Preparation Of Salts

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Notes

What is a Salt?

A **salt** is a compound formed when the hydrogen atom in an acid is replaced by a **metal** or **ammonium ion**.
Example: Replacing H in HCl with K gives KCl (potassium chloride).
The name of a salt has two parts: first part from the metal/metal oxide/carbonate, second part from the acid.
Hydrochloric acid produces chlorides; sulfuric acid produces sulfates; nitric acid produces nitrates.

Solubility Rules

All **sodium, potassium, and ammonium** salts are soluble.
All **nitrates** are soluble.
Most **chlorides** are soluble except **silver chloride** and **lead(II) chloride**.
Most **sulfates** are soluble except **barium sulfate**, **calcium sulfate**, and **lead(II) sulfate**.
Most **carbonates** are insoluble except those of sodium, potassium, and ammonium.
Most **hydroxides** are insoluble except those of sodium, potassium, and ammonium (calcium hydroxide is sparingly soluble).

Preparing Soluble Salts – Method A (Excess Solid)

Use **excess solid reactant** (metal, insoluble base, or insoluble carbonate) with dilute acid.
Add solid until no more reacts, indicating excess.
**Filter** to remove unreacted solid; the filtrate is the salt solution.
**Evaporate** the filtrate gently until saturated (test with cold glass rod – crystals form).
**Crystallise** by leaving in a warm place, then **decant** excess liquid and **dry** crystals with filter paper.
Example (metal): Mg + H₂SO₄ → MgSO₄ + H₂.
Example (insoluble base): CuO + H₂SO₄ → CuSO₄ + H₂O.
Example (insoluble carbonate): CaCO₃ + 2HCl → CaCl₂ + CO₂ + H₂O.

Preparing Soluble Salts – Method B (Titration)

Use **titration** when the base is soluble (alkali).
Pipette a known volume of alkali into a conical flask, add a few drops of **indicator** (e.g., phenolphthalein).
Fill a burette with acid, add slowly while swirling until the indicator changes colour (end point).
Repeat without indicator using the same volumes to obtain a neutral solution.
**Evaporate**, **crystallise**, **decant**, and **dry** the crystals as in Method A.
Example: NaOH + HCl → NaCl + H₂O.

Preparing Insoluble Salts (Extended Tier)

Insoluble salts are prepared by **precipitation**.
Mix aqueous solutions of two soluble salts that react to form the insoluble salt (precipitate).
**Filter** to collect the precipitate.
**Wash** the precipitate with distilled water to remove soluble impurities.
**Dry** the solid (e.g., in a warm oven).
Example: Pb(NO₃)₂(aq) + K₂SO₄(aq) → PbSO₄(s) + 2KNO₃(aq).

Hydrated & Anhydrous Salts

**Hydrated salts** contain water of crystallisation within their structure (e.g., CuSO₄·5H₂O, blue).
**Anhydrous salts** contain no water (e.g., CuSO₄, white).
Heating a hydrated salt drives off water of crystallisation, forming the anhydrous salt.
Adding water to an anhydrous salt rehydrates it (reversible reaction).
Water of crystallisation is shown in the formula with a dot: e.g., CoCl₂·6H₂O.

Particle arrangement in solids, liquids, and gases – relevant to understanding solubility and crystallisation.

