---
title: "Metals: Reactions with Acids and Corrosion"
book: "School Chemistry — Grades 1 to 12"
subject: chemistry
language: en
chapter: 19
exercises: 12
source: https://one-course.com/books/chemistry/1/en/chapter/19-metals-reactions-with-acids-and-corrosion
license: CC-BY-NC-SA-4.0
credit: "One Chemistry Book, One Course (one-course.com)"
---

# Chapter 19 — Metals: Reactions with Acids and Corrosion

In a harbour, the steel hull of an old fishing boat is streaked with orange [rust](#def-g9-metals-acids-corrosion-corrosion). On the quay, a zinc-coated gate has stood in the rain for thirty years and still shines dull grey. In a museum, a gold ring buried for three thousand years is as bright as the day it was made. Metals do not all face the world in the same way: some are attacked by water, [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) and acids, others hardly at all.

**You already know.**

[Ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) are charged [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom); metals give [cations](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) such as $\ce{Fe^{2+}}$, $\ce{Zn^{2+}}$, $\ce{Al^{3+}}$, identified by [precipitation](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-precipitate) tests ([Chapter 17](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#ch-g9-ions)). An [acidic solution](https://one-course.com/books/chemistry/1/en/chapter/18-acids-bases-and-ph#def-g9-acids-bases-ph-acidic) contains many $\ce{H+(aq)}$ [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) ([Chapter 18](https://one-course.com/books/chemistry/1/en/chapter/18-acids-bases-and-ph#ch-g9-acids-bases-ph)). Hydrogen burns with a squeaky pop at the mouth of a test tube ([Chapter 10](https://one-course.com/books/chemistry/1/en/chapter/10-identifying-substances#ch-g7-identifying-substances)).

![The hull of an old boat: the steel is rusting.](https://one-course.com/images/onecourse/chapters/chemistry-1/g9-metals-acids-corrosion/img-abc3f3038cbc.jpg)

*The hull of an old boat: the steel is rusting.*

## 19.1 Metals in hydrochloric acid

**In the lab — Three metals in acid.**

The teacher drops a small piece of iron, of zinc and of copper into three test tubes of dilute hydrochloric acid. Bubbles rise from the iron and the zinc, slowly from the iron, briskly from the zinc, and the metals slowly disappear; nothing happens to the copper. The gas, collected in a small tube, gives a squeaky pop: it is hydrogen. With sodium hydroxide the liquid of the first tube gives a green [precipitate](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-precipitate) ($\ce{Fe^{2+}}$ [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion)), that of the second a white one ($\ce{Zn^{2+}}$ [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion)).

![Iron, zinc and copper in dilute hydrochloric acid: iron and zinc give off hydrogen; copper does not react.](https://one-course.com/images/onecourse/chapters/chemistry-1/g9-metals-acids-corrosion/fig-31f675205049.svg)

*Iron, zinc and copper in dilute hydrochloric acid: iron and zinc give off hydrogen; copper does not react.*

**Proposition 19.1 (Metal plus acid).**

Many metals — iron, zinc, aluminium, magnesium — react with the $\ce{H+}$ [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) of an acid: the metal disappears as metal [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion), which stay dissolved, and hydrogen gas is given off. Copper, silver and gold do not react with hydrochloric acid.

## 19.2 Writing the equation with ions

**Definition 19.2 (Spectator ion).**

An [ion](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) that is present in a solution during a reaction but takes no part in it, and is found unchanged at the end, is a *spectator ion*. It is left out of the equation.

**Example 19.3 (Iron and zinc in hydrochloric acid).**

Hydrochloric acid contains $\ce{H+(aq)}$ and $\ce{Cl-(aq)}$ [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion). The chloride [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) are found unchanged at the end: they are spectators. The equations are

$$
\ce{Fe(s) + 2H+(aq) -> Fe^{2+}(aq) + H2(g)}, \qquad
  \ce{Zn(s) + 2H+(aq) -> Zn^{2+}(aq) + H2(g)}.
$$

They balance both the [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) and the charges: $+2$ on each side.

**Method 19.4 (Writing the equation of a metal with an acid).**

1. [Reactants](https://one-course.com/books/chemistry/1/en/chapter/12-chemical-reactions-reactants-and-products#def-g7-chemical-reactions-reactant) : the metal (s) and the $\ce{H+(aq)}$ [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) .
2. Products: the metal [ion](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) (aq), with its usual charge, and hydrogen $\ce{H2(g)}$ .
3. Balance the [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) , then check that the total charge is the same on both sides.

For aluminium: $\ce{2Al(s) + 6H+(aq) -> 2Al^{3+}(aq) + 3H2(g)}$; charge $+6$ on each side.

**Safety.**

![](https://one-course.com/images/onecourse/chapters/chemistry-1/g9-metals-acids-corrosion/fig-25de6905b70f.svg)

![](https://one-course.com/images/onecourse/chapters/chemistry-1/g9-metals-acids-corrosion/fig-fb377c177908.svg)

The hydrogen given off is extremely flammable. The reaction is run in small test tubes, by the teacher, far from any flame except the one used for the pop test.

