---
title: "Ions and Ionic Solutions"
book: "School Chemistry — Grades 1 to 12"
subject: chemistry
language: en
chapter: 17
exercises: 12
source: https://one-course.com/books/chemistry/1/en/chapter/17-ions-and-ionic-solutions
license: CC-BY-NC-SA-4.0
credit: "One Chemistry Book, One Course (one-course.com)"
---

# Chapter 17 — Ions and Ionic Solutions

Two clear drinks stand on a gym bench: a bottle of sports drink, sold for its “electrolytes”, and a glass of sugared water. In a laboratory, two metal plates connected to a battery and a small bulb are dipped into each in turn. In the sports drink the bulb lights up; in the sugared water it stays dark. Something in the first liquid carries electric charge, and the sugar does not. That something is [ions](#def-g9-ions-ion).

**You already know.**

An [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) has a [nucleus](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) with $Z$ [protons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-proton), each of charge $+e$, and $Z$ [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) around it, each of charge $-e$: the [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) is neutral ([Chapter 16](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#ch-g9-inside-the-atom)). A [solute](https://one-course.com/books/chemistry/1/en/chapter/9-solutions-and-solubility#def-g6-solutions-and-solubility-solute) dissolved in water gives an [aqueous solution](https://one-course.com/books/chemistry/1/en/chapter/9-solutions-and-solubility#def-g6-solutions-and-solubility-solute) ([Chapter 9](https://one-course.com/books/chemistry/1/en/chapter/9-solutions-and-solubility#ch-g6-solutions-and-solubility)). A [characteristic test](https://one-course.com/books/chemistry/1/en/chapter/10-identifying-substances#def-g7-identifying-substances-characteristic-test) detects a species by a visible result ([Chapter 10](https://one-course.com/books/chemistry/1/en/chapter/10-identifying-substances#ch-g7-identifying-substances)).

![A sports drink and a glass of water: the drink contains dissolved ions.](https://one-course.com/images/onecourse/chapters/chemistry-1/g9-ions/img-2f163e213984.jpg)

*A sports drink and a glass of water: the drink contains dissolved [ions](#def-g9-ions-ion).*

## 17.1 Atoms that gain or lose electrons

**Definition 17.1 (Ion, cation, anion).**

An *ion* is an [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom), or a group of [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom), that has lost or gained one or more [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus), and so carries an electric charge. An ion with a positive charge, having lost [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus), is a *cation*; an ion with a negative charge, having gained [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus), is an *anion*. The charge is written at the top right of the formula: $\ce{Na+}$, $\ce{Cu^{2+}}$, $\ce{Cl-}$.

**Example 17.2 (Sodium and chlorine).**

A sodium [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom), 11 [protons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-proton) and 11 [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus), loses one [electron](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus): it becomes the sodium [ion](#def-g9-ions-ion) $\ce{Na+}$, 11 [protons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-proton) and 10 [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus), charge $+1$:

$$
\ce{Na -> Na+ + e-}.
$$

A chlorine [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom), 17 [protons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-proton) and 17 [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus), gains one [electron](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus): it becomes the chloride [ion](#def-g9-ions-ion) $\ce{Cl-}$, 17 [protons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-proton) and 18 [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus):

$$
\ce{Cl + e- -> Cl-}.
$$

![A sodium atom becomes a sodium ion by losing an electron; a chlorine atom becomes a chloride ion by gaining one. The protons do not change.](https://one-course.com/images/onecourse/chapters/chemistry-1/g9-ions/fig-225984458690.svg)

*A sodium [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) becomes a sodium [ion](#def-g9-ions-ion) by losing an [electron](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus); a chlorine [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) becomes a chloride [ion](#def-g9-ions-ion) by gaining one. The [protons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-proton) do not change.*

**Proposition 17.3 (Some common ions).**

Monatomic [ions](#def-g9-ions-ion) are made from one [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom); polyatomic [ions](#def-g9-ions-ion) from a group of [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) that stays together:

| [cations](#def-g9-ions-ion) |  | [anions](#def-g9-ions-ion) |  |
| --- | --- | --- | --- |
| sodium | $\ce{Na+}$ | chloride | $\ce{Cl-}$ |
| potassium | $\ce{K+}$ | hydroxide | $\ce{OH-}$ |
| calcium | $\ce{Ca^{2+}}$ | nitrate | $\ce{NO3-}$ |
| magnesium | $\ce{Mg^{2+}}$ | carbonate | $\ce{CO3^{2-}}$ |
| copper(II) | $\ce{Cu^{2+}}$ | sulfate | $\ce{SO4^{2-}}$ |
| iron(II), iron(III) | $\ce{Fe^{2+}}$, $\ce{Fe^{3+}}$ |  |  |
| zinc | $\ce{Zn^{2+}}$ |  |  |
| aluminium | $\ce{Al^{3+}}$ |  |  |
| ammonium | $\ce{NH4+}$ |  |  |