Practice questions

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1.When solid sodium chloride is mixed with water, sodium chloride solution forms. What name is given to the process of mixing a solid with water to form a solution?
Easy
- ACrystallising
- BDiluting
- CDissolving
- DMelting
2.Which salt is insoluble in water?
Easy
- ASodium chloride
- BPotassium nitrate
- CSilver chloride
- DAmmonium sulfate
3.A student reacts excess iron powder with sulfuric acid to prepare iron(II) sulfate. How would the student know when all the sulfuric acid has reacted?
Medium
- ANo more gas is produced and the mixture becomes neutral
- BThe solution turns blue
- CThe iron powder dissolves completely
- DA precipitate forms
4.Lead(II) sulfate is prepared by mixing aqueous lead(II) nitrate and aqueous potassium sulfate. What is the state symbol for lead(II) sulfate in the equation?
Hard
- A(aq)
- B(s)
- C(l)
- D(g)
5.What is the name of the salt formed when magnesium oxide reacts with sulfuric acid?
Easy
- AMagnesium sulfate
- BMagnesium sulfide
- CMagnesium sulfite
- DMagnesium oxide
6.In the preparation of a soluble salt using an excess solid method, what is the purpose of adding the solid in excess?
Medium
- ATo increase the rate of reaction
- BTo ensure all the acid reacts
- CTo produce more salt crystals
- DTo make the solution saturated
7.Which method should be used to prepare a soluble salt from a soluble base (alkali) and an acid?
Medium
- AExcess metal method
- BExcess insoluble base method
- CTitration method
- DPrecipitation method
8.Hydrated copper(II) sulfate is blue. When heated, it turns white. What is the white substance?
Hard
- AAnhydrous copper(II) sulfate
- BHydrated copper(II) sulfate
- CCopper(II) oxide
- DCopper metal

