What is pH?
The p in pH comes from the German word potenz (power), and H stands for hydrogen ion. pH measures the concentration of H⁺ (H₃O⁺) ions in a solution on a scale from 0 to 14.
- pH < 7: acidic (more H⁺ than OH⁻)
- pH = 7: neutral (equal H⁺ and OH⁻), like pure water
- pH > 7: basic (more OH⁻ than H⁺)
More H⁺ means a lower pH. The scale is logarithmic: each drop of 1 in pH means 10 times more H⁺ ions.
Universal indicator and pH paper
Litmus only says acid or base. A universal indicator is a mixture of several indicators; it shows a different colour at each pH, so we can read an approximate pH value. Red means strongly acidic, orange-yellow weakly acidic, green neutral, blue weakly basic and violet strongly basic.
| Liquid | Approx. pH |
|---|---|
| Gastric juice | 1.5–2 |
| Lemon juice | 2.2 |
| Vinegar | 3 |
| Tomato juice | 4.2 |
| Milk | 6.5 |
| Pure water | 7 |
| Blood | 7.4 |
| Baking soda solution | 8.3 |
| Milk of magnesia | 10 |
| Sodium hydroxide solution | 14 |
Strong and weak acids and bases
Strength is about how many H⁺ or OH⁻ ions a substance gives in water.
- Strong acids give many H⁺ ions: HCl, H₂SO₄, HNO₃.
- Weak acids give few H⁺ ions: acetic acid, citric acid, carbonic acid.
- Strong bases (strong alkalis): NaOH, KOH. Weak bases: NH₄OH, Mg(OH)₂.
Strength is different from concentration. A dilute strong acid still breaks up fully; a concentrated weak acid still gives few ions. So 0.1 M HCl has a lower pH (about 1) than 0.1 M acetic acid (about 3).
Importance of pH in everyday life
Plants, animals and our body
Our body works in a narrow pH range, about 7.0–7.8. Blood stays near 7.4. Living things survive only in a small pH window.
Acid rain
When rain water has pH below 5.6, it is called acid rain (from sulphur and nitrogen oxides in air). When it flows into rivers, the lower pH harms fish and other water life.
Soil pH
Plants grow best in a certain pH range. Soil that is too acidic is treated with quicklime, slaked lime or chalk; too basic soil is treated with organic matter.
Digestion
The stomach makes HCl to help digestion. Extra acid causes pain and irritation (acidity); mild bases such as milk of magnesia neutralise it.
Tooth decay
Bacteria break down sugar and food bits in the mouth into acids. Below pH 5.5, tooth enamel (calcium hydroxyapatite, the hardest substance in the body) starts to corrode. Toothpaste is basic and neutralises this acid.
Self-defence by animals and plants
A bee sting injects an acid, so a mild base like baking soda paste gives relief. Nettle leaves have stinging hairs with methanoic acid; the leaf of the dock plant, which often grows nearby, is traditionally rubbed on the sting.
Key formulas and definitions
- pH < 7 acidic, pH = 7 neutral, pH > 7 basic
- Lower pH = more H⁺ ions; 1 pH unit = 10 times the H⁺
- Ratio of H⁺ between two solutions = 10^(difference in pH)
- Strong acids: HCl, H₂SO₄, HNO₃; weak acids: CH₃COOH, citric, carbonic
- Strong bases: NaOH, KOH; weak bases: NH₄OH, Mg(OH)₂
- Tooth enamel dissolves below pH 5.5; acid rain has pH below 5.6
Worked examples
1. Solution A has pH 3 and solution B has pH 5. Which is more acidic, and by how many times?
Lower pH is more acidic, so A. Difference = 5 − 3 = 2, so A has 10² = 100 times more H⁺ ions than B.
2. Arrange in increasing pH: NaOH solution, blood, lemon juice, pure water.
Lemon juice (about 2.2) < pure water (7) < blood (7.4) < NaOH solution (about 14).
3. A universal indicator turns green in solution P and violet in solution Q. What can you say?
Green means about pH 7, so P is neutral. Violet means about pH 13–14, so Q is strongly basic.
4. Why does a person with acidity take milk of magnesia and not vinegar?
Acidity means too much HCl in the stomach. Milk of magnesia, Mg(OH)₂, is a mild base that neutralises the extra acid. Vinegar is an acid and would make it worse.
5. A student dilutes an acid of pH 2 by adding water. Will the pH go up or down? Can it cross 7?
Dilution lowers the concentration of H⁺, so pH rises (towards 7). But it cannot cross 7 by adding pure water; it only gets closer to neutral.
6. Two 0.1 M solutions: HCl gives pH about 1 and acetic acid gives pH about 3. Explain.
HCl is a strong acid and splits fully into ions; acetic acid is weak and splits only a little. At the same concentration, HCl gives about 100 times more H⁺, so its pH is 2 units lower.
Common mistakes
- Thinking a higher pH means more acidic. It is the opposite: lower pH, more acidic.
- Thinking pH 4 is only a little more acidic than pH 5. It has 10 times more H⁺.
- Mixing up strong with concentrated. Strong means fully ionised; concentrated means a lot of acid in little water.
- Believing acid rain is any rain with pH below 7. Normal rain is already slightly acidic (about 5.6) due to CO₂; acid rain is below 5.6.