The pH scale measures how many hydrogen ions (H⁺) a solution has. It runs from 0 to 14.
| pH | Nature | Examples |
|---|---|---|
| 0 – 7 | acidic | 1 M HCl (≈0), lemon juice, tomato juice (4.6) |
| 7 | neutral | pure water |
| 7 – 14 | basic | baking soda solution, milk of magnesia, 1 M NaOH (≈14) |
The rule that decides most questions: the more H⁺ ions a solution produces, the lower its pH. So the strongest acid has the lowest pH number, and the strongest base the highest. Comparing four solutions is a standard 1-mark MCQ — read it as "which has fewest H⁺ ions?" and pick the largest pH.
Strong and weak. A strong acid gives all of its H⁺ ions in water (HCl, H₂SO₄, HNO₃). A weak acid gives only some (acetic acid in vinegar, carbonic acid). The same distinction holds for bases: NaOH and KOH are strong, ammonium hydroxide is weak. Strength is about how completely it ionises, not how concentrated it is — a point the board tests directly.
Why it matters in everyday life
These five are where the marks are. Each has a number attached, and the number is the answer.
Living things are pH sensitive. Our body works within a narrow range of 7.0 to 7.8. Organisms survive only small changes.
Acid rain. When rainwater has a pH below 5.6, it is called acid rain. Flowing into a river, it lowers the river's pH and aquatic life struggles to survive.
Tooth decay. Begins when the pH of the mouth falls below 5.5. Tooth enamel — calcium hydroxyapatite, a crystalline form of calcium phosphate, the hardest substance in the body — does not dissolve in water but is corroded below that pH. Bacteria in the mouth make acids by degrading leftover sugar. Cleaning after eating, with a basic toothpaste, neutralises the acid.
Indigestion. The stomach produces hydrochloric acid to digest food. Too much causes pain, and an antacid — a mild base such as magnesium hydroxide (milk of magnesia) — neutralises the excess.
Stings. A bee sting injects an acid; baking soda, a mild base, relieves it. Nettle stinging hairs inject methanoic acid — rubbing a dock leaf, which is basic, helps.
Soil pH
Plants need a specific pH range. To test soil: shake about 2 g of soil with 5 mL of water, filter, and check the filtrate with universal indicator paper. Universal indicator gives the approximate pH as a colour; litmus only tells you acidic or basic.
What the board actually asks
Across 2023–2026 this topic carried about 3 marks a paper, mostly as case-based questions with a pH-scale figure, plus MCQs.
- Rank solutions by pH from a table or a scale diagram, then name which is most acidic or basic.
- Tomato juice has pH 4.6 — how will it taste, and why? Sour, because pH below 7 means acidic.
- Under what condition is rainwater called acid rain, and what is its effect? Below 5.6; it lowers river pH and harms aquatic life.
- Distinguish strong from weak acids — answer in terms of the number of H⁺ ions produced, not concentration.
A frequent slip: saying a solution of pH 3 is "three times" more acidic than one of pH 5. Say more acidic, and justify it by H⁺ ion concentration — the scale is not linear, and the board's marking wants the reason, not a ratio.