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Magnesium reacts with hydrochloric acid as Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g), giving a salt and hydrogen. Free notes and 12 questions answered.
Acid-Metal Reactions: Products and Patterns is part of Reactions of acids in GCSE AQA Higher Chemistry.
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Magnesium reacts with hydrochloric acid as Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g), giving a salt and hydrogen. Free notes and 12 questions answered.
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Use this lesson to stay inside Reactions of acids while connecting the detail back to the wider Topic 4: Chemical changes topic.
Read the core explanations from Acid-Metal Reactions: Products and Patterns before testing yourself from memory.
Drop a piece of zinc into hydrochloric acid and watch it fizz violently. Try the same with copper - nothing happens. Why do some metals react with acids while others don't?
The answer lies in a fundamental rule of chemistry: only metals more reactive than hydrogen will displace it from acids. This creates a predictable pattern that chemists use every day.
Picture the reactivity series as a ladder. Metals higher up are more reactive and can 'kick out' metals lower down from their compounds. Hydrogen sits in the middle of this ladder.
Key reactivity order: Potassium > Sodium > Lithium > Calcium > Magnesium > Aluminium > Zinc > Iron > Hydrogen > Copper > Silver > Gold
The golden rule: Metals above hydrogen react with acids. Metals below hydrogen don't react with acids. This is why zinc fizzes in acid but copper doesn't.
When a reactive metal meets an acid, two products always form: a salt and hydrogen gas. The general equation is:
The salt name follows a simple pattern. With hydrochloric acid, you get chlorides. With sulfuric acid, you get sulphates. With dilute hydrochloric acid you get chlorides, and with dilute sulfuric acid you get sulfates. Nitric acid is usually treated separately because it does not follow the simple acid-metal hydrogen pattern.
Worked example: Writing a balanced equation for magnesium and hydrochloric acid
Write the balanced equation for magnesium reacting with hydrochloric acid.
Answer: $Mg(s) + 2HCl(aq) \rightarrow MgCl_2(aq) + H_2(g)$
Every acid-metal reaction produces hydrogen gas, and there's a classic test to prove it. Hold a lit splint near the mouth of the test tube - you'll hear a distinctive 'squeaky pop' as the hydrogen ignites.
This happens because hydrogen is highly flammable. When it meets the flame, it reacts explosively with oxygen in the air: $2H_2(g) + O_2(g) \rightarrow 2H_2O(g)$
Safety note: The 'pop' can be quite loud, so warn everyone nearby. Never collect large volumes of hydrogen - it's explosive in high concentrations.
Knowing how much salt forms is crucial for industrial processes. The calculation follows standard mole ratios from the balanced equation.
Worked example: Calculating mass of salt produced
Calculate the mass of zinc chloride produced when 6.5g of zinc reacts completely with hydrochloric acid. [Ar: Zn = 65, Cl = 35.5]
Answer: 13.6g of zinc chloride
Acid-metal questions appear regularly in GCSE papers. Here's how to maximise your marks and avoid the common traps that catch students out.
Common exam mistakes: Students often forget that copper doesn't react with acids, or they write H instead of H₂ for hydrogen gas. Remember - hydrogen is diatomic, so it's always H₂.
Mark-winning phrases: 'More reactive than hydrogen', 'displaces hydrogen from the acid', 'squeaky pop test confirms hydrogen gas'. These show you understand the underlying science.
Exam precision note: In Acid-Metal Reactions: Products and Patterns - Reactions of acids with metals - Reactions of acids, always identify the key chemical idea first, then link it to evidence, data, or an equation. A high-scoring GCSE answer names the particles involved, states what changes, and justifies the conclusion using correct scientific language. Where calculations are used, show each step with units and keep significant figures sensible. Finally, check that terminology matches AQA wording so your answer is unambiguous and mark-scheme friendly.
When using the reactivity series to predict reactions with acids, the key comparison is whether the metal is above or below hydrogen. Metals above hydrogen usually react with dilute acids to produce a salt and hydrogen gas. Metals below hydrogen, such as copper, do not react in that simple way. Students should also be careful not to overgeneralise nitric acid, because GCSE questions typically use hydrochloric or sulfuric acid when teaching the simple acid-metal pattern.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Hydrogen is highly flammable and explosive in high concentrations, so large volumes should never be collected. In the squeaky pop test the pop can be quite loud, so everyone nearby should be warned.
The balanced equation for magnesium reacting with hydrochloric acid is Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g). Two HCl are needed because each Mg²⁺ ion needs two Cl⁻ ions, and the products are magnesium chloride and hydrogen gas.
Hydrogen gas is always H₂ because hydrogen is diatomic, with two hydrogen atoms joined together. Writing H instead of H₂ for hydrogen gas is a common mistake in acid-metal equations.
The equation for hydrogen burning is 2H₂(g) + O₂(g) → 2H₂O(g). Hydrogen reacts with oxygen in the air when it meets a flame, which causes the squeaky pop in the test for hydrogen.
Only metals more reactive than hydrogen react with dilute acids, displacing hydrogen to form a salt and hydrogen gas. Magnesium, zinc and iron react with hydrochloric and sulfuric acids, but copper, silver and gold do not.
Hydrogen makes a squeaky pop with a lit splint because it is highly flammable and reacts explosively with oxygen in the air: 2H₂ + O₂ → 2H₂O. Every acid-metal reaction produces hydrogen, so this test confirms the gas.
The mass of salt is found using moles. For 6.5g of zinc in Zn + 2HCl → ZnCl₂ + H₂, moles of Zn = 6.5 ÷ 65 = 0.1 mol, giving 0.1 mol of ZnCl₂; Mr of ZnCl₂ = 65 + (2 × 35.5) = 136, so mass = 0.1 × 136 = 13.6g.
Metals make chlorides with dilute hydrochloric acid and sulfates with dilute sulfuric acid, as in magnesium + hydrochloric acid → magnesium chloride + hydrogen. Nitric acid is usually treated separately because it does not follow the simple acid-metal hydrogen pattern.
The Mr of zinc chloride, ZnCl₂, is 136. With Zn = 65 and Cl = 35.5, the calculation is 65 + (2 × 35.5) = 136, so 0.1 mol of zinc chloride has a mass of 13.6g.
Nitric acid is usually treated separately because it does not follow the simple acid-metal pattern of making a salt and hydrogen. GCSE questions on this pattern typically use hydrochloric acid, which makes chlorides, or sulfuric acid, which makes sulfates.
Iron reacts with dilute sulfuric acid because iron is more reactive than hydrogen. Like magnesium and zinc, it displaces hydrogen from the acid, producing a sulfate salt and hydrogen gas.
Zinc fizzes in acid because it is more reactive than hydrogen, so it displaces hydrogen from the acid and the hydrogen gas forms bubbles. Copper is less reactive than hydrogen, so nothing happens when it is put in acid.