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Rub a drop of soapy water between your fingers and it feels slippery. Taste something with baking soda in it and there’s a faint bitterness. Pour drain cleaner into a clogged sink and hair and grease dissolve. All three effects come from the same kind of substance: a base.
Bases are the chemical opposite of acids. Where an acid gives away hydrogen ions, a base takes them in. Put the two together and they cancel each other out. That simple idea runs through digestion, cleaning, agriculture, medicine and a large part of the chemical industry.
The short answer
A base is a substance that accepts hydrogen ions (protons), or that produces hydroxide ions (OH⁻) in water.
The two descriptions overlap. Hydroxide ions are themselves excellent proton acceptors: OH⁻ + H⁺ → H₂O. So a substance that releases hydroxide into water is acting as a base, and so is a substance that grabs protons directly.
Sodium hydroxide is the classic example. It is an ionic solid, and when it dissolves the ions separate:
NaOH(s) → Na⁺(aq) + OH⁻(aq)
The solution is full of hydroxide ions, ready to react with any acid they meet.
Bases that contain no hydroxide
Here is where many students get stuck. Ammonia, NH₃, is a well-known base, but its formula has no OH in it. How can it make a solution basic?
The answer is that ammonia takes a proton from water:
NH₃ + H₂O ⇌ NH₄⁺ + OH⁻
The nitrogen atom in ammonia has a lone pair of electrons, and that lone pair bonds to a hydrogen ion pulled off a water molecule. What remains of the water molecule is a hydroxide ion. So ammonia generates hydroxide rather than containing it.
This is why the Brønsted–Lowry definition (a base is a proton acceptor) is more useful than the older Arrhenius definition (a base produces OH⁻). It covers ammonia, carbonate ions, the ethanoate ion and many other bases in one sentence. See acid–base definitions for how the three definitions compare.
Bases vs alkalis
The two words are often used as if they mean the same thing, but they don’t.
- A base is any substance that neutralises an acid. Many bases are insoluble: copper(II) oxide, iron(III) hydroxide and magnesium oxide barely dissolve in water.
- An alkali is a base that dissolves in water to give OH⁻ ions. Sodium hydroxide, potassium hydroxide, calcium hydroxide and aqueous ammonia are alkalis.
So every alkali is a base, but not every base is an alkali. Copper(II) oxide reacts with sulfuric acid to make copper(II) sulfate. It’s clearly a base, but it will never turn your indicator purple, because it doesn’t dissolve enough to release hydroxide into the water.
Types of bases
| Type | Examples | How it reacts with acid |
|---|---|---|
| Metal oxides | CaO, MgO, CuO | Oxide ion accepts protons → water |
| Metal hydroxides | NaOH, KOH, Ca(OH)₂ | Hydroxide accepts protons → water |
| Metal carbonates | Na₂CO₃, CaCO₃ | Carbonate accepts protons → CO₂ + water |
| Hydrogencarbonates | NaHCO₃ | Accepts a proton → CO₂ + water |
| Ammonia and amines | NH₃, CH₃NH₂ | Lone pair on N accepts a proton |
Oxides and hydroxides of metals are basic because the oxide ion (O²⁻) and hydroxide ion (OH⁻) are strong proton acceptors. Non-metal oxides go the other way: carbon dioxide and sulfur dioxide dissolve to form acidic solutions. This contrast between metal and non-metal oxides is one of the clearest trends across the periodic table.
Properties of bases
They feel slippery or soapy. Strong alkalis react with the fats and oils in your skin and turn them into soap, a reaction called saponification. The slippery feel is literally a thin layer of soap forming, which is also why strong alkalis damage skin.
They taste bitter. Baking soda and tonic water taste bitter. (Again, never taste lab chemicals.)
They change indicator colours. Alkalis turn red litmus blue, universal indicator blue or purple, and phenolphthalein pink.
They neutralise acids. Base + acid → salt + water. For a hydroxide, the core reaction is H⁺ + OH⁻ → H₂O.
Their solutions conduct electricity. Dissolved ions carry charge.
Alkaline solutions have a pH above 7. The more hydroxide there is, the higher the pH, up to about 14 for concentrated sodium hydroxide.
Strong and weak bases
Just like acids, bases differ in how completely they react with water.
A strong base dissociates completely. Group 1 hydroxides (lithium, sodium, potassium, rubidium and caesium hydroxide) are strong bases. So are the soluble Group 2 hydroxides of calcium, strontium and barium, although calcium hydroxide doesn’t dissolve very much.
A weak base only partly reacts with water. In a solution of ammonia, only a small fraction of NH₃ molecules have picked up a proton at any moment; the rest are still NH₃. That’s why household ammonia is less dangerous than drain cleaner of a similar concentration.
Bases around the home
| Substance | Base it contains | Approximate pH |
|---|---|---|
| Baking soda solution | Sodium hydrogencarbonate | ≈ 8.3 |
| Toothpaste | Various, often carbonates | ≈ 7–9 |
| Hand soap | Salts of fatty acids | ≈ 9–10 |
| Milk of magnesia | Magnesium hydroxide | ≈ 10.5 |
| Household ammonia | Ammonia | ≈ 11–11.5 |
| Bleach | Sodium hypochlorite + some NaOH | ≈ 11–13 |
| Oven and drain cleaner | Sodium hydroxide | ≈ 13–14 |
Antacid tablets use weak bases such as calcium carbonate or magnesium hydroxide to neutralise excess stomach acid. Gardeners spread lime (calcium hydroxide or calcium carbonate) on acidic soil to raise its pH. Bakers rely on the reaction between baking soda and acidic ingredients to release carbon dioxide and make cakes rise.
Bases in industry
Sodium hydroxide is one of the most produced chemicals in the world. It is made by the electrolysis of brine and used to make soap, paper, aluminium and countless other products. Ammonia, made by the Haber process, goes mostly into fertilisers. Calcium oxide (quicklime) and calcium hydroxide (slaked lime) are used in steelmaking, water treatment and construction.
Safety
Strong alkalis are at least as dangerous to the eyes as strong acids, and often more so. Alkalis dissolve the fats in cell membranes and keep penetrating tissue, while acid damage tends to form a barrier that slows itself down. Always wear eye protection, and wash any splash off skin with plenty of water straight away.
Never mix bleach with ammonia-based cleaners. The reaction produces toxic chloramine gases.
Common misconceptions
- “Bases are safer than acids.” Concentrated sodium hydroxide causes severe burns and can blind.
- “Alkali and base are the same word.” An alkali is specifically a soluble base.
- “A base must contain OH.” Ammonia, carbonates and oxides are all bases without an OH group in their formula.
- “Neutralisation always gives pH 7.” It gives pH 7 only when a strong acid meets a strong base in exactly matching amounts. A weak acid neutralised by a strong base gives a slightly basic salt solution.
Key takeaways
- A base accepts protons. An alkali is a base that dissolves in water to give OH⁻ ions.
- Metal oxides, hydroxides, carbonates and hydrogencarbonates, plus ammonia and amines, are all bases.
- Bases feel soapy, taste bitter, turn red litmus blue, neutralise acids and give a pH above 7 in solution.
- Strong bases dissociate completely; weak bases react only partly with water.
- Try the pH calculator to see how hydroxide concentration sets pH, and read the pH scale explained for the maths behind it.
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