# Reactivity Series and Corrosion

> Chemistry · CIE IGCSE 0620
> Source: https://www.owlsprep.com/study/cie-0620-u9-reactivity-series-and-corrosion/

This guide covers the CIE IGCSE 0620 reactivity series, displacement reaction prediction rules, conditions for iron rusting, and core/Extended corrosion prevention methods aligned with the 2026-2028 syllabus.

**Prerequisites:** [Basic properties of metals (Unit 9.1)](https://www.owlsprep.com/study/cie-0620-u9-basic-metal-properties/); [Writing word and balanced symbol equations](https://www.owlsprep.com/study/cie-0620-u1-chemical-equations/)

## Learning objectives

- Recall the full reactivity series order including carbon and hydrogen
- Predict metal reactions with water, dilute acids, and metal compounds using the series
- Explain the conditions for iron rusting and core prevention methods
- Identify valid displacement reactions using reactivity series rankings
- (Extended) Explain sacrificial protection as a redox corrosion prevention method

## Core: Reactivity Series Order & Reaction Rules

**Reactivity Series** — A ranked list of metals (plus non-metals carbon and hydrogen) ordered from most reactive to least reactive, used to predict chemical reaction outcomes.

1. Potassium (K)
2. Sodium (Na)
3. Calcium (Ca)
4. Magnesium (Mg)
5. Aluminium (Al)
6. Carbon (C)
7. Zinc (Zn)
8. Iron (Fe)
9. Lead (Pb)
10. Hydrogen (H)
11. Copper (Cu)
12. Silver (Ag)
13. Gold (Au)

> **mnemonic**
>
> Please Stop Calling Me A Careless Zebra Instead Try Learning How Copper Saves Gold

**Worked example:** Use the reactivity series to state if magnesium will react with dilute hydrochloric acid, and write the word equation if the reaction occurs.

1. Locate magnesium and hydrogen in the series: magnesium is positioned above hydrogen.
2. Apply the rule: metals above hydrogen react with dilute acids to produce a salt and hydrogen gas.
3. Reaction occurs, word equation: magnesium + hydrochloric acid → magnesium chloride + hydrogen

> **Exam tip:** You must memorize the full series including carbon and hydrogen, as they are required for extraction and acid reaction questions.

## Core: Displacement Reactions

**Displacement Reaction** — A reaction where a more reactive metal displaces (replaces) a less reactive metal from a solution of its salt, or from its solid oxide.

A displacement reaction only occurs if the uncombined metal is higher in the reactivity series than the metal in the compound. For example, iron will displace copper from copper sulfate solution, but copper will not displace iron from iron sulfate solution.

**Worked example:** A student adds zinc powder to pale green iron(II) sulfate solution. State the observations you would expect, and explain why the reaction occurs using the reactivity series.

1. Locate zinc and iron in the series: zinc is above iron, so displacement will occur.
2. Observations: grey zinc powder dissolves, pale green solution turns colourless, dark grey iron solid deposits at the bottom of the test tube.
3. Explanation: Zinc is more reactive than iron, so it displaces iron from its sulfate salt. Word equation: zinc + iron(II) sulfate → zinc sulfate + iron

## Core: Rusting & Prevention Methods

**Rusting** — The specific corrosion of iron, where iron reacts with oxygen and liquid water to form soft, flaky hydrated iron(III) oxide (rust) that falls off to expose more iron to further corrosion.

Rusting requires **both oxygen (from air) and liquid water** to occur. If either substance is absent (e.g. iron stored in dry air, or boiled deoxygenated water), rusting cannot happen.

- **Barrier methods**: Coat iron with paint, oil, grease, or plastic to block contact with oxygen and water
- **Galvanising**: Coat iron with a layer of zinc, which acts as a barrier and provides additional protection if scratched
- **Alloying**: Mix iron with chromium and nickel to form stainless steel, which does not rust

**Worked example:** An iron garden gate is showing early signs of rust. State two effective methods to prevent further rusting, and explain how each works.

1. Method 1: Paint the entire gate surface. This creates an airtight, waterproof barrier that stops oxygen and water from touching the iron, preventing rust formation.
2. Method 2: Galvanise the gate. The zinc coating acts as a physical barrier, and will also provide sacrificial protection if the coating is scratched (see Extended section for full explanation).

