pH Scale

• pH scale 0–14 — measure of acidity/alkalinity at 298 K.
• pH < 7 acidic; pH = 7 neutral; pH > 7 basic.
• Indicators — litmus, phenolphthalein, methyl orange, universal indicator.
• pH paper / pH meter for approximate measurement.
• Importance: soil pH, stomach acid, tooth decay, living systems.

1. pH — potential of hydrogen; pH = −log[H⁺] (conceptual at Class 10).
2. Strong vs weak acid — HCl strong; acetic acid weak (same pH definition).
3. Litmus — red in acid, blue in base; purple neutral.
4. Phenolphthalein — colourless in acid, pink in base.
5. Methyl orange — red in acid, yellow in base.
6. Universal indicator — continuous colour change shows approximate pH.
7. Neutral — pure water pH 7 at 25 °C.
8. Tooth decay — mouth pH below 5.5 damages enamel.
9. Soil pH — affects crop growth; lime added to acidic soil.
10. Sting relief — bee acid (formic) vs wasp base — different remedies (NCERT context).

Classify solutions: pH 2, 7, 9, 13

pH 2 — strongly **acidic** (e.g. lemon juice ~2)
pH 7 — **neutral** (pure water)
pH 9 — **basic** (e.g. baking soda solution)
pH 13 — **strongly basic** (e.g. NaOH solution)
Conclusion: lower pH = more acidic; higher pH = more basic.

• Thinking pH 0 impossible (very strong acids exist below pH 1).
• Misconception: all acids dangerous at any concentration (dilute acetic acid in vinegar safe).
• Confusing pH with acidity strength without concentration context.
• Using same indicator for all pH ranges (universal indicator better).
• Believing pH 7 for all salts (depends on hydrolysis — see salts topic).

• What pH range is acidic? Basic? Neutral?
• Which indicator turns pink in sodium hydroxide?
• Why is tooth enamel damaged below pH 5.5?
• Name one natural acid and its approximate pH.

Open the Practice tab for graded questions on pH Scale.