## 17.2 Ionic compounds and their formulas

**Definition 17.4 (Ionic compound).**

An *ionic compound* is made of [cations](#def-g9-ions-ion) and [anions](#def-g9-ions-ion) in such numbers that the whole carries no charge. Its formula gives the proportion of the [ions](#def-g9-ions-ion), without their charges: $\ce{NaCl}$ for sodium chloride, one $\ce{Na+}$ for each $\ce{Cl-}$; $\ce{CaCl2}$ for calcium chloride, two $\ce{Cl-}$ for each $\ce{Ca^{2+}}$.

**Method 17.5 (Writing the formula of an ionic compound).**

1. Write the two [ions](#def-g9-ions-ion) with their charges, [cation](#def-g9-ions-ion) first.
2. Find the smallest numbers of each that make the total charge zero.
3. Write these numbers as [subscripts](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-formula) ; a polyatomic [ion](#def-g9-ions-ion) needing a [subscript](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-formula) goes in brackets.

Aluminium oxide: $\ce{Al^{3+}}$ and $\ce{O^{2-}}$; $2 \times (+3) + 3 \times
(-2) = 0$, so $\ce{Al2O3}$. Iron(III) hydroxide: $\ce{Fe^{3+}}$ and three $\ce{OH-}$: $\ce{Fe(OH)3}$.

**Proposition 17.6 (An ionic solid is a stack of ions).**

In a crystal of salt, sodium [ions](#def-g9-ions-ion) and chloride [ions](#def-g9-ions-ion) alternate in a regular three-dimensional stack, each [ion](#def-g9-ions-ion) surrounded by [ions](#def-g9-ions-ion) of the opposite charge, which hold it in place. A grain of table salt holds billions of billions of them; its cubic shape reflects the cubic stack.

![](https://one-course.com/images/onecourse/chapters/chemistry-1/g9-ions/fig-b5771fb8ee75.svg)

![](https://one-course.com/images/onecourse/chapters/chemistry-1/g9-ions/img-04cfee820721.jpg)

*One layer of a salt crystal: sodium [ions](#def-g9-ions-ion) (purple, smaller) and chloride [ions](#def-g9-ions-ion) (green) alternate.*

*Halite, natural rock salt. Photo: Robert M. Lavinsky, CC BY-SA 3.0.*

## 17.3 Ionic solutions conduct

**Notation 17.7 (The state symbol (aq)).**

An [ion](#def-g9-ions-ion) or a [molecule](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) dissolved in water is marked (aq), for aqueous: $\ce{Na+(aq)}$, $\ce{Cl-(aq)}$.

**Proposition 17.8 (Ionic solutions conduct electricity).**

When an [ionic compound](#def-g9-ions-ionic-compound) [dissolves](https://one-course.com/books/chemistry/1/en/chapter/2-mixing-and-dissolving#def-g2-mixing-and-dissolving-dissolve) in water, its [ions](#def-g9-ions-ion) separate and move freely in the solution: salt water contains $\ce{Na+(aq)}$ and $\ce{Cl-(aq)}$. Moving [ions](#def-g9-ions-ion) carry electric charge, so an ionic solution conducts electricity. A solution of sugar, whose [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) carry no charge, does not; nor does very pure water.

**In the lab — Which liquids conduct?.**

Two metal plates, a low-voltage battery and a small bulb are joined in a circuit by the teacher; the plates are dipped in turn into distilled water, sugar water, salt water and a copper sulfate solution, and rinsed between each. The bulb lights only with the two ionic solutions.

![The conductivity test: with salt water, ions carry the current and the bulb lights; sugar water contains no ions and the bulb stays dark.](https://one-course.com/images/onecourse/chapters/chemistry-1/g9-ions/fig-356663e87b32.svg)

*The conductivity test: with salt water, [ions](#def-g9-ions-ion) carry the current and the bulb lights; sugar water contains no [ions](#def-g9-ions-ion) and the bulb stays dark.*

## 17.4 Precipitation tests for ions

**Definition 17.9 (Precipitate).**

When two solutions are mixed, a [cation](#def-g9-ions-ion) of one and an [anion](#def-g9-ions-ion) of the other may form an [ionic compound](#def-g9-ions-ionic-compound) that does not [dissolve](https://one-course.com/books/chemistry/1/en/chapter/2-mixing-and-dissolving#def-g2-mixing-and-dissolving-dissolve). It appears as a fine solid that clouds the liquid and settles: a *precipitate*. The reaction is a *precipitation*.

