The pH scale and why pH matters
pH tells how acidic or basic a solution is, on a scale from 0 to 14. Below 7 is acidic, 7 is neutral and above 7 is basic.
Strong and weak
The "p" in pH stands for potenz, a German word for power. A higher concentration of H⁺ ions means a lower pH. Acids that give many H⁺ ions, like HCl, are strong acids. Acids that give fewer, like acetic acid, are weak acids. In the same way, NaOH is a strong base and ammonium hydroxide is a weak base. Universal indicator is a mixture of indicators that shows a different colour at each pH.
pH of some common substances (about)
| Substance | pH (about) | Nature |
|---|---|---|
| Gastric juice | 1.2 | Strongly acidic |
| Lemon juice | 2.2 | Acidic |
| Pure water | 7 | Neutral |
| Blood | 7.4 | Slightly basic |
| Milk of magnesia | 10 | Basic |
| Sodium hydroxide solution | 14 | Strongly basic |
Activity: finding the pH of samples
- 1Take small amounts of the samples in test tubes: dilute HCl, dilute NaOH, dilute ethanoic acid, lemon juice, water and dilute sodium hydrogencarbonate solution.
- 2Dip a strip of pH paper in each, or add a few drops of universal indicator.
- 3Match the colour with the colour chart on the pH paper box.
- 4★Write down each pH and say whether the sample is acidic, neutral or basic.HCl, ethanoic acid and lemon juice: below 7. Water: about 7. Sodium hydrogencarbonate and NaOH: above 7
★ key step
pH in everyday life
| Where | What happens |
|---|---|
| Our body | Works within pH 7.0 to 7.8. Living things can survive only in a narrow pH range. |
| Acid rain | Rain with pH below 5.6. When it flows into rivers, it lowers their pH and makes it hard for aquatic animals to survive. |
| Soil | Plants need a suitable pH to grow well. Farmers test the soil, and add a base such as lime to soil that is too acidic. |
| Digestion | The stomach makes hydrochloric acid to help digest food. Too much acid causes pain and irritation (indigestion). An antacid such as milk of magnesia, Mg(OH)₂, a mild base, neutralises it. |
| Tooth decay | Starts when the pH in the mouth falls below 5.5. Bacteria break down sugar and food left on teeth into acids that attack the enamel (calcium hydroxyapatite). Basic toothpaste neutralises the acid. |
| Stings | A bee sting leaves an acid that causes pain. Rubbing on baking soda, a mild base, gives relief. An ant bite and a nettle sting inject methanoic acid; for nettle, a leaf of the dock plant, which grows near it, is a traditional remedy. |
Acids in nature
| Source | Acid |
|---|---|
| Vinegar | Acetic acid (ethanoic acid) |
| Lemon, orange | Citric acid |
| Tamarind | Tartaric acid |
| Tomato | Oxalic acid |
| Sour milk (curd) | Lactic acid |
| Ant sting, nettle sting | Methanoic acid |
Sweets and food stuck in the teeth let bacteria make acid, so brush after meals. When the stomach burns after a heavy, spicy meal, an antacid neutralises the extra acid.
1 marks: “One solution has pH 3 and another has pH 6. Which has more H⁺ ions?” The one with pH 3. A lower pH means more H⁺ ions.
2 marks: “What is acid rain? How does it affect aquatic life?” Rain with pH below 5.6. When it flows into rivers, it lowers their pH, making it hard for fish and other aquatic animals to survive.
3 marks: “How does tooth decay happen, and how can it be prevented?” Bacteria turn sugar and food left in the mouth into acids. Below pH 5.5 the enamel (calcium hydroxyapatite) is attacked. Brushing with basic toothpaste after meals neutralises the acid.
Low pH, lots of H⁺. Below 7 sour, 7 pure water, above 7 soapy.