More questions

9.Which of the following is an insoluble salt?
Easy
- ASodium carbonate
- BPotassium chloride
- CBarium sulfate
- DAmmonium nitrate
10.In the preparation of copper(II) sulfate crystals, copper(II) oxide is added to warm dilute sulfuric acid. After filtering, the filtrate is heated until concentrated. How can you test if the solution is saturated?
Medium
- ADip a cold glass rod into the solution and see if crystals form
- BMeasure the pH of the solution
- CAdd more copper(II) oxide and see if it dissolves
- DWeigh the solution
11.Which of the following is the correct method to obtain pure dry lead(II) sulfate from a mixture of lead(II) sulfate and water?
Hard
- AEvaporate the water to dryness
- BFilter, wash the residue with distilled water, and dry
- CDecant the liquid and heat the residue
- DCrystallise by cooling
12.What is the formula of hydrated copper(II) sulfate?
Easy
- ACuSO4
- BCuSO4·5H2O
- CCuSO4·H2O
- DCuSO4·10H2O
13.A student prepares magnesium sulfate crystals by reacting excess magnesium oxide with warm dilute sulfuric acid. After filtering, the filtrate is heated until a drop placed on a cold glass rod forms crystals. The solution is then left to crystallise. Which step is used to check that the solution is saturated?
Medium
- AHeating the solution until it boils
- BDipping a cold glass rod into the solution and looking for crystals
- CAdding a few drops of indicator
- DMeasuring the pH of the solution
14.A student prepares lead(II) sulfate by mixing aqueous lead(II) nitrate and aqueous potassium sulfate. The precipitate is filtered, washed with distilled water, and dried. Why is the precipitate washed with distilled water?
Hard
- ATo remove any soluble impurities such as potassium nitrate
- BTo dissolve the lead(II) sulfate
- CTo prevent the lead(II) sulfate from decomposing
- DTo increase the yield of lead(II) sulfate
15.Which of the following salts is insoluble in water?
Easy
- ASodium chloride
- BPotassium nitrate
- CSilver chloride
- DAmmonium sulfate
16.A student uses titration to prepare sodium chloride crystals from sodium hydroxide and hydrochloric acid. Which piece of apparatus is used to measure the volume of acid added during the titration?
Medium
- APipette
- BBurette
- CMeasuring cylinder
- DConical flask
17.Hydrated copper(II) sulfate is heated until it turns white. The white solid is then left exposed to air and slowly turns blue again. Which statement explains this observation?
Hard
- AThe white solid absorbs water from the air to reform hydrated copper(II) sulfate
- BThe white solid reacts with oxygen in the air to form hydrated copper(II) sulfate
- CThe white solid decomposes to copper oxide and then rehydrates
- DThe white solid is anhydrous and remains white indefinitely
18.What is the name of the salt formed when zinc oxide reacts with sulfuric acid?
Easy
- AZinc sulfate
- BZinc sulfide
- CZinc sulfite
- DZinc oxide sulfate
19.Which method should be used to prepare a pure sample of calcium chloride from calcium carbonate and dilute hydrochloric acid?
Medium
- ATitration using an indicator
- BAdding excess calcium carbonate to the acid, filtering, then crystallising
- CMixing aqueous solutions of two soluble salts and filtering
- DHeating the mixture to dryness
20.A student prepares a sample of potassium nitrate by neutralising nitric acid with potassium hydroxide solution. The pH is measured after each addition of KOH. The results show a sharp pH change from 2.7 to 11.0 between 24.5 cm³ and 25.5 cm³ of KOH added. What is the approximate volume of KOH needed to neutralise the acid?
Hard
- A24.5 cm³
- B25.0 cm³
- C25.5 cm³
- D30.0 cm³
21.In the preparation of copper(II) sulfate crystals, copper(II) oxide is added to warm dilute sulfuric acid until no more dissolves. After filtering, the filtrate is heated gently to concentrate it. Why must the solution not be heated to dryness?
Medium
- AThe salt would decompose
- BThe salt would become anhydrous and lose water of crystallisation
- CThe salt would react with the air
- DThe salt would not crystallise properly
22.A student prepares magnesium sulfate crystals by reacting excess magnesium with dilute sulfuric acid. After the reaction stops, the mixture is filtered. The filtrate is then heated until a small sample forms crystals on a cold glass rod. What is the purpose of this test?
Hard
- ATo check that all the acid has reacted
- BTo check that the solution is saturated
- CTo check that the magnesium is in excess
- DTo check that the crystals are dry
23.Which of the following is a hydrated salt?
Easy
- ACuSO₄
- BCuSO₄·5H₂O
- CNaCl
- DKNO₃
24.A student prepares lead(II) sulfate by mixing aqueous lead(II) nitrate and aqueous potassium sulfate. The mixture is filtered, and the residue is washed with distilled water and dried. What is the purpose of filtration?
Medium
- ATo separate the soluble lead(II) sulfate from the solution
- BTo separate the insoluble lead(II) sulfate from the solution
- CTo remove the potassium nitrate from the mixture
- DTo collect the potassium sulfate
25.A student prepares a solution of magnesium sulfate by reacting excess magnesium oxide with warm dilute sulfuric acid. After filtering, the filtrate is heated until it is concentrated. How can the student check that the solution is saturated?
Medium
