What Is the pH Scale?
The pH scale is a numbered scale from 0 to 14 that measures whether a solution is acidic, neutral, or alkaline:
- pH 0–6: Acidic (lower number = stronger acid)
- pH 7: Neutral (e.g. pure water)
- pH 8–14: Alkaline (higher number = stronger alkali)
The word "pH" stands for "potential of Hydrogen" — it measures the concentration of hydrogen ions (H⁺) in the solution.
The pH Scale in Detail
| pH Value | Solution | Example |
|---|---|---|
| 0–1 | Strongly acidic | Battery acid (H₂SO₄) |
| 2–3 | Moderately acidic | Stomach acid, lemon juice |
| 4–5 | Weakly acidic | Vinegar, coffee |
| 6 | Very weakly acidic | Milk, urine |
| 7 | Neutral | Pure water |
| 8–9 | Weakly alkaline | Seawater, baking soda |
| 10–11 | Moderately alkaline | Soapy water, antacids |
| 12–13 | Strongly alkaline | Bleach, oven cleaner |
| 14 | Very strongly alkaline | Sodium hydroxide solution |
The pH scale is logarithmic — each step of 1 pH unit represents a tenfold change in H⁺ concentration. This means a pH 1 solution has ten times more H⁺ ions than a pH 2 solution, and 100 times more than a pH 3 solution. Even small changes in pH can represent large changes in acidity.
Measuring pH
Universal indicator: A mixture of several indicators that changes colour across the full pH range. Colours range from red/orange (acidic) through green (neutral) to blue/purple (alkaline).
pH meter: An electronic device that measures the exact pH value. More accurate than indicators.
pH paper/strips: Paper coated with indicator that changes colour. Compared against a colour chart to find the approximate pH.
pH and Human Biology
pH is tightly controlled in living things:
- Blood: pH 7.35–7.45 (slightly alkaline). If pH shifts outside this range by even 0.4, it can be fatal.
- Stomach: pH 1.5–3.5 (strongly acidic — for digestion and killing bacteria in food).
- Soil: Most crops grow best between pH 6 and 7.
You can make a simple pH indicator at home from red cabbage. Boil red cabbage in water, then let it cool. The purple liquid changes colour based on pH — it turns red/pink in acids and green/yellow in alkalis. Add a drop to lemon juice or bicarbonate of soda and watch the colours change!
Real world: Blood pH and the body's buffer system
Human blood must be kept between pH 7.35 and 7.45. Even a shift to pH 7.2 (acidosis) or pH 7.6 (alkalosis) can be life-threatening, because enzymes and haemoglobin stop working at the wrong pH. Your body uses a carbonic acid/bicarbonate buffer system to maintain this narrow range — exhaling more CO₂ raises blood pH; holding your breath lowers it. This is real-time pH control chemistry keeping you alive.
Real world: Stomach acid vs antacids
Stomach acid (HCl) sits at pH 1–2 — acidic enough to corrode metal. This is necessary for digestion and killing bacteria in food. When excess acid is produced (from stress, spicy food, or caffeine), it causes painful heartburn. Antacid medicines bring the stomach pH up to around 3–4, still acidic enough for digestion but no longer painful. Just one pH unit change (from pH 2 to pH 3) means ten times fewer H⁺ ions — a massive difference on a logarithmic scale.
Real world: Swimming pool pH management
Swimming pool water must be maintained at pH 7.2–7.8. Below pH 7, the water becomes too acidic: it irritates swimmers' eyes, corrodes metal fittings, and damages pool liners. Above pH 8, chlorine disinfection loses efficiency and bacteria can survive. Pool operators test pH daily and add acid (hydrochloric acid or sodium bisulfate) to lower pH, or soda ash (sodium carbonate) to raise it. Proper pH management keeps swimmers safe.
Where do you see this in real life? Blood pH kept at 7.4 (life depends on it), stomach acid at pH 1 (digesting food), swimming pools at pH 7.2–7.8 (safe for swimming), soil pH 6–7 (crops growing well), lemon juice at pH 2 (souring food) — the pH scale is the most important number in biology, agriculture, and water treatment.
Practice Problems
A student tests two solutions with universal indicator. Solution A turns yellow-orange (pH ≈ 4) and Solution B turns green (pH ≈ 7). Which is more acidic, and how do you know?
Show answer
Why is it important that the pH of human blood is kept very close to 7.4?