Last reviewed 30 Sept 2026 · 8 min read
Acids
An acid is a substance that gives hydrogen ions (H⁺) (hydronium ions, H₃O⁺) in water. Acids are sour in taste, turn blue litmus red, and react with metals, carbonates and bases.
Properties
- Acid + metal → salt + hydrogen: . (The hydrogen gas burns with a "pop" sound.)
- Acid + metal carbonate/bicarbonate → salt + water + carbon dioxide (brisk effervescence; the gas turns lime water milky).
- Acid + base → salt + water (neutralisation).
- Acids conduct electricity in solution, since they produce ions.
- Concentrated acids are corrosive; dilute an acid by adding the acid slowly to water (never water to acid), since the process gives out a lot of heat.
Common acids
| Acid | Formula | Found in |
|---|---|---|
| Hydrochloric acid | HCl | gastric juice in the stomach (helps digestion) |
| Sulphuric acid | H₂SO₄ | lead–acid batteries; "king of chemicals" — the most widely made industrial chemical; fertilisers |
| Nitric acid | HNO₃ | fertilisers, explosives |
| Acetic acid | CH₃COOH | vinegar (4–8 % solution) |
| Citric acid | lemon, orange and other citrus fruits | |
| Tartaric acid | tamarind, grapes, unripe mangoes | |
| Lactic acid | sour milk, curd | |
| Oxalic acid | tomato, spinach | |
| Formic (methanoic) acid | HCOOH | ant and nettle stings |
| Malic acid | apples | |
| Ascorbic acid | Vitamin C — amla, citrus fruits | |
| Carbonic acid | H₂CO₃ | soda water, soft drinks |
| Amino acids, fatty acids | proteins and fats | |
| Uric acid | urine | |
| Aqua regia | 3 HCl : 1 HNO₃ | dissolves gold and platinum |
Basicity: the number of replaceable H⁺ ions: HCl (monobasic), H₂SO₄ (dibasic), H₃PO₄ (tribasic).
Bases and alkalis
A base gives hydroxide ions (OH⁻) in water, or accepts H⁺. A base soluble in water is an alkali (NaOH, KOH, Ca(OH)₂, NH₄OH). Bases are bitter, feel soapy, turn red litmus blue, and conduct electricity in solution.
- Base + acid → salt + water.
- Base + ammonium salts liberates ammonia.
- Strong alkalis are corrosive.
Common bases
| Base | Formula | Use |
|---|---|---|
| Sodium hydroxide (caustic soda) | NaOH | soap, paper, detergents, purification of bauxite |
| Potassium hydroxide (caustic potash) | KOH | soft soap |
| Calcium hydroxide (slaked lime) | Ca(OH)₂ | whitewash, cement, neutralising acidic soils, lime water for testing CO₂ |
| Magnesium hydroxide (milk of magnesia) | Mg(OH)₂ | antacid for acidity |
| Ammonium hydroxide | NH₄OH | window cleaner, fertilisers |
Antacids (sodium bicarbonate, magnesium hydroxide, aluminium hydroxide) relieve the excess acid in the stomach.
Indicators
An indicator changes colour in acid or alkaline solution.
| Indicator | In acid | In base |
|---|---|---|
| Blue litmus | turns red | stays blue |
| Red litmus | stays red | turns blue |
| Phenolphthalein | colourless | pink |
| Methyl orange | red / pink | yellow |
| Turmeric | yellow | reddish-brown (used to test soap) |
| China rose (hibiscus) | dark pink / magenta | green |
| Red cabbage extract | red | green / yellow |
| Universal indicator | red → orange → yellow | green → blue → violet |
Olfactory indicators: onion, vanilla and clove oils lose their smell in a base.
The pH scale
The pH is a measure of the hydrogen-ion concentration: . The scale runs from 0 to 14:
| pH | Nature |
|---|---|
| < 7 | acidic (the lower, the stronger) |
| = 7 | neutral (pure water) |
| > 7 | basic (alkaline) (the higher, the stronger) |
A change of one pH unit corresponds to a tenfold change in . The pH is measured with pH paper or a pH meter.
| Substance | Approx. pH |
|---|---|
| Gastric juice | 1.5–3.0 |
| Lemon juice | about 2.2 |
| Vinegar | about 2.5–3 |
| Tomato | about 4.2 |
| Black coffee | about 5 |
| Normal rain | about 5.6 |
| Milk | about 6.4–6.8 |
| Saliva | about 6.5–7.5 |
| Pure water | 7 |
| Blood | 7.35–7.45 (slightly basic) |
| Sea water | about 8 |
| Baking soda solution | about 8.3 |
| Milk of magnesia | about 10 |
| Washing soda | about 11 |
| Household bleach / lime water | about 12–13 |
| Sodium hydroxide (strong) | about 14 |
Acid rain has a pH below 5.6, caused by sulphur dioxide and nitrogen oxides; it damages buildings (the Taj Mahal), plants and aquatic life.
pH in daily life
- Tooth decay starts when the pH in the mouth falls below about 5.5; toothpastes are basic and neutralise the acid.
- Soil pH decides plant growth: acidic soils are treated with lime; alkaline soils with gypsum or organic matter.
- Indigestion is due to excess HCl in the stomach and is relieved by antacids.
- Bee sting (methanoic acid) is treated with baking soda; a wasp sting (alkaline) with vinegar.
- The pH of blood is maintained by buffers.
A solution has a hydrogen ion concentration of mol/L. Then — an acid. A solution with has pH 9, a base.