic nature:
Each unit change in pH represents a 10-fold change in [H⁺].
- pH 3 is 10 times more acidic than pH 4
- pH 2 is 100 times more acidic than pH 4
- pH 1 is 1000 times more acidic than pH 4
This is why even a small pH change is significant. The pH of stomach acid (pH ≈ 2) is 100,000 times more acidic than pure water (pH = 7). The human body maintains blood pH between 7.35–7.45 — a pH of 7.0 (only 0.4 units below normal) can be fatal.
Why This Works
The logarithmic definition was proposed by Søren Sørensen in 1909 because hydrogen ion concentrations span an enormous range ( to mol/L). Expressing this as a 0–14 scale makes it manageable. The negative sign converts the small positive exponents to positive pH numbers (so we don’t have to say “negative 7” for neutral).
The self-ionisation of water () has an equilibrium constant at 25°C. This is why pH + pOH = 14, and why neutral pH = 7.
Alternative Method
Quick reference pH scale:
| pH | [H⁺] in mol/L | Example |
|---|---|---|
| 0 | 1 | Battery acid (H₂SO₄, conc.) |
| 1–2 | 0.1–0.01 | Stomach acid, lemon juice |
| 3–4 | 0.001–0.0001 | Vinegar, orange juice |
| 5–6 | – | Coffee, rainwater |
| 7 | Pure water, blood | |
| 8–9 | – | Seawater, soap |
| 10–11 | – | Milk of magnesia, baking soda |
| 12–13 | – | Bleach, household ammonia |
| 14 | 1M NaOH |
Common Mistake
The most common error is reversing the relationship: students write “higher pH means more acidic.” It’s exactly the opposite — higher pH means LESS H⁺, means MORE basic/alkaline. Low pH = acidic; high pH = basic. Remember: pH 0 is extremely acidic (stomach acid territory); pH 14 is extremely basic (drain cleaner territory).
For CBSE Class 10, you need to know: pH 7 = neutral, below 7 = acidic, above 7 = basic, and the real-life examples. For CBSE Class 11–12 and JEE, you also need the mathematical definition (), the ionic product of water (), and the relation pH + pOH = 14.