**Proposition 17.10 (Tests for common ions).**

A few drops of a [reagent](https://one-course.com/books/chemistry/1/en/chapter/10-identifying-substances#def-g7-identifying-substances-characteristic-test) solution identify an [ion](#def-g9-ions-ion) by the colour of the [precipitate](#def-g9-ions-precipitate) it forms:

| [ion](#def-g9-ions-ion) looked for | [reagent](https://one-course.com/books/chemistry/1/en/chapter/10-identifying-substances#def-g7-identifying-substances-characteristic-test) | [precipitate](#def-g9-ions-precipitate) | colour |
| --- | --- | --- | --- |
| copper(II) $\ce{Cu^{2+}}$ | sodium hydroxide | $\ce{Cu(OH)2}$ | blue |
| iron(II) $\ce{Fe^{2+}}$ | sodium hydroxide | $\ce{Fe(OH)2}$ | green |
| iron(III) $\ce{Fe^{3+}}$ | sodium hydroxide | $\ce{Fe(OH)3}$ | orange-brown |
| zinc $\ce{Zn^{2+}}$ | sodium hydroxide | $\ce{Zn(OH)2}$ | white |
| aluminium $\ce{Al^{3+}}$ | sodium hydroxide | $\ce{Al(OH)3}$ | white |
| chloride $\ce{Cl-}$ | silver nitrate | $\ce{AgCl}$ | white, darkens in light |

**Example 17.11 (Writing a precipitation).**

Only the [ions](#def-g9-ions-ion) that form the solid are written; the others stay dissolved and take no part:

$$
\ce{Cu^{2+}(aq) + 2OH-(aq) -> Cu(OH)2(s)}, \qquad
  \ce{Ag+(aq) + Cl-(aq) -> AgCl(s)}.
$$

![Precipitates formed with sodium hydroxide (first five tubes) and with silver nitrate (last tube).](https://one-course.com/images/onecourse/chapters/chemistry-1/g9-ions/fig-ee779bbc234b.svg)

*[Precipitates](#def-g9-ions-precipitate) formed with sodium hydroxide (first five tubes) and with silver nitrate (last tube).*

**Safety.**

![](https://one-course.com/images/onecourse/chapters/chemistry-1/g9-ions/fig-93c17238ab7b.svg)

![](https://one-course.com/images/onecourse/chapters/chemistry-1/g9-ions/fig-0df833fa40ae.svg)

Sodium hydroxide: causes severe skin burns and eye damage, and may be corrosive to metals (GHS05); irritating to skin and eyes in dilute form (GHS07). Silver nitrate, the chloride [reagent](https://one-course.com/books/chemistry/1/en/chapter/10-identifying-substances#def-g7-identifying-substances-characteristic-test), is an oxidiser, corrosive and very toxic to aquatic life. Goggles, gloves; [reagents](https://one-course.com/books/chemistry/1/en/chapter/10-identifying-substances#def-g7-identifying-substances-characteristic-test) dispensed by drops by the teacher.

**Method 17.12 (Testing for an ion).**

1. Pour a little of the solution to be tested into a test tube.
2. Add the [reagent](https://one-course.com/books/chemistry/1/en/chapter/10-identifying-substances#def-g7-identifying-substances-characteristic-test) drop by drop.
3. Note the colour of any [precipitate](#def-g9-ions-precipitate) and compare with the table.
4. Conclude, and remember that a test identifies an [ion](#def-g9-ions-ion) , not a compound: the iron(III) chloride solution gives both the $\ce{Fe^{3+}}$ test and the $\ce{Cl-}$ test.

## 17.5 Exercises

**Exercise 17.1 ★.**

What is an [ion](#def-g9-ions-ion)? What is the difference between a [cation](#def-g9-ions-ion) and an [anion](#def-g9-ions-ion)?

**Solution of Exercise 17.1.**

An [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) or group of [atoms](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) that has lost or gained [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) and carries a charge. A [cation](#def-g9-ions-ion) is positive ([electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) lost), an [anion](#def-g9-ions-ion) negative ([electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) gained).

**Exercise 17.2 ★.**

A magnesium [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) (12 [protons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-proton)) loses two [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus). Write the formula of the [ion](#def-g9-ions-ion) formed, and give its numbers of [protons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-proton) and [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus).

**Solution of Exercise 17.2.**

$\ce{Mg^{2+}}$: 12 [protons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-proton), 10 [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus).

