What Is a Base?
A base is a substance that can neutralise an acid. Bases accept hydrogen ions (H⁺) from acids. Metal oxides and metal hydroxides are common bases.
Examples of bases:
- Copper oxide (CuO)
- Calcium oxide (CaO) — quicklime
- Magnesium oxide (MgO)
- Sodium hydroxide (NaOH)
- Calcium hydroxide (Ca(OH)₂)
What Is an Alkali?
An alkali is a base that dissolves in water to produce hydroxide ions (OH⁻). All alkalis are bases, but not all bases are alkalis — some bases (like copper oxide) do not dissolve in water.
Common alkalis:
- Sodium hydroxide (NaOH) — used in drain cleaners
- Potassium hydroxide (KOH) — used in batteries
- Calcium hydroxide (Ca(OH)₂) — limewater; used to neutralise acidic soil
- Ammonia solution (NH₃(aq)) — found in cleaning products
The name "alkali" comes from the Arabic word "al-qali", meaning the ashes of plants. For centuries, wood ash (which contains potassium and sodium compounds) was used to make soap by reacting with fats or oils — a process that still works today. The ash dissolved in water produces an alkaline solution.
Properties of Alkalis
| Property | Description |
|---|---|
| pH | Above 7 (7–14) |
| Taste | Bitter (do not taste chemicals!) |
| Feel | Slippery or soapy |
| Litmus | Turn red litmus paper blue |
| Corrosive | Strong alkalis are corrosive (NaOH can cause chemical burns) |
Strong alkalis like sodium hydroxide (NaOH) are just as dangerous as strong acids. They cause chemical burns and can be more difficult to detect because they have a soapy feeling rather than a painful sting at first. Always handle alkalis with the same care as acids — wear goggles and gloves.
Alkalis in Everyday Life
| Alkali | Everyday use |
|---|---|
| Sodium hydroxide | Oven cleaners, drain cleaners |
| Ammonia solution | Floor and window cleaners |
| Calcium hydroxide | Treating acidic soil in farming |
| Magnesium hydroxide | Antacid medicine (neutralises stomach acid) |
| Sodium bicarbonate (NaHCO₃) | Baking; also a mild antacid |
Strong and Weak Alkalis
Just like acids, alkalis can be strong or weak:
- Strong alkalis (e.g. NaOH) fully dissociate in water, releasing many OH⁻ ions. pH is typically 13–14.
- Weak alkalis (e.g. ammonia solution) only partially dissociate. pH is typically 9–11.
Real world: Drain cleaners and oven cleaners
Drain cleaner and oven cleaner contain sodium hydroxide (NaOH, pH 13–14). NaOH is a strong alkali that saponifies fats and oils (converts them to soap and glycerol), dissolving grease and organic blockages. It also hydrolyses hair proteins, which is why it unblocks drains clogged with hair. The "soapy" or "slippery" feeling you notice when handling any alkali is because the NaOH is reacting with the oils on your skin — wash it off immediately.
Real world: Antacid tablets neutralise stomach acid
When excess stomach acid causes heartburn, antacid tablets containing magnesium hydroxide (Mg(OH)₂) or calcium carbonate (CaCO₃) provide fast relief. These are bases that neutralise the hydrochloric acid: Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O. The pH of the stomach rises towards neutral, stopping the burning sensation. The salt and water produced are harmless. This is neutralisation chemistry curing real discomfort.
Real world: Farmers liming acidic soil
Prolonged rainfall leaches basic minerals from soil, making it increasingly acidic. Many crops grow poorly below pH 6. Farmers spread calcium hydroxide (slaked lime) or calcium carbonate (ground limestone) on fields to neutralise the acid: Ca(OH)₂ + H₂SO₄ → CaSO₄ + 2H₂O. This raises the pH, improving nutrient availability and crop yield. The same principle is used to treat acidified lakes damaged by acid rain.
Where do you see this in real life? Drain cleaners unclogging pipes (NaOH dissolving fat), antacid tablets relieving heartburn (Mg(OH)₂ neutralising HCl), lime spread on farm fields (Ca(OH)₂ raising soil pH), ammonia-based window cleaners — bases and alkalis neutralise acids in your kitchen, garden, and medicine cabinet.
Practice Problems
Explain the difference between a base and an alkali.
Show answer
Why is calcium hydroxide solution (limewater) put on farmland?