- ADip a cold glass rod into the solution and see if crystals form on the end.
- BMeasure the temperature of the solution.
- CAdd a small amount of water and see if it dissolves.
- DTest the pH of the solution.
26.A student plans to prepare pure dry crystals of hydrated magnesium sulfate, MgSO₄·7H₂O, from a solution of magnesium sulfate. Which sequence of steps is correct?
Hard
- AHeat until concentrated, then leave to crystallise, decant excess liquid, dry with filter paper.
- BEvaporate to dryness, then scrape out the solid.
- CFilter, then heat the filtrate to dryness.
- DLeave to crystallise, then filter and wash with distilled water.
27.Which salt can be prepared by mixing two aqueous solutions and collecting the precipitate?
Medium
- ALead(II) sulfate
- BSodium chloride
- CPotassium nitrate
- DCopper(II) sulfate
28.A student adds excess zinc oxide to warm dilute nitric acid until no more reacts. After filtering, what is the name of the salt in the filtrate?
Easy
- AZinc nitrate
- BZinc nitrite
- CZinc nitride
- DZinc oxide
29.In the preparation of copper(II) sulfate crystals, why is the dilute sulfuric acid warmed before adding copper(II) oxide?
Medium
- ATo increase the rate of reaction
- BTo make the acid more concentrated
- CTo prevent the formation of anhydrous salt
- DTo dissolve the copper(II) oxide more easily
30.A student uses titration to prepare sodium chloride crystals from sodium hydroxide solution and hydrochloric acid. Which indicator is used to determine the end point?
Hard
- APhenolphthalein
- BMethyl orange
- CUniversal indicator
- DLitmus
31.A student prepares lead(II) sulfate by mixing aqueous lead(II) nitrate and aqueous potassium sulfate. After filtering, the residue is washed with distilled water. Why is the residue washed?
Medium
- ATo remove traces of soluble potassium nitrate
- BTo dissolve any remaining lead(II) sulfate
- CTo dry the precipitate
- DTo prevent decomposition of the precipitate
32.A student prepares a sample of potassium nitrate by neutralising nitric acid with potassium hydroxide solution. The pH is measured after each addition of KOH. The results show a sharp rise in pH from 2.7 to 11.0 when 24.5 cm³ to 25.5 cm³ of KOH is added. What volume of KOH is needed to neutralise the acid?
Hard
- A25.0 cm³
- B24.5 cm³
- C25.5 cm³
- D24.0 cm³
33.A student reacts excess iron powder with dilute sulfuric acid to prepare iron(II) sulfate. How does the student know when all the acid has reacted?
Medium
- ANo more gas is produced and the solid iron remains in excess.
- BThe solution turns colourless.
- CThe iron powder disappears completely.
- DThe solution becomes warm.
34.A student wants to prepare pure dry crystals of hydrated copper(II) sulfate from copper(II) oxide and dilute sulfuric acid. Which method should be used?
Hard
- AAdd excess copper(II) oxide to warm acid, filter, then crystallise the filtrate.
- BAdd excess acid to copper(II) oxide, then evaporate to dryness.
- CMix copper(II) oxide and acid, then filter and wash the residue.
- DTitrate the acid with copper(II) oxide using an indicator.
35.Hydrated copper(II) sulfate is blue. When heated, it turns white. Which statement explains this change?
Medium
- AWater of crystallisation is lost, forming anhydrous copper(II) sulfate.
- BThe copper(II) ions are reduced to copper(I).
- CSulfur dioxide gas is released.
- DThe sulfate ions decompose to sulfur and oxygen.
36.A student prepares lead(II) sulfate by mixing aqueous lead(II) nitrate and aqueous potassium sulfate. After filtering, the residue is washed with distilled water and dried. What is the purpose of washing the residue?
Medium
- Ato remove soluble impurities such as potassium nitrate
- Bto dissolve the lead(II) sulfate
- Cto increase the yield of lead(II) sulfate
- Dto dry the precipitate
37.In the preparation of copper(II) sulfate crystals, copper(II) oxide is added to warm dilute sulfuric acid until no more dissolves. After filtering, the filtrate is heated until concentrated and then left to cool. Why is the acid warmed before adding the oxide?
Medium
- Ato increase the rate of reaction
- Bto make the acid more concentrated
- Cto prevent the formation of hydrogen gas
- Dto ensure the oxide dissolves
38.A student uses titration to prepare sodium chloride crystals from sodium hydroxide and hydrochloric acid. Which piece of apparatus is used to measure the volume of acid added accurately?
Medium
- Aburette
- Bpipette
- Cmeasuring cylinder
- Dconical flask
39.Hydrated copper(II) sulfate is heated until it turns white. The white solid is then left exposed to air and slowly turns blue again. Which statement explains this change?
Hard
- AThe anhydrous salt absorbs water from the air to re-form the hydrated salt.
- BThe white solid reacts with carbon dioxide in the air.
- CThe anhydrous salt decomposes to form a blue compound.
- DThe white solid is oxidised by oxygen in the air.
40.A student plans to prepare pure dry crystals of hydrated magnesium sulfate, MgSO4·7H2O, from a solution of magnesium sulfate. Which sequence of steps is correct?
Hard
- Aheat to evaporate water until crystals form, then dry with filter paper
- Bheat to dryness, then add water
- Cfilter, then heat to evaporate all water
- Dleave to crystallise, then heat to remove water of crystallisation