**Exercise 17.3 ★.**

Write the formulas of potassium chloride, magnesium chloride and calcium oxide (oxide [ion](#def-g9-ions-ion): $\ce{O^{2-}}$).

**Solution of Exercise 17.3.**

$\ce{KCl}$, $\ce{MgCl2}$, $\ce{CaO}$.

**Exercise 17.4 ★.**

Why does salt water conduct electricity while sugar water does not?

**Solution of Exercise 17.4.**

Salt water contains [ions](#def-g9-ions-ion), $\ce{Na+(aq)}$ and $\ce{Cl-(aq)}$, which move and carry charge. Sugar [molecules](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-molecule) carry no charge.

**Exercise 17.5 ★★.**

Write the formulas of copper(II) sulfate, aluminium chloride and sodium carbonate.

**Solution of Exercise 17.5.**

$\ce{CuSO4}$, $\ce{AlCl3}$, $\ce{Na2CO3}$.

**Exercise 17.6 ★★.**

Sodium hydroxide added to a solution gives an orange-brown [precipitate](#def-g9-ions-precipitate). Which [ion](#def-g9-ions-ion) does the solution contain? Write the equation of the [precipitation](#def-g9-ions-precipitate).

**Solution of Exercise 17.6.**

Iron(III), $\ce{Fe^{3+}}$: $\ce{Fe^{3+}(aq) + 3OH-(aq) -> Fe(OH)3(s)}$.

**Exercise 17.7 ★★.**

How many [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) does the sulfate [ion](#def-g9-ions-ion) $\ce{SO4^{2-}}$ carry in excess of its [protons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-proton)? The nitrate [ion](#def-g9-ions-ion) $\ce{NO3-}$?

**Solution of Exercise 17.7.**

Sulfate: 2 extra [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus). Nitrate: 1.

**Exercise 17.8 ★★.**

A solution gives a white [precipitate](#def-g9-ions-precipitate) with silver nitrate and a blue one with sodium hydroxide. Name the dissolved [ionic compound](#def-g9-ions-ionic-compound) and write its formula.

**Solution of Exercise 17.8.**

White with silver nitrate: $\ce{Cl-}$. Blue with sodium hydroxide: $\ce{Cu^{2+}}$. Copper(II) chloride, $\ce{CuCl2}$.

**Exercise 17.9 ★★.**

Look at the figure of the salt crystal layer. How many neighbours of opposite charge does an [ion](#def-g9-ions-ion) in the middle of the layer have in this layer?

**Solution of Exercise 17.9.**

Four: one on each side (left, right, above, below).

**Exercise 17.10 ★★.**

Write the equation of the [precipitation](#def-g9-ions-precipitate) of iron(II) hydroxide from $\ce{Fe^{2+}}$ and $\ce{OH-}$ [ions](#def-g9-ions-ion).

**Solution of Exercise 17.10.**

$\ce{Fe^{2+}(aq) + 2OH-(aq) -> Fe(OH)2(s)}$.

**Exercise 17.11 ★★★.**

A white powder may be zinc chloride or sodium chloride. Dissolved in water, it gives a white [precipitate](#def-g9-ions-precipitate) with sodium hydroxide. Which is it? Write the equation.

**Solution of Exercise 17.11.**

Zinc chloride: sodium [ions](#def-g9-ions-ion) give no [precipitate](#def-g9-ions-precipitate) with sodium hydroxide, zinc [ions](#def-g9-ions-ion) give a white one: $\ce{Zn^{2+}(aq) + 2OH-(aq) -> Zn(OH)2(s)}$.

**Exercise 17.12 ★★★.**

In a crystal of calcium chloride, how many chloride [ions](#def-g9-ions-ion) are there for 1000 calcium [ions](#def-g9-ions-ion)? Check that the crystal is neutral.

**Solution of Exercise 17.12.**

$\ce{CaCl2}$: 2000 chloride [ions](#def-g9-ions-ion). Charges: $1000 \times (+2) + 2000 \times
(-1) = 0$.

## 17.6 Problem: The Rusty Well

**Problem 17.1.**

Weekend problem — orange stains in a farmhouse sink, two tests, and the number of iron ions in a litre of well water

The water of a farmhouse well leaves orange stains in the sink. A laboratory receives a sample. Freshly drawn, the water is clear; with sodium hydroxide it gives a green [precipitate](#def-g9-ions-precipitate), which turns orange-brown within an hour at the surface, where it meets 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 laboratory measures $0.30\,\mathrm{mg}$ of iron, in the form of iron [ions](#def-g9-ions-ion), per litre. Take the mass of a [nucleon](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-proton) as $1.67 \times 10^{-27}\,\mathrm{kg}$, and 56 [nucleons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-proton) for an iron [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom).

