Salts

IP Chemistry (6092), Sec 3

By the end of this topic, you can answer

1

Table salt is only one salt. What makes a compound a salt?

2

Which salts dissolve in water, and which do not?

3

Why do we add an excess of zinc when we make zinc sulfate?

4

Why is sodium nitrate made by titration and not with sodium metal?

5

How do you make an insoluble salt such as barium sulfate?

6

Given a list of chemicals, how do you plan the preparation of a salt?

Starter

PredictYou want to make blue copper(II) sulfate crystals. You have copper metal, copper(II) oxide and dilute sulfuric acid.

Which starting materials would you use, and how would you get crystals at the end?

NoteNot every pair of chemicals works, and the right method depends on whether each substance dissolves in water. This topic shows how to choose.

1

Salts and Solubility

1.1What a salt is

A salt is an ionic compound made of a cation (positive ion) and an anion (negative ion).

A salt forms when the hydrogen ions of an acid are replaced by metal ions or ammonium ions.
Sulfuric acid gives sulfates, nitric acid gives nitrates and hydrochloric acid gives chlorides.

AnalogyA salt's name is like a person's name: the first name comes from the metal, and the surname from the acid. Copper(II) sulfate takes "copper" from copper(II) oxide and "sulfate" from sulfuric acid.

1

Salts and Solubility

1.2Solubility rules

Salts	Soluble in water	Insoluble in water
sodium, potassium and ammonium salts	all	none
nitrates	all	none
chlorides	most	AgCl and PbCl₂
sulfates	most	BaSO₄, PbSO₄ and CaSO₄ (sparingly soluble)
carbonates	sodium, potassium and ammonium only	all others
hydroxides	sodium, potassium and ammonium (calcium hydroxide slightly)	all others

AnalogyThink of a "SNAP" pass: Sodium, Nitrate, Ammonium and Potassium salts always get into the water. Everyone else has to check the rules.

1

Salts and Solubility

Quick checkWhich salt is insoluble in water?

Aammonium sulfate

Blead(II) nitrate

Csilver chloride

Dsodium carbonate

NoteC: silver chloride is one of the two insoluble chlorides. All ammonium salts, all nitrates and all sodium salts are soluble.

2

Preparing Salts

2.1Choosing a method

2

Preparing Salts

2.2Method 1: an acid and an excess insoluble substance

Add the solid in excess, so all the acid is used up. The excess is insoluble, so it is filtered off.
Heat the filtrate until saturated: small crystals form on a glass rod dipped in and removed.
Cool to crystallise, then filter, wash with a little cold distilled water and dry the crystals.

2

Preparing Salts

2.2Acids with metals, bases and carbonates

H₂SO₄(aq) + Zn(s) → ZnSO₄(aq) + H₂(g)

2HNO₃(aq) + CuO(s) → Cu(NO₃)₂(aq) + H₂O(l)

2HCl(aq) + MgCO₃(s) → MgCl₂(aq) + CO₂(g) + H₂O(l)

Do not use very reactive metals: they react violently with acids.
Unreactive metals such as copper do not react with dilute acids. Use copper(II) oxide or copper(II) carbonate instead.

AnalogyAdding the solid in excess is like topping up a drink with ice until the last cube stops melting. When some solid is left over, you know all the acid is used up.

2

Preparing Salts

2.3Method 2: titration

Used for sodium, potassium and ammonium salts: all the starting materials are soluble.
An excess cannot be filtered off, so the volumes must be exact.
Add the acid until the indicator just changes colour: the end-point.
Repeat with the same volumes without the indicator, then crystallise.

HNO₃(aq) + NaOH(aq) → NaNO₃(aq) + H₂O(l)

2

Preparing Salts

2.4Method 3: precipitation

An insoluble salt is made by mixing a solution of its cation with a solution of its anion.

Mix the solutions and stir until no more precipitate forms.
Filter to collect the precipitate as the residue.
Wash with distilled water, then dry between sheets of filter paper.

2

Preparing Salts

2.4Writing the equations

Ba(NO₃)₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaNO₃(aq)

Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

The sodium ions and nitrate ions are spectators: they stay in solution and pass through the filter paper.

AnalogyMixing the solutions is like letting two crowds into one hall. Most people keep moving, but every barium ion that meets a sulfate ion locks arms with it and sits down: the precipitate.

2

Preparing Salts

2.5Planning a preparation

Salt	Soluble?	Method	Starting materials
zinc sulfate	yes	excess insoluble substance	dilute sulfuric acid + excess zinc, zinc oxide or zinc carbonate
copper(II) nitrate	yes	excess insoluble substance	dilute nitric acid + excess copper(II) oxide or carbonate (not copper)
sodium nitrate	yes	titration	dilute nitric acid + aqueous sodium hydroxide or carbonate (not sodium)
barium sulfate	no	precipitation	aqueous barium nitrate + dilute sulfuric acid or any soluble sulfate

2

Preparing Salts

Worked exampleDescribe how you would prepare lead(II) chloride from lead(II) carbonate, dilute nitric acid and aqueous potassium chloride.

Step 1Lead(II) chloride is insoluble, so use precipitation. First turn insoluble lead(II) carbonate into soluble lead(II) nitrate.

Step 2Add excess lead(II) carbonate to dilute nitric acid, then filter off the unreacted carbonate.

Step 3Add excess aqueous potassium chloride to the filtrate: Pb(NO₃)₂(aq) + 2KCl(aq) → PbCl₂(s) + 2KNO₃(aq).

Step 4Filter, wash the residue with distilled water and dry it between sheets of filter paper.

2

Preparing Salts

2.6Common mistakes

Mistake	Why it is wrong
Using sodium or potassium metal with an acid	Very reactive metals react violently with acids.
Using copper or silver with a dilute acid	Unreactive metals do not react with dilute acids.
Not adding the solid in excess (method 1)	Some acid is left, so the salt is contaminated with acid.
Leaving the indicator in (method 2)	The indicator makes the salt impure.
Using method 1 for a sodium salt	Sodium compounds are soluble, so an excess cannot be filtered off.
Washing crystals with a lot of water	Use a little cold distilled water, or the crystals dissolve.

2

Preparing Salts

Quick checkWhich method is best for preparing potassium sulfate?

Apotassium metal with dilute sulfuric acid

Bexcess potassium carbonate with dilute sulfuric acid, then filter

Ctitration of aqueous potassium hydroxide with dilute sulfuric acid

Dprecipitation from two solutions

NoteC: potassium sulfate and all its starting materials are soluble, so titration is used. Potassium reacts violently with acids, an excess of soluble potassium carbonate cannot be filtered off, and precipitation is only for insoluble salts.

Exit ticket

1Name two insoluble sulfates.

Any two of barium sulfate, lead(II) sulfate and calcium sulfate.[1]

2Why is the solid added in excess in method 1?

So that all the acid is used up and the salt is not contaminated with acid.[1]

3Which method is used to make sodium chloride?

Titration of dilute hydrochloric acid with aqueous sodium hydroxide.[1]

4Write the ionic equation for the precipitation of barium sulfate.

Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)[1]