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5.4 Salts from Metals

GCSE Chemistry · AQA · Chapter 5: Chemical Changes

A free GCSE Chemistry lesson for AQA (8462) from OBA School: reacting metals with dilute acids to make salts and hydrogen, naming the salts, and seeing these reactions as redox. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • Describe the reactions of magnesium, zinc and iron with dilute hydrochloric and sulfuric acids
  • Name the salt made from a given metal and acid, and work out its formula from the ions
  • Describe the test for hydrogen and how to obtain pure, dry crystals of the salt
  • Explain metal–acid reactions as redox reactions and write ionic and half equations (Higher tier)
  • Calculate masses of salt and volumes of hydrogen from a mass of metal (Higher tier)

Key words

  • Acid: A substance that forms hydrogen ions, H⁺(aq), when it dissolves in water.
  • Salt: A compound formed when the hydrogen in an acid is replaced by a metal (or ammonium).
  • Chloride: A salt made from hydrochloric acid, HCl. Contains Cl− ions.
  • Sulfate: A salt made from sulfuric acid, H₂SO₄. Contains SO₄²− ions.
  • Nitrate: A salt made from nitric acid, HNO₃. Contains NO₃− ions.
  • Redox: A reaction where one substance is oxidised and another is reduced.

Metal + acid → salt + hydrogen

Metals above hydrogen in the reactivity series react with dilute acids. The metal dissolves, bubbles of hydrogen are given off, and a salt is left in solution. The metal replaces the hydrogen in the acid.

magnesium + hydrochloric acid → magnesium chloride + hydrogen Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)

zinc + sulfuric acid → zinc sulfate + hydrogen Zn(s) + H₂SO₄(aq) → ZnSO₄(aq) + H₂(g)

iron + hydrochloric acid → iron(II) chloride + hydrogen Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)

Key fact: Test for hydrogen: hold a lighted splint at the mouth of the test tube. Hydrogen burns with a squeaky pop.

Naming salts and writing formulae

The first part of the name comes from the metal; the second part comes from the acid.

  • hydrochloric acid; Formula HCl; Salts are called chlorides; Example with magnesium magnesium chloride, MgCl₂
  • sulfuric acid; Formula H₂SO₄; Salts are called sulfates; Example with magnesium magnesium sulfate, MgSO₄
  • nitric acid; Formula HNO₃; Salts are called nitrates; Example with magnesium magnesium nitrate, Mg(NO₃)₂

Worked example: formula of a salt

Zinc chloride: ions are Zn²⁺ and Cl−.
Charges must cancel: one Zn²⁺ (+2) needs two Cl− (−2).

Answer: ZnCl₂

Iron forms iron(II) salts with dilute acids, such as iron(II) sulfate, FeSO₄. Copper, silver and gold do not react with dilute acids, so their salts must be made in other ways (5.5–5.6). Metals such as sodium and potassium react too violently with acids to be safe.

Getting pure, dry crystals of the salt

After the reaction the salt is dissolved in water. To collect it as crystals:

  • Use excess metal. Keep adding small amounts until the fizzing stops and some metal is left over. Then all the acid has reacted, so no acid ends up in the crystals.
  • Filter off the leftover metal. The filtrate is a solution of the salt.
  • Evaporate some of the water in an evaporating basin. A water bath (a beaker of boiling water under the basin) heats more gently than a flame and stops spitting. Stop when the solution reaches its point of crystallisation: crystals appear at the edge, or a glass rod dipped in and taken out grows tiny crystals as the drop on it cools.
  • Leave to crystallise at room temperature. The slower the water evaporates, the larger the crystals. Pour off any liquid and dab the crystals dry between filter papers.
  • potassium, sodium, lithium, calcium; Suitable for making a salt with dilute acid? No – the reaction is violent and dangerous.
  • magnesium, aluminium (oxide layer removed), zinc, iron; Suitable for making a salt with dilute acid? Yes – they react at a safe, steady rate.
  • copper, silver, gold; Suitable for making a salt with dilute acid? No – they are below hydrogen, so there is no reaction. Use the metal oxide or carbonate instead (5.5–5.6).

Metal–acid reactions as redox (Higher tier)

The chloride or sulfate ions do not change – they are spectator ions. The ionic equation shows what really happens:

Mg(s) + 2H⁺(aq) → Mg²⁺(aq) + H₂(g)

oxidation: Mg → Mg²⁺ + 2e− (magnesium atoms lose electrons)

reduction: 2H⁺ + 2e− → H₂ (hydrogen ions gain electrons)

Each hydrogen ion gains one electron to become a hydrogen atom, and two hydrogen atoms then share a pair of electrons to form an H₂ molecule. Because the metal atoms always give electrons to the hydrogen ions, every metal + acid reaction is a redox reaction.

Worked example: masses and volumes (Higher tier)

0.48 g of magnesium reacts with excess hydrochloric acid. (Ar: Mg = 24, Cl = 35.5)
moles of Mg = 0.48 ÷ 24 = 0.020 mol, the ratio is 1 : 1 : 1, so 0.020 mol of MgCl₂ and of H₂ form
Mr of MgCl₂ = 24 + (2 × 35.5) = 95, so mass of salt = 0.020 × 95 = 1.9 g
volume of H₂ at room temperature and pressure = 0.020 × 24 = 0.48 dm³

Answer: 1.9 g of MgCl₂ and 480 cm³ of hydrogen

Comparing rates

A student added the same number of moles of magnesium and of zinc to excess acid and recorded the hydrogen volume every 10 seconds.

The steeper the line, the faster the reaction, so magnesium is more reactive. The final volume is the same because each metal atom gives one H₂ molecule, and the same number of moles of each metal was used.

Common mistakes

  • Writing “magnesium chlorine” or “magnesium sulfur”. The salt names end in -ide or -ate: chloride, sulfate, nitrate.
  • Thinking copper reacts with dilute acids. Copper is below hydrogen in the reactivity series.
  • Saying the hydrogen test gives a “pop” with a glowing splint. Use a lighted splint (a glowing splint is the test for oxygen).
  • In half equations, saying the hydrogen ions are oxidised. They gain electrons, so they are reduced.

Revision questions

Try these, then check your answers below each one.

Question 1. Name the salt formed when zinc reacts with dilute sulfuric acid.

Answer: zinc sulfate

Working: Zn + H₂SO₄ → ZnSO₄ + H₂. The salt is zinc sulfate.

Question 2. What is the positive test for hydrogen gas? (Choose from: a lighted splint burns with a squeaky pop, a glowing splint relights, limewater turns milky, damp litmus paper is bleached.)

Answer: a lighted splint burns with a squeaky pop

Working: Hydrogen burns with a squeaky pop when a lighted splint is held near it. (Glowing splint = oxygen; limewater = carbon dioxide; bleached litmus = chlorine.)

Question 3. Iron reacts with hydrochloric acid to form iron(II) chloride. What is its formula? (Choose from: FeCl₂, FeCl₃, FeCl, Fe₂Cl.)

Answer: FeCl₂

Working: One Fe²⁺ needs two Cl−, so the formula is FeCl₂.

Related lessons

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This lesson is section 5.4 (AGCCH0504) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.

 

 

 

 

 

 

 

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