## 19.3 Corrosion

**Definition 19.5 (Corrosion and rust).**

*Corrosion* is the slow attack of a metal by the substances around it — oxygen, water, acids, salts — which turns the metal into [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) or into compounds such as oxides. The corrosion of iron and steel produces *rust*, a brown, crumbly, hydrated iron oxide that flakes off and lets the attack go on deeper.

**Proposition 19.6 (What iron needs to rust).**

Iron [rusts](#def-g9-metals-acids-corrosion-corrosion) only when both oxygen and water reach it. Salt dissolved in the water, as on roads salted in winter or by the sea, makes rusting faster.

**In the lab — Three nails, one week.**

Three clean iron nails are placed in three test tubes: the first with dry [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) and a drying agent that removes water vapour; the second in water boiled to remove its dissolved [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air), covered with a layer of oil; the third half in tap water, half in [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air). After a week, only the third nail is rusty.

![The three-nail experiment: iron rusts only where water and oxygen reach it together, here at the surface of the water.](https://one-course.com/images/onecourse/chapters/chemistry-1/g9-metals-acids-corrosion/fig-eab4a5a4af3c.svg)

*The three-nail experiment: iron [rusts](#def-g9-metals-acids-corrosion-corrosion) only where water and oxygen reach it together, here at the surface of the water.*

**Example 19.7 (Aluminium and copper protect themselves).**

Aluminium is attacked by the oxygen of the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) at once, but the thin layer of oxide formed sticks to the metal and seals it from the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air): aluminium window frames last for decades. Copper, slowly attacked by [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air), water and carbon dioxide, becomes covered with a green crust, the patina, which also sticks and protects the metal below. [Rust](#def-g9-metals-acids-corrosion-corrosion), by contrast, flakes off.

![The Statue of Liberty: its copper skin is covered with a green patina. Photo: Elcobbola, public domain.](https://one-course.com/images/onecourse/chapters/chemistry-1/g9-metals-acids-corrosion/img-f28d67d6bfc0.jpg)

*The Statue of Liberty: its copper skin is covered with a green patina. Photo: Elcobbola, public domain.*

## 19.4 Protecting metals

**Definition 19.8 (Galvanising).**

*Galvanising* is coating steel with a layer of zinc, by dipping it in molten zinc. Zinc is attacked by [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) and water rather than the steel below it: even where the coating is scratched, the nearby zinc is corroded first and the steel is spared.

**Proposition 19.9 (Ways to protect iron and steel).**

- keep water and [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) away: paint, oil, grease, a [plastic](https://one-course.com/books/chemistry/1/en/chapter/15-plastics-and-synthetic-materials#def-g8-plastics-plastic) coating;
- galvanise: a zinc layer that corrodes in place of the steel;
- use stainless steel, an alloy of iron with chromium, which protects itself with a thin sealing oxide layer;
- fix blocks of zinc to ship hulls: the zinc is slowly eaten away instead of the steel, and is replaced from time to time (how this works is explained in a university volume).

![](https://one-course.com/images/onecourse/chapters/chemistry-1/g9-metals-acids-corrosion/fig-10baaad26b7c.svg)

![](https://one-course.com/images/onecourse/chapters/chemistry-1/g9-metals-acids-corrosion/img-6491011a1785.jpg)

*A scratched galvanised sheet.*

*Galvanised steel railings.*

## 19.5 Exercises

**Exercise 19.1 ★.**

Which gas is given off when zinc reacts with hydrochloric acid? How is it identified?

**Solution of Exercise 19.1.**

Hydrogen: a lighted splint at the mouth of the tube gives a squeaky pop.

**Exercise 19.2 ★.**

Which of these metals react with dilute hydrochloric acid: iron, copper, zinc, gold, aluminium?

**Solution of Exercise 19.2.**

Iron, zinc and aluminium react; copper and gold do not.

**Exercise 19.3 ★.**

What does iron need in order to [rust](#def-g9-metals-acids-corrosion-corrosion)?

**Solution of Exercise 19.3.**

Both oxygen and water.

**Exercise 19.4 ★.**

What is a [spectator ion](#def-g9-metals-acids-corrosion-spectator)? Name the [spectator ion](#def-g9-metals-acids-corrosion-spectator) when iron reacts with hydrochloric acid.

**Solution of Exercise 19.4.**

An [ion](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) present during a reaction that takes no part in it. With hydrochloric acid: the chloride [ion](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) $\ce{Cl-}$.

**Exercise 19.5 ★★.**

Write the equation of the reaction of magnesium with the $\ce{H+}$ [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) of an acid (the magnesium [ion](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) is $\ce{Mg^{2+}}$). Check the charges.

**Solution of Exercise 19.5.**

$\ce{Mg(s) + 2H+(aq) -> Mg^{2+}(aq) + H2(g)}$; charge $+2$ on each side.

**Exercise 19.6 ★★.**

After iron has reacted with hydrochloric acid, how can you show that the solution contains iron(II) [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion)?

**Solution of Exercise 19.6.**

Add sodium hydroxide: a green [precipitate](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-precipitate) of $\ce{Fe(OH)2}$ shows iron(II) [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion).