**Part I — The tests.**

1. Which [ion](#def-g9-ions-ion) does the green [precipitate](#def-g9-ions-precipitate) show? Give the formula of the [precipitate](#def-g9-ions-precipitate) .
2. Which [ion](#def-g9-ions-ion) does the orange-brown [precipitate](#def-g9-ions-precipitate) show? Give its formula.
3. An iron(II) [ion](#def-g9-ions-ion) and an iron(III) [ion](#def-g9-ions-ion) differ by one [electron](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) . Which has more [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) ? How many [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus) does each have (iron: $Z = 26$ )?
4. Write the equation of the [precipitation](#def-g9-ions-precipitate) of iron(III) hydroxide.

**Part II — The stains.**

5. In the sink, the clear water slowly turns cloudy and orange. Which [ion](#def-g9-ions-ion) does the water hold when it comes out of the well, and which does it end up holding?
6. Write the formula of the [ionic compound](#def-g9-ions-ionic-compound) formed by iron(III) [ions](#def-g9-ions-ion) and hydroxide [ions](#def-g9-ions-ion) , and check that it is neutral.
7. Can the iron of the water be removed by [filtering](https://one-course.com/books/chemistry/1/en/chapter/3-separating-mixtures#def-g3-separating-mixtures-filter) the water as it comes out of the well? After it has turned orange? Explain.
8. Is a water containing iron [ions](#def-g9-ions-ion) a [pure substance](https://one-course.com/books/chemistry/1/en/chapter/8-pure-substances-and-mixtures#def-g6-pure-substances-and-mixtures-pure-substance) ? A [homogeneous mixture](https://one-course.com/books/chemistry/1/en/chapter/8-pure-substances-and-mixtures#def-g6-pure-substances-and-mixtures-homogeneous) ?

**Part III — Counting the [ions](#def-g9-ions-ion).**

9. Compute the mass of an iron [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) , in kilograms.
10. Convert $0.30\,\mathrm{mg}$ into kilograms.
11. Why may the mass of an iron [ion](#def-g9-ions-ion) be taken equal to that of an iron [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) ?
12. Compute the number of iron [ions](#def-g9-ions-ion) in a litre of the well water.

**Solution of Problem 17.1.**

**1.** Iron(II), $\ce{Fe^{2+}}$: $\ce{Fe(OH)2}$.

**2.** Iron(III), $\ce{Fe^{3+}}$: $\ce{Fe(OH)3}$.

**3.** $\ce{Fe^{2+}}$ has one more: $26 - 2 = 24$ [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus), against $26 - 3 = 23$ for $\ce{Fe^{3+}}$.

**4.** $\ce{Fe^{3+}(aq) + 3OH-(aq) -> Fe(OH)3(s)}$.

**5.** It comes out holding iron(II) [ions](#def-g9-ions-ion); in contact with 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) they become iron(III) [ions](#def-g9-ions-ion), which form the orange solid.

**6.** $\ce{Fe(OH)3}$: $(+3) + 3 \times (-1) = 0$.

**7.** Not as it comes out: the iron [ions](#def-g9-ions-ion) are dissolved and pass through a filter. Once the orange solid has formed, yes: it is a [precipitate](#def-g9-ions-precipitate), held back by a filter.

**8.** No: a [mixture](https://one-course.com/books/chemistry/1/en/chapter/8-pure-substances-and-mixtures#def-g6-pure-substances-and-mixtures-mixture) (water and dissolved [ions](#def-g9-ions-ion)). Freshly drawn and clear, it is homogeneous.

**9.** $56 \times 1.67 \times 10^{-27} = 9.35 \times 10^{-26}\,\mathrm{kg}$.

**10.** $0.30\,\mathrm{mg} = 0.30 \times 10^{-6}\,\mathrm{kg} = 3.0 \times 10^{-7}\,\mathrm{kg}$.

**11.** An [ion](#def-g9-ions-ion) differs from the [atom](https://one-course.com/books/chemistry/1/en/chapter/11-atoms-and-molecules#def-g7-atoms-and-molecules-atom) by two or three [electrons](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus), whose mass is negligible beside the [nucleus](https://one-course.com/books/chemistry/1/en/chapter/16-inside-the-atom#def-g9-inside-the-atom-nucleus).

**12.** $3.0 \times 10^{-7} / 9.35 \times 10^{-26} \approx 3.2
\times 10^{18}$ iron [ions](#def-g9-ions-ion) per litre.