Reactions of Metals

Three reaction families (cold water, steam, dilute acid) carry the experimental weight of the Metals topic, and the marks split evenly between equations and observations. The pattern examiners flag: students write hydroxide where the product is an oxide (or vice versa), and describe vigour (“it reacts a lot”) instead of observations (“rapid effervescence”). Both habits are fixable, and this page fixes them.

With cold water: hydroxide + hydrogen

Only the metals at the top of the reactivity series react with cold water, and the products are always the metal hydroxide and hydrogen:

• 2K + 2H2O → 2KOH + H2 (violent; lilac flame)
• 2Na + 2H2O → 2NaOH + H2 (rapid; metal melts into a ball, darts across the surface)
• Ca + 2H2O → Ca(OH)2 + H2 (steady fizzing; the mixture turns cloudy because calcium hydroxide is only slightly soluble)

The cloudiness with calcium is a favourite observation mark; limewater is dilute calcium hydroxide solution. Magnesium with cold water reacts so slowly that bubbles take days to appear; 0620 treats it as “very slow / almost no reaction”.

With steam: oxide + hydrogen

Heat the metal and pass steam over it, and the moderately reactive metals react. The product changes to the oxide, not the hydroxide:

• Mg + H2O(g) → MgO + H2 (fast; bright white light, white solid formed)
• Zn + H2O(g) → ZnO + H2 (slower; zinc oxide is yellow hot, white cold)
• 3Fe + 4H2O(g) → Fe3O4 + 4H2 (slow, needs constant heating)

The hydroxide/oxide switch is the single most-lost mark in this subtopic. Cold water → hydroxide. Steam → oxide. The iron-steam equation with Fe3O4 is Extended-level detail; at Core, “iron reacts slowly with steam to form an oxide and hydrogen” is sufficient.

With dilute acids: salt + hydrogen

Metals above hydrogen in the series react with dilute hydrochloric or sulfuric acid to give a salt and hydrogen:

Metal	Observation with dilute HCl	Equation
Magnesium	Vigorous effervescence, tube gets hot, metal dissolves	Mg + 2HCl → MgCl2 + H2
Zinc	Steady effervescence	Zn + 2HCl → ZnCl2 + H2
Iron	Slow effervescence	Fe + 2HCl → FeCl2 + H2
Copper	No reaction	No reaction

With sulfuric acid the salts are sulfates: Mg + H2SO4 → MgSO4 + H2. Note iron gives iron(II) chloride, FeCl2; the iron(III) salt is wrong here. Confirm the gas with a lighted splint: a squeaky pop identifies hydrogen. Salt naming and preparation continue in acids, bases and salts.

(Supplement) Each of these is a redox reaction: the metal atom is oxidised (Mg → Mg2+ + 2e−) and hydrogen ions are reduced (2H+ + 2e− → H2). The ionic equation Mg + 2H+ → Mg2+ + H2 is worth learning for Paper 4.

Placing an unknown metal

The deduction question gives you observations and asks for a position. Work through the gates in order:

1. Reacts with cold water? → above magnesium (top three).
2. No reaction with cold water but reacts with steam? → magnesium-to-iron band.
3. Reacts with dilute acid? → above hydrogen. Compare fizzing speed against zinc and iron to narrow it.
4. No reaction with acid? → below hydrogen: copper, silver, gold.

State the gate you used: “X reacts with steam but not cold water, so X is less reactive than calcium but more reactive than copper.”

Worked exam question

A student adds a piece of magnesium ribbon to dilute sulfuric acid. (a) State two observations. [2] (b) Write a balanced symbol equation for the reaction. [2] (c) Name the gas produced and describe the test for it. [2] (d) Explain why copper shows no reaction with dilute sulfuric acid. [1]

Model answer: (a) Effervescence / bubbles (1); the magnesium dissolves/disappears (or: the tube becomes warm) (1). (b) Mg + H2SO4 → MgSO4 + H2: formulae (1), balanced (1). (c) Hydrogen (1); a lighted splint gives a squeaky pop (1). (d) Copper is below hydrogen in the reactivity series, so it cannot displace hydrogen from the acid (1).

Mark-by-mark: (a) “it reacts quickly” is not an observation; bubbles, dissolving, and heat are. (b) MgSO4 must be correct; Mg(SO4)2 is a common formula error. (c) the test mark needs both the lighted splint and the squeaky pop. (d) the scoring idea is the position relative to hydrogen, not “copper is unreactive”.

The mistakes that cost marks

1. Hydroxide/oxide confusion. Cold water gives the hydroxide; steam gives the oxide. Writing MgOH or “magnesium hydroxide” for the steam reaction loses the product mark.
2. Observations that are conclusions. “Hydrogen is given off” is a deduction. The observation is effervescence/bubbling. Cambridge separates the two ruthlessly, especially on Paper 6.
3. Iron(III) salts from acid reactions. Iron with dilute acids forms iron(II) salts (FeCl2, FeSO4). The 3+ state belongs to other contexts, such as rust.
4. Forgetting copper’s non-reaction. Questions include copper precisely to test whether you will write “no reaction.” Inventing CuCl2 + H2 fails the question’s whole point.

How examiners want it phrased

Student wording	Mark-scheme wording
”It fizzes like crazy"	"Rapid effervescence; the metal dissolves and the tube becomes warm"
"Magnesium and steam make magnesium hydroxide"	"Magnesium reacts with steam to form magnesium oxide and hydrogen"
"The gas pops"	"A lighted splint ignites the gas with a squeaky pop, showing it is hydrogen"
"Copper is too weak to react"	"Copper is below hydrogen in the reactivity series, so it does not react with dilute acids”

Equations, observations, gates: practise each separately, then together on past-paper questions. If the hydroxide/oxide switch still catches you under time pressure, a free trial lesson will drill it until it cannot.

Test yourself

Check cold water vs steam before each product, then click to mark yourself.

Q1 (2 marks). Calcium is added to cold water. Write the balanced symbol equation and state one observation.

Show answer

• Ca + 2H2O → Ca(OH)2 + H2 [1] • effervescence / the mixture turns cloudy, because calcium hydroxide is only slightly soluble [1]

Q2 (2 marks). Steam is passed over heated zinc. Write the balanced symbol equation and describe the appearance of the solid product.

Show answer

• Zn + H2O → ZnO + H2 (steam gives the oxide, not the hydroxide) [1] • zinc oxide is yellow when hot and white when cold [1]

Q3 (3 marks). Metal X does not react with cold water, reacts with steam, and fizzes faster than iron in dilute hydrochloric acid. Deduce the position of X in the reactivity series, explaining each step.

Show answer

• no reaction with cold water places X below calcium [1] • reaction with steam places X in the magnesium-to-iron band [1] • faster fizzing than iron with dilute acid places X above iron, so X sits between magnesium and iron (zinc fits) [1]