A reactive metal reacts with a dilute acid to give a salt and hydrogen gas, for example Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g); the ionic equation is Mg(s) + 2H+(aq) → Mg2+(aq) + H2(g).
When a reactive metal is added to a dilute acid, it displaces hydrogen from the acid, and the products are a salt and hydrogen gas. This is one of the standard ways to make a salt and one of the standard ways to prepare and test hydrogen, so it appears again and again in 4541.
The balanced equation
The general word equation is: metal + acid → salt + hydrogen. With correct formulae and state symbols, common examples are:
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)
2Al(s) + 6HCl(aq) → 2AlCl3(aq) + 3H2(g)
Fe(s) + 2HCl(aq) → FeCl2(aq) + H2(g)
Notice that iron forms iron(II) chloride with dilute acid, and that aluminium needs six moles of acid for two moles of metal. The ionic equation removes the spectator acid anion and shows the real change, the metal atom gives electrons to hydrogen ions:
Mg(s) + 2H+(aq) → Mg2+(aq) + H2(g)
The same ionic pattern holds for zinc and iron, with the correct charge on the metal ion.
Conditions required
The metal must be above hydrogen in the reactivity series, magnesium, aluminium, zinc and iron all react, while copper, silver and gold do not react with dilute acid at all. A dilute acid is used, and no heating is required for the more reactive metals, although warming speeds up a slow one. Dilute hydrochloric acid or dilute sulfuric acid is chosen because nitric acid is an oxidising acid and does not give a simple hydrogen product.
What you observe
You see effervescence, bubbles of a colourless gas, as the metal dissolves and the piece grows smaller. The mixture warms up, because the reaction is exothermic. The gas is hydrogen, confirmed by the test: a lighted wooden splint held at the mouth of the tube gives a squeaky “pop”. The solution is colourless for magnesium, zinc and aluminium salts, but pale green when iron(II) salts form. With a very reactive metal such as magnesium the fizzing is vigorous; with a less reactive metal such as iron it is slow.
Where it appears in the SPM exam
In 4541/1 this is tested as the gas test for hydrogen and as the ordering of metals by how fast they fizz. In 4541/2 it supports salt-preparation questions and the writing of full and ionic equations. In the practical paper 4541/3, the reaction of magnesium with hydrochloric acid is a classic rate-of-reaction experiment: you measure the volume of hydrogen collected against time, or the loss in mass, and draw a rate graph. Knowing the products lets you predict and explain every one of these.
How we teach it
We make sure you can name the salt from the acid used (chloride from HCl, sulfate from H2SO4), balance the hydrogen correctly, and give the exact wording of the hydrogen test, a lighted splint and a “pop”, never a glowing splint. Students most often lose marks by writing FeCl3 instead of FeCl2 with dilute acid, or by forgetting that copper does not react. Anchoring the equation to the reactivity series keeps all of these answers consistent and correct.
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