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Aluminium and Its Amphoteric Oxide and Hydroxide

Aluminium (Al, atomic number 13) is a light, silvery metal with 3 outer electrons, so it forms Al³⁺. In air it grows a thin, tough skin of Al₂O₃ that protects it. Both Al₂O₃ and Al(OH)₃ are amphoteric: they react with acids to make aluminium salts and with strong alkalis to make aluminate salts. The metal itself also gives hydrogen with both dilute acid and hot alkali.

🎬 Step-by-step story

  1. An aluminium atom has 3 electrons in its outer shell. It gives all 3 away easily and becomes Al³⁺.
  2. In air, a thin hard skin of aluminium oxide, Al₂O₃, forms on the shiny metal. This skin guards the metal inside.
  3. Dip it in acid. The skin dissolves first. Then the metal gives hydrogen bubbles and makes a salt, AlCl₃.
  4. Now dip it in alkali. The skin dissolves again and hydrogen bubbles again. This time the salt is sodium aluminate.
  5. It reacts with acid and with alkali. A substance that can do both is called amphoteric.
  6. Free play. Choose water, acid or alkali. Scratch the skin off or leave it on. Watch for bubbles.

Tip: drag the 3D scene to turn it. Use two fingers to zoom.

🤔 Common doubts, cleared

Why does aluminium give away exactly 3 electrons?

It has 3 electrons in the outer shell. Losing them leaves a full inner shell, which is stable. So it forms Al³⁺.

If aluminium is reactive, why do pans last so long?

The oxide skin that forms in air is thin but tight. It keeps oxygen and water from the metal below, so the reaction stops.

Why do bubbles start a little late in acid?

The acid must first dissolve the oxide skin. Only then does it touch the metal and make hydrogen.

How can the same metal react with both acid and alkali?

Its oxide and hydroxide can act as a base with acid and as an acid with alkali. This two-way behaviour is called amphoteric.

Does scratching the skin change what happens in water?

In cold water, no bubbles even then, because a new skin forms at once. In acid or alkali, scratching makes bubbles start sooner.

The aluminium atom and where it is found

Aluminium is in group 13 of the periodic table. Its atom has 3 outer electrons. It loses all three and forms the ion Al³⁺, so its common compounds have the formula pattern AlX₃ or Al₂X₃ (AlCl₃, Al₂O₃).

It is the most common metal in the Earth's crust, but it is never found as a free metal. It is found in ores like bauxite (mostly Al₂O₃) and is made by electrolysis. The metal is silvery, light (density about 2.7 g/cm³), soft, and a good conductor of heat and electricity.

Why aluminium does not corrode: the oxide skin

Aluminium is a reactive metal, yet a shiny pan lasts for years. The reason is the skin. Oxygen in the air reacts with the surface: 4Al + 3O₂ → 2Al₂O₃. The skin is only a few nanometres thick, but it is hard, tightly stuck and does not let air or water through. So it stops the reaction going deeper. If you scratch it, a new skin forms at once. (Anodising makes this skin thicker on purpose.)

Aluminium oxide is amphoteric

An amphoteric oxide acts like a base towards an acid and like an acid towards a strong alkali. Al₂O₃ does both:

With acid: Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O (salt and water).

With alkali: Al₂O₃ + 2NaOH + 3H₂O → 2Na[Al(OH)₄] (sodium aluminate; some books write NaAlO₂ + H₂O).

Other amphoteric oxides: ZnO, PbO, SnO. Not amphoteric: Na₂O (only basic) and CO₂ (only acidic).

Aluminium hydroxide is amphoteric too

Add a little sodium hydroxide to an aluminium salt solution. A white, jelly-like solid appears: Al³⁺ + 3OH⁻ → Al(OH)₃. It does not dissolve in water. It reacts with both sides:

With acid: Al(OH)₃ + 3HCl → AlCl₃ + 3H₂O.

With extra alkali: Al(OH)₃ + NaOH → Na[Al(OH)₄], so the white solid dissolves when you add more NaOH.

Check with ammonia solution: it is a weak alkali. The white solid forms but does not dissolve in excess ammonia. This difference helps tell Al³⁺ from Zn²⁺ (zinc hydroxide dissolves in both).