Slides

What is a Salt?

A salt is a compound formed when the hydrogen atom in an acid is replaced by a metal or ammonium ion.
For example, replacing H in HCl with potassium gives potassium chloride (KCl).
The name of a salt has two parts: the first comes from the metal, metal oxide or carbonate; the second comes from the acid.

Which of the following is a salt?

HCl
NaCl
H2SO4
NaOH

Solubility Rules

All sodium, potassium and ammonium salts are soluble.
All nitrates are soluble.
Most chlorides are soluble, except silver and lead(II) chlorides.
Most sulfates are soluble, except barium, calcium and lead(II) sulfates.
Most carbonates and hydroxides are insoluble, except those of sodium, potassium and ammonium.

Which of these salts are soluble in water?

Sodium nitrate
Silver chloride
Potassium carbonate
Lead(II) sulfate

Preparing Soluble Salts – Method A (Excess Solid)

Used when the reactant is a solid (metal, insoluble base or insoluble carbonate) added in excess.
Steps: add excess solid to warm acid, stir until no more reacts, filter to remove excess solid, crystallise the filtrate.
Example: magnesium + sulfuric acid → magnesium sulfate + hydrogen.

Why is the solid reactant added in excess when preparing a soluble salt?

To speed up the reaction
To ensure all the acid reacts
To make the salt more soluble
To prevent crystallisation

Method A2: Using Excess Insoluble Base

For insoluble bases like metal oxides (e.g., copper(II) oxide).
Warm dilute acid, add the base slowly while stirring until no more reacts.
Filter to remove excess base, then crystallise the salt solution.
Example: CuO + H2SO4 → CuSO4 + H2O.

In Method A2, the excess insoluble base is removed by ____.

Method B: Using an Alkali (Titration)

Used when the base is soluble (alkali), e.g., sodium hydroxide.
Steps: pipette alkali into conical flask, add indicator, titrate with acid from burette until colour change.
Repeat without indicator using the same volumes, then crystallise the neutral solution.
Example: NaOH + HCl → NaCl + H2O.

What indicator is commonly used in the titration method for preparing a soluble salt?

Universal indicator
Phenolphthalein
Litmus
Methyl orange

Preparing Insoluble Salts (Extended)

Insoluble salts are prepared by precipitation reaction.
Mix aqueous solutions of two soluble salts; the insoluble salt forms as a precipitate.
Filter to collect the precipitate, wash with distilled water, and dry.
Example: Pb(NO3)2(aq) + K2SO4(aq) → PbSO4(s) + 2KNO3(aq).

Which steps are needed to prepare an insoluble salt?

Mix two soluble salt solutions
Heat the mixture to evaporate water
Filter to collect the precipitate
Wash the precipitate with distilled water

Hydrated and Anhydrous Salts

Hydrated salts contain water of crystallisation within their structure (e.g., CuSO4·5H2O, blue).
Anhydrous salts contain no water (e.g., CuSO4, white).
Heating a hydrated salt removes water of crystallisation, forming the anhydrous salt.
Adding water to an anhydrous salt reverses the process.

What is the colour of hydrated copper(II) sulfate?

White
Blue
Green
Yellow

Crystallisation Technique

After filtration, heat the salt solution gently in an evaporating basin until concentrated.
Check saturation by dipping a cold glass rod into the solution; crystals forming on the rod indicate saturation.
Leave the basin in a warm place to allow crystals to form slowly.
Decant excess liquid and dry crystals with filter paper.

To check if a solution is saturated, dip a cold ____ into it and see if crystals form.

Summary of Methods

Soluble salt + insoluble solid → Method A (excess solid, filter, crystallise).
Soluble salt + soluble alkali → Method B (titration, then crystallise).
Insoluble salt → precipitation reaction (mix two soluble salts, filter, wash, dry).

Which method is used to prepare sodium chloride from sodium hydroxide and hydrochloric acid?

Method A1 (excess metal)
Method A2 (excess base)
Method B (titration)
Precipitation

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