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“Is copper oxide an alkali?” It’s a question that turns up in exams every year, and a lot of students get it wrong. The confusion comes from the way the two words are used in everyday speech, where “alkaline” and “basic” are treated as synonyms. In chemistry they aren’t quite the same, and the difference comes down to one property: whether the substance dissolves in water.
The one-sentence answer
A base is any substance that reacts with an acid to form a salt and water. An alkali is a base that dissolves in water, producing hydroxide ions (OH⁻).
Picture the set of all bases as a large circle. Alkalis are a smaller circle inside it. Everything in the small circle is also in the large one, but plenty of bases sit outside the small circle because they won’t dissolve.
Side-by-side comparison
| Base | Alkali | |
|---|---|---|
| Definition | Neutralises acids; proton acceptor | Soluble base that releases OH⁻ in water |
| Must dissolve in water? | No | Yes |
| Changes indicator colour directly? | Only if it dissolves | Yes |
| pH measurable? | Not for insoluble bases | Yes, above 7 |
| Examples | CuO, MgO, Fe(OH)₃, ZnO, CaCO₃, NaOH | NaOH, KOH, LiOH, Ca(OH)₂, aqueous NH₃ |
| Feels soapy? | Only soluble ones | Yes, when concentrated |
Why solubility makes the difference
Indicators, pH meters and the soapy feel of alkalis all depend on hydroxide ions in solution. If a base won’t dissolve, it can’t put hydroxide ions into the water, so none of these effects appear.
Copper(II) oxide is a good example. Drop the black powder into water with universal indicator and the colour stays green. Nothing happens because almost no copper oxide dissolves. But warm the same powder with dilute sulfuric acid and it disappears, leaving a blue solution of copper(II) sulfate:
CuO + H₂SO₄ → CuSO₄ + H₂O
It has neutralised the acid, so it’s a base. It just isn’t an alkali.
Which metal hydroxides are alkalis?
Solubility follows clear patterns on the periodic table.
Group 1 hydroxides are all soluble, so they’re all alkalis: lithium, sodium, potassium, rubidium and caesium hydroxide. They’re also strong bases.
Group 2 hydroxides get more soluble going down the group:
| Hydroxide | Solubility in water | Alkali? |
|---|---|---|
| Be(OH)₂ | Insoluble (and amphoteric) | No |
| Mg(OH)₂ | Very slightly soluble | Weakly; pH of a saturated solution only ≈ 10 |
| Ca(OH)₂ | Slightly soluble | Yes: limewater |
| Sr(OH)₂ | Moderately soluble | Yes |
| Ba(OH)₂ | Fairly soluble | Yes |
Transition metal hydroxides are insoluble. Copper(II) hydroxide (blue), iron(II) hydroxide (green) and iron(III) hydroxide (orange-brown) all form as precipitates when you add sodium hydroxide to solutions of their salts. That’s why they’re used in tests for metal ions. They’re bases, not alkalis.
Ammonia is a special case. It’s a gas that dissolves very well in water and reacts partly to form hydroxide ions, so aqueous ammonia is an alkali, and a weak one.
Metal oxides follow the same logic
Metal oxides are bases because the oxide ion, O²⁻, is a strong proton acceptor. Whether an oxide counts as an alkali depends on whether it reacts with water to form a soluble hydroxide:
- Na₂O + H₂O → 2NaOH (soluble → alkaline solution)
- CaO + H₂O → Ca(OH)₂ (slightly soluble → limewater)
- CuO + H₂O → no reaction (stays a solid base)
Sodium oxide and calcium oxide are sometimes loosely called “alkaline oxides” because of this, although strictly the oxide itself is a base that forms an alkali in water.
How to test whether a base is an alkali
- Add a small amount to distilled water and stir.
- Add a few drops of universal indicator (or dip in red litmus paper).
- Blue or purple colour → hydroxide ions are present → alkali.
- Stays green → the base hasn’t dissolved (or is neutral) → not an alkali.
- To confirm it is a base at all, add a little to dilute acid with indicator. If the acid colour fades towards green as the solid dissolves, the substance has neutralised the acid.
Wear eye protection. Even dilute alkalis irritate the eyes.
Everyday examples
| Substance | Base or alkali? | Why |
|---|---|---|
| Drain cleaner (NaOH) | Alkali | Very soluble, strong base |
| Limewater (Ca(OH)₂) | Alkali | Slightly soluble hydroxide |
| Milk of magnesia (Mg(OH)₂) | Base (barely an alkali) | Very low solubility makes it gentle on the stomach |
| Chalk (CaCO₃) | Base | Insoluble but neutralises acid, fizzing |
| Baking soda (NaHCO₃) | Weakly alkaline salt | Soluble, solution pH ≈ 8.3 |
| Rust (iron oxides) | Base | Insoluble, dissolves in acids |
| Household ammonia | Alkali | Soluble weak base |
The antacid example is worth a closer look. Magnesium hydroxide is effective because it’s mostly insoluble. It sits in the stomach as a suspension and neutralises acid gradually, without making the stomach contents strongly alkaline. A soluble strong alkali like sodium hydroxide would do the same job violently and damage the stomach lining.
Carbonates: bases that fizz
Carbonates and hydrogencarbonates neutralise acids too, so they are bases, but they react differently from oxides and hydroxides:
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
The fizzing is carbon dioxide escaping. Calcium carbonate is insoluble, so it’s a base but not an alkali. Sodium carbonate dissolves well and gives a distinctly alkaline solution (pH around 11), so it behaves as an alkali, which is why washing soda is used for cleaning.
Exam-style questions
1. Explain why copper(II) oxide is described as a base but not an alkali. Model answer: Copper(II) oxide neutralises acids to form a salt and water, so it is a base. It is insoluble in water, so it cannot release hydroxide ions into solution, which means it is not an alkali.
2. Which ion is present in all alkaline solutions? Answer: The hydroxide ion, OH⁻.
3. Magnesium oxide is added to water containing universal indicator. The indicator turns pale blue. Explain. Answer: A small amount of magnesium oxide reacts with water to form magnesium hydroxide, which is very slightly soluble. The few hydroxide ions released make the solution weakly alkaline.
4. Name a base that produces a gas when it reacts with hydrochloric acid. Answer: Any carbonate or hydrogencarbonate, for example calcium carbonate. The gas is carbon dioxide.
Common mistakes
- Calling every base an alkali. Check solubility first.
- Saying alkalis “contain” OH⁻ in the solid. Ammonia contains none; it generates OH⁻ by reacting with water.
- Assuming insoluble bases don’t react. They neutralise acids perfectly well; you just can’t measure their pH in water.
- Forgetting that “strong” is about dissociation, not solubility. Calcium hydroxide is a strong base (what dissolves dissociates fully) even though little of it dissolves.
Key takeaways
- Alkalis are the subset of bases that dissolve in water to give OH⁻.
- Group 1 hydroxides and the heavier Group 2 hydroxides are alkalis; most transition metal hydroxides and oxides are insoluble bases.
- To classify a base, test its solution with an indicator; to confirm it’s a base, see whether it neutralises an acid.
- For more background, read what is a base? and acids vs bases.
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