Alkalis on metals and oxides

What counts as an alkali. An alkali is simply a base that dissolves in water, releasing hydroxide ions, OH⁻. Every alkali is a base, but not every base is an alkali — insoluble bases like copper(II) oxide or iron(III) hydroxide are bases without being alkalis. The two alkalis used throughout ICSE Class 10 analytical chemistry are sodium hydroxide (NaOH) and ammonium hydroxide (NH₄OH), and it is their reaction with metals and oxides that sets up everything that follows in cation testing.

Most metals ignore alkalis. Reactive and moderately reactive metals — iron, copper, calcium, magnesium — show no reaction when dropped into NaOH solution, even on warming. This matches everyday chemical intuition: metals are expected to react with acids (releasing hydrogen), not with bases.

A handful of metals break that rule. Aluminium, zinc and lead are exceptions. Because they are amphoteric, they can also act like a weak acid towards a strong base, which lets them react with concentrated NaOH to liberate hydrogen gas and form a soluble salt called a zincate, aluminate, or plumbate:

where: Zn and Al are the amphoteric metals reacting; NaOH is the strong alkali; Na₂ZnO₂ and NaAlO₂ are the soluble sodium salts formed (zincate and aluminate respectively); H₂↑ is hydrogen gas, identified by giving a "pop" with a lit splint.

Metal oxides are usually basic — but not always. Most metal oxides react only with acids, forming a salt and water, which is why they are classed as basic oxides (for example calcium oxide, CaO). A small set of metal oxides, however, are amphoteric oxides: they react with both acids and strong alkalis. For this syllabus the two key amphoteric oxides are aluminium oxide (Al₂O₃) and zinc oxide (ZnO):

where: Al₂O₃ and ZnO are the amphoteric oxides; NaOH is the alkali they react with to give an aluminate or zincate salt plus water; HCl is the acid they react with, giving the metal chloride plus water — the same oxide behaves as a base towards the acid and as an "acid" towards the alkali. The figure below places these amphoteric metals and oxides against the ordinary basic oxides and metals that react only with acids, showing exactly where the dual behaviour sits.

Amphoteric character comes down to how firmly a metal ion holds onto its oxide or hydroxide layer. Aluminium and zinc ions sit at a borderline point where their oxide can accept extra hydroxide ions from a strong alkali just as readily as it can accept protons from an acid, so the same solid dissolves in either direction. Al₂O₃ therefore behaves like a base when HCl attacks it — H⁺ from the acid pulls oxide ions away as water, releasing Al³⁺ into solution as AlCl₃. But when strong NaOH attacks it, the oxide instead accepts OH⁻ ions to form the soluble aluminate ion, AlO₂⁻, dissolving in the opposite direction. This is exactly what "amphoteric" means: the compound plays acid in one reaction and base in the other, and both reactions give a salt and water. This dual pathway is also why aluminium and zinc metal themselves can react with hot concentrated NaOH — the thin protective oxide layer on the metal surface is itself amphoteric, so it dissolves in the alkali, exposing fresh metal that then reacts further with the strong base to liberate hydrogen. Ordinary metals like iron and copper have oxide layers that are purely basic, so alkali simply cannot attack them, and the metal underneath stays untouched. This same amphoteric chemistry reappears later in qualitative analysis: when NaOH is added to a solution of an Al³⁺ or Zn²⁺ salt, a white gelatinous hydroxide precipitate first forms — but keep adding NaOH and the precipitate redissolves, because the amphoteric hydroxide accepts more OH⁻ to form the soluble aluminate or zincate. Precipitates such as Fe(OH)₃ or Cu(OH)₂ are not amphoteric, so they stay insoluble in excess alkali, which is precisely how these ions are told apart in the confirmatory tests.

Question: Explain, with equations, what happens when zinc oxide is treated separately with (a) dilute hydrochloric acid and (b) excess sodium hydroxide solution, and state what this shows about the nature of ZnO.

Step 1. Identify the nature of ZnO. Zinc oxide is one of the two amphoteric oxides in this syllabus (along with Al₂O₃), so it is expected to react with both an acid and a strong alkali, each time producing a salt and water.

Step 2. Reaction with dilute HCl. Here ZnO behaves as an ordinary basic oxide:

The white solid dissolves, giving a colourless solution of zinc chloride.

Step 3. Reaction with excess NaOH. Here ZnO behaves as though it were an acidic oxide:

The white solid again dissolves, this time giving a colourless solution of sodium zincate.

Step 4. Interpret the result. Because ZnO reacts with both an acid and a base, forming a salt and water in each case, it is classified as an amphoteric oxide. Answer: ZnO dissolves in dilute HCl to give ZnCl₂ + H₂O, and in excess NaOH to give Na₂ZnO₂ + H₂O, confirming zinc oxide is amphoteric.

Competitive stretch

A student instead drops a strip of aluminium metal into excess hot NaOH solution. Effervescence is seen, and the colourless gas gives a "pop" with a burning splint. Step 1: recognise aluminium as an amphoteric metal alongside zinc and lead. Step 2: the pop test identifies the gas as hydrogen. Step 3: write the balanced equation, $2\text{Al} + 2\text{NaOH} + 2\text{H}_2\text{O} \rightarrow 2\text{NaAlO}_2 + 3\text{H}_2\uparrow$. Answer: aluminium dissolves in excess NaOH, releasing hydrogen gas and forming soluble sodium aluminate.

• Assuming all metals react with alkalis to give hydrogen — only Al, Zn and Pb do this; Fe, Cu, Ca and Mg do not react with NaOH at all.
• Treating "alkali" and "base" as the same word — every alkali is a base, but a base must be water-soluble to be called an alkali.
• Thinking amphoteric means reacting with just one of acid or alkali — it specifically means reacting with both, giving a salt and water each time.
• Mixing up product suffixes — zinc gives a zincate, aluminium gives an aluminate, lead gives a plumbate; writing the wrong ending is a common exam slip.

Ask the tutor: Want to see how this amphoteric behaviour plays out step by step in the actual cation confirmatory tests for Al³⁺ and Zn²⁺? Ask your AI tutor to walk through both tests side by side.