**Exercise 19.7 ★★.**

Look at the three-nail figure. Why is the water of the second tube boiled and covered with oil?

**Solution of Exercise 19.7.**

Boiling removes the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) dissolved in the water, and the oil stops [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) from dissolving again: the nail has water but no oxygen.

**Exercise 19.8 ★★.**

Why does a car [rust](#def-g9-metals-acids-corrosion-corrosion) faster in a town where roads are salted in winter?

**Solution of Exercise 19.8.**

Salt dissolved in the water splashed on the car makes rusting faster.

**Exercise 19.9 ★★.**

Why do aluminium window frames not [rust](#def-g9-metals-acids-corrosion-corrosion) away, although aluminium reacts with the oxygen of the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air)?

**Solution of Exercise 19.9.**

The oxide layer formed sticks to the metal and seals it from the [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air): the attack stops at the surface.

**Exercise 19.10 ★★.**

Give three ways of protecting a steel bicycle frame from [rust](#def-g9-metals-acids-corrosion-corrosion).

**Solution of Exercise 19.10.**

Paint it, oil or grease it, galvanise it (or make it of stainless steel).

**Exercise 19.11 ★★★.**

Write and check the equation of the reaction of aluminium with the $\ce{H+}$ [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) of an acid. How many hydrogen [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) are formed for 200 aluminium [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom)?

**Solution of Exercise 19.11.**

$\ce{2Al(s) + 6H+(aq) -> 2Al^{3+}(aq) + 3H2(g)}$: 2 Al and 6 H on each side, charge $+6$ on each side. For 200 Al [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom): $200 \times \frac{3}{2} = 300$ hydrogen [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule).

**Exercise 19.12 ★★★.**

A galvanised steel bucket is scratched to the steel. Explain why the steel at the bottom of the scratch does not [rust](#def-g9-metals-acids-corrosion-corrosion) for a long time.

**Solution of Exercise 19.12.**

The zinc around the scratch is corroded in place of the steel: as long as zinc remains nearby, the steel is spared.

## 19.6 Problem: How Thick Is the Zinc?

**Problem 19.1.**

Weekend problem — dissolving the zinc coat of a galvanised plate to find out how thick it was

A galvanised steel plate, a square of side $10.0\,\mathrm{cm}$, is coated with zinc on both faces (the edges are neglected). In a laboratory, the teacher weighs it, $125.6\,\mathrm{g}$, then dips it in hydrochloric acid containing an additive that stops the acid from attacking the steel. When the bubbling stops, the plate is rinsed, dried and weighed again: $113.5\,\mathrm{g}$. The mass of $1\,\mathrm{cm}^{3}$ of zinc is $7.13\,\mathrm{g}$.

**Part I — The reaction.**

1. Write the equation of the reaction of zinc with the $\ce{H+}$ [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) of the acid.
2. Which gas causes the bubbling? Describe its test.
3. Which test would show the zinc [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) in the solution afterwards?
4. Name the [spectator ions](#def-g9-metals-acids-corrosion-spectator) .

**Part II — The zinc.**

5. What mass of zinc has dissolved?
6. Why is the bubbling a sign that the zinc has not all been dissolved yet?
7. What would happen to the steel if the acid did not contain the additive? Which test would then show it?
8. Why does the zinc protect the steel of the plate as long as it is there?

**Part III — The thickness.**

9. What is the total coated area of the plate, in $\mathrm{cm}^{2}$ ?
10. What volume did the zinc occupy?
11. Deduce the thickness of the zinc layer, in centimetres.
12. Express this thickness in micrometres ( $1\,\mathrm{cm}$ $= 10\,000\,\text{µ}\mathrm{m}$ ).

**Solution of Problem 19.1.**

**1.** $\ce{Zn(s) + 2H+(aq) -> Zn^{2+}(aq) + H2(g)}$.

**2.** Hydrogen: a squeaky pop with a lighted splint.

**3.** Sodium hydroxide gives a white [precipitate](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-precipitate) of $\ce{Zn(OH)2}$.

**4.** The chloride [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion) $\ce{Cl-}$.

**5.** $125.6 - 113.5 = 12.1\,\mathrm{g}$.

**6.** Hydrogen is given off as long as zinc reacts with the acid; when no zinc is left, the bubbling stops.

**7.** The iron of the steel would react too, giving hydrogen and $\ce{Fe^{2+}}$ [ions](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-ion); sodium hydroxide would then give a green [precipitate](https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions#def-g9-ions-precipitate).

**8.** The zinc is attacked in place of the steel and keeps water and [air](https://one-course.com/books/chemistry/1/en/chapter/4-air-a-mixture-of-gases#def-g4-air-a-mixture-of-gases-air) away from it.

**9.** $2 \times 10.0 \times 10.0 = 200\,\mathrm{cm}^{2}$.

**10.** $12.1 \div 7.13 \approx 1.70\,\mathrm{cm}^{3}$.

**11.** $1.70 \div 200 = 0.0085\,\mathrm{cm}$.

**12.** $0.0085 \times 10\,000 = 85\,\text{µ}\mathrm{m}$: the zinc layer was about $85\,\text{µ}\mathrm{m}$ thick.