Reactions of the metal with acid, alkali and water

Dilute acid: 2Al + 6HCl → 2AlCl₃ + 3H₂. Hot strong alkali: 2Al + 2NaOH + 6H₂O → 2Na[Al(OH)₄] + 3H₂. In both cases hydrogen gas is made.

Water: nothing visible, because of the oxide skin. Conc. nitric acid makes the skin thicker (passivation), so it can be carried in aluminium tanks. Aluminium also burns brightly in oxygen when powdered, and it takes oxygen away from iron oxide in the thermite reaction: 2Al + Fe₂O₃ → Al₂O₃ + 2Fe (used to weld rails).

Uses of aluminium and its compounds

Light and strong alloys (duralumin) for aircraft and bikes; power cables (light and conducts well); foil and drink cans; pots and pans; window frames. Al₂O₃ is used for gem stones (ruby, sapphire), as an abrasive and in furnace linings. Alum (a double sulphate) clears muddy water. Al(OH)₃ is used in some antacids and in water purification.

Try it: scratch and dip

In the 3D, choose a liquid and tick the box to scratch the skin off. Predict first: will bubbles start sooner? Then check. At home: put a small piece of aluminium foil in a glass of plain water and another in a little lemon juice (acid). Look after an hour. Only the acid should show tiny bubbles. Do not use drain cleaner or strong alkali at home.

Key formulas and definitions

Worked examples

1. Write the equation for aluminium oxide reacting with dilute hydrochloric acid.

Al₂O₃ is a base-like oxide here. It gives a salt and water: Al₂O₃ + 6HCl → 2AlCl₃ + 3H₂O. Check atoms: 2 Al, 3 O, 6 H and 6 Cl on both sides.

2. A white jelly appears when NaOH is added slowly to aluminium chloride solution, then vanishes with extra NaOH. Explain.

First: AlCl₃ + 3NaOH → Al(OH)₃ + 3NaCl, the white jelly. Al(OH)₃ is amphoteric, so extra alkali reacts with it: Al(OH)₃ + NaOH → Na[Al(OH)₄], which dissolves. That is why the jelly vanishes.

3. Why can a foil of aluminium sit in water for days without any hydrogen?

The thin Al₂O₃ skin keeps water away from the metal. Without contact, Al cannot react. The skin is not damaged by neutral water, so no hydrogen appears.

4. How many grams of Al are needed to make 6.72 L of H₂ at STP from dilute acid?

6.72 L ÷ 22.4 = 0.3 mol H₂. From 2Al : 3H₂, moles of Al = 0.3 × 2/3 = 0.2 mol. Mass = 0.2 × 27 = 5.4 g.

5. How much HCl (in grams) reacts with 10.2 g of Al₂O₃?

Moles Al₂O₃ = 10.2 ÷ 102 = 0.1 mol. 1 mol Al₂O₃ needs 6 mol HCl, so 0.6 mol HCl. Mass = 0.6 × 36.5 = 21.9 g.

6. A student says: "ZnO and Al₂O₃ both dissolve in NaOH and in HCl, so I cannot tell Zn²⁺ from Al³⁺ using NaOH alone." How can ammonia solve this?

Add excess ammonia solution to each. Zn(OH)₂ dissolves in excess ammonia (it forms a complex ion), but Al(OH)₃ does not dissolve. So the solid that remains is aluminium hydroxide.

Common mistakes

Practice quiz

1. What does amphoteric mean?
2. Which gas is made when Al reacts with dilute HCl?
3. What protects an aluminium pan from corrosion?
4. Which oxide is NOT amphoteric?
5. Excess NaOH is added to white Al(OH)₃. What happens?

Practice: answer these yourself

Type or choose your answer, then press Check. Use a hint if you are stuck; the full solution appears after you answer.

Frequently asked questions

Why is aluminium oxide called amphoteric?

Because it reacts with acids to give salt and water, and also with strong alkalis to give an aluminate salt. It behaves as a base with acid and as an acid with alkali.

Does aluminium react with water?

Not at normal conditions, because of its oxide skin. If the skin is removed (for example by mercury amalgam), the metal reacts slowly with water and gives hydrogen.

Is it safe to use aluminium foil for acidic or salty food?

Short contact is fine. Long contact with very acidic or very alkaline food can dissolve the skin and some metal, so glass or steel is a better choice for storing such food.

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