## Extended Only: Sacrificial Protection

**Sacrificial Protection** — A corrosion prevention method where a more reactive metal (e.g. zinc or magnesium) is attached to an iron object. The more reactive metal oxidises (corrodes) preferentially, sacrificing itself to protect the iron from rusting.

This method works even if the protective metal layer is damaged, unlike barrier methods. The more reactive metal loses electrons more easily than iron, acting as a reducing agent that donates electrons to any iron that would otherwise oxidise to form rust.

**Worked example:** Iron ship hulls are often fitted with blocks of magnesium to prevent rusting. Explain how this works, with reference to the reactivity series.

1. Locate magnesium and iron in the reactivity series: magnesium is significantly more reactive than iron.
2. When exposed to salt water and oxygen, magnesium corrodes (oxidises) preferentially, losing electrons to form magnesium ions.
3. Any iron atom that would lose electrons to form rust immediately gains electrons from the corroding magnesium, so it remains as neutral iron metal and does not rust.
4. Magnesium blocks are replaced periodically once they have fully corroded.

## Common pitfalls

- **Wrong:** Forgetting to include carbon and hydrogen when using the reactivity series
  - Why it fails: Carbon is required to predict metal extraction from oxides, and hydrogen is required to predict dilute acid reaction outcomes.
  - Correct: Always memorize the full series including both non-metals in the correct order.
- **Wrong:** Stating that all metals react with dilute acids
  - Why it fails: Metals below hydrogen (copper, silver, gold) cannot displace hydrogen from dilute acids, so no reaction occurs.
  - Correct: Only predict acid reactions for metals positioned above hydrogen in the reactivity series.
- **Wrong:** Using the terms corrosion and rusting interchangeably
  - Why it fails: Corrosion is the general breakdown of any metal, while rusting is the specific corrosion of iron only. Other metals like aluminium form protective corrosion layers that stop further damage.
  - Correct: Use rusting exclusively when referring to iron corrosion, and corrosion for all other metals.
- **Wrong:** Claiming barrier methods like paint work even if scratched
  - Why it fails: Scratches break the barrier, allowing oxygen and water to reach the iron surface, so rusting starts at the scratch site.
  - Correct: Use galvanising or sacrificial protection for objects where the coating may be damaged, as these methods work even if the surface is scratched.
- **Wrong:** Stating rusting only requires water or only oxygen
  - Why it fails: Both reactants are required for the rusting reaction to proceed; iron will not rust in dry air or deoxygenated water.
  - Correct: Always state that both oxygen (from air) and liquid water are necessary for rusting to occur.

## Cheatsheet

| Concept | Core Rule | Extended Rule |
| --- | --- | --- |
| Reactivity Series Order | K > Na > Ca > Mg > Al > C > Zn > Fe > Pb > H > Cu > Ag > Au | More reactive metals are stronger reducing agents, lose electrons more easily |
| Dilute Acid Reactions | Metals above H produce salt + H₂ | Reactions are redox: metal oxidised, H⁺ ions reduced |
| Displacement Reactions | More reactive metal displaces less reactive metal from its compound | Redox process: more reactive metal is oxidised, less reactive metal ion is reduced |
| Rusting Conditions | Requires both O₂ and liquid water | Rusting is redox: Fe oxidised, O₂ reduced |
| Rust Prevention | Barrier methods, galvanising, alloying | Sacrificial protection uses Zn/Mg to corrode preferentially and protect Fe |
| Metal Extraction | Metals below C extracted by reduction with C; metals above C extracted by electrolysis | Extraction reactions are redox processes |

## What's next

Now that you have mastered the reactivity series and corrosion, you are ready to move on to metal extraction processes, which rely entirely on reactivity series rankings to select the correct extraction method for each metal. If you are studying the Extended tier, you can also practice writing ionic half equations for displacement and corrosion reactions to prepare for longer answer questions in Paper 2 and 4. Make sure to test your knowledge with structured practice questions on this topic, as it is frequently examined across both Core and Extended papers.

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