(g) Acids, bases and salt preparations Archives

2:34 know the general rules for predicting the solubility of ionic compounds in water: common sodium, potassium and ammonium compounds are soluble, all nitrates are soluble, common chlorides are soluble, except those of silver and lead(II), common sulfates are soluble, except for those of barium, calcium and lead(II), common carbonates are insoluble, except for those of sodium, potassium and ammonium, common hydroxides are insoluble except for those of sodium, potassium and calcium (calcium hydroxide is slightly soluble)

Salt	Solubility	Exceptions
sodium (Na⁺), potassium (K⁺) and ammonium (NH₄⁺)	soluble	none
nitrates (NO₃^-)	soluble	none
chlorides (Cl^-)	soluble	silver chloride (AgCl) and lead (II) chloride (PbCl₂)
sulfates (SO₄^2-)	soluble	barium sulfate (BaSO₄), calcium sulfate (CaSO₄) and lead (II) sulfate (PbSO₄)
carbonates (CO₃^2-)	insoluble	sodium carbonate (Na₂CO₃), potassium carbonate (K₂CO₃) and ammonium carbonate ((NH₄)₂CO₃)
hydroxides (OH^-)	insoluble	sodium hydroxide (NaOH), potassium hydroxide (KOH) and calcium hydroxide (Ca(OH)₂) (calcium hydroxide is slightly soluble)

Hydr0Gen2023-01-10T11:40:19+00:00Categories: (g) Acids, bases and salt preparations, 2 Inorganic chemistry, Edexcel iGCSE Chemistry|Tags: 2:34, Chemistry, Double, GCSE, GCSE_Chemistry_SpecPoint, SpecPoint, Topic: Acids & Salts, Topic: Acids & Salts (D)|

2:35 understand acids and bases in terms of proton transfer

An acid is a proton (H⁺) donor.

A base is a proton (H⁺) acceptor.

A proton is the same as a hydrogen ion. A good way to think about that is to realise that a hydrogen atom is just one proton and zero neutrons surrounded by only one electron. If that atom becomes an ion by the removal of the electron, then only one proton is left.

When sulfuric acid reacts with copper (II) oxide (CuO):

Cu²⁺O²⁻ (s) + H₂SO₄ (aq) → Cu²⁺ (aq) + SO₄²⁻ (aq) + H₂O (l)

H₂SO₄ is an acid. It donates protons (H⁺) to CuO, the base.

Hydr0Gen2023-01-10T11:41:24+00:00Categories: (g) Acids, bases and salt preparations, 2 Inorganic chemistry, Edexcel iGCSE Chemistry|Tags: 2:35, Chemistry, ChemistrySpecPointsNew2019, Double, GCSE, GCSE_Chemistry_SpecPoint, SpecPoint, Topic: Acids & Salts, Topic: Acids & Salts (D)|

2:36 understand that an acid is a proton donor and a base is a proton acceptor

An acid is a proton donor.

A base is a proton acceptor.

A proton is the same as a hydrogen ion. A good way to think about that is to realise that a hydrogen atom is just one proton and zero neutrons surrounded by only one electron. If that atom becomes an ion by the removal of the electron, then only one proton is left.

Hydr0Gen2023-01-10T11:41:38+00:00Categories: (g) Acids, bases and salt preparations, 2 Inorganic chemistry, Edexcel iGCSE Chemistry|Tags: 2:36, Chemistry, ChemistrySpecPointsNew2019, Double, GCSE, GCSE_Chemistry_SpecPoint, SpecPoint, Topic: Acids & Salts, Topic: Acids & Salts (D)|

2:37 describe the reactions of hydrochloric acid, sulfuric acid and nitric acid with metals, bases and metal carbonates (excluding the reactions between nitric acid and metals) to form salts

Acid reactions summary

alkali + acid → water + salt

base + acid → water + salt

carbonate + acid → water + salt + carbon dioxide

metal + acid → salt + hydrogen

To assist remembering this list, many pupils find it useful to remember this horrid looking but very effective mnemonic:

AAWS

BAWS

CAWS CoD

MASH

Acids are a source of hydrogen ions (H⁺) when in solution. When the hydrogen in an acid is replaced by a metal, the compound is called a salt. The name of the salt depends on the acid used. For example if sulfuric acid is used then a sulfate salt will be formed.

Parent acid	Formula	Salt	Formula ion
sulfuric acid	H₂SO₄	sulfate	SO₄^2-
hydrochloric acid	HCl	chloride	Cl^-
nitric acid	HNO₃	nitrate	NO₃^-

Acid + Alkali and Acid + Base

A base is a substance that can neutralise an acid, forming a salt and water only.

Alkalis are soluble bases. When they react with acids, a salt and water is formed. The salt formed is often as a colourless solution. Alkalis are a source of hydroxide ions (OH⁻) when in solution.

alkali + acid → water + salt

base + acid → water + salt

Examples of acid + alkali reactions:

sodium hydroxide + hydrochloric acid → sodium chloride + water
NaOH (aq) + HCl (aq) → NaCl (aq) + H₂O (l)

potassium hydroxide + sulfuric acid → potassium sulfate + water
2KOH (aq) + H₂SO₄ (aq) → K₂SO₄ (aq) + 2H₂O (l)

Example of an acid + base reaction:

CuO (s) + H₂SO₄ (aq) → CuSO₄ (aq) + H₂O (l)

Acid + Carbonate

carbonate + acid → water + salt + carbon dioxide

A carbonate is a compound made up of metal ions and carbonate ions. Examples of metal carbonates are sodium carbonate, copper carbonate and magnesium carbonate.

When carbonates react with acids, bubbling is observed which is the carbon dioxide being produced. If the acid is in excess the carbonate will disappear.

Examples of acid + carbonate reactions:

calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide
CaCO₃ (s) + 2HCl (aq) → CaCl₂ (aq) + H₂O (l) + CO₂ (g)

potassium carbonate + hydrochloric acid → potassium chloride + water + carbon dioxide
K₂CO₃ (aq) + 2HCl (aq) → 2KCl (aq) + H₂O (l) + CO₂ (g)

Acid + Metal

metal + acid → salt + hydrogen

Metals will react with an acid if the metal is above hydrogen in the reactivity series.

When metals react with acids, bubbling is observed which is the hydrogen being produced. If the acid is in excess the metal will disappear.

Examples of acid + metal reactions:

magnesium + sulfuric acid → magnesium sulfate + hydrogen
Mg (s) + H₂SO₄ (aq) → MgSO₄ (aq) + H₂ (g)

aluminium + hydrochloric acid → aluminium chloride + hydrogen
2Al (s) + 6HCl (aq) → 2AlCl₃ (aq) + 3H₂ (g)

copper + hydrochloric acid → no reaction (since copper is below hydrogen in the reactivity series)

Hydr0Gen2023-01-10T11:41:49+00:00Categories: (g) Acids, bases and salt preparations, 2 Inorganic chemistry, Edexcel iGCSE Chemistry|Tags: 2:37, Chemistry, Double, GCSE, GCSE_Chemistry_SpecPoint, SpecPoint, Topic: Acids & Salts, Topic: Acids & Salts (D), Topic: Key Reactions, Topic: Specification experiments|

2:38 know that metal oxides, metal hydroxides and ammonia can act as bases, and that alkalis are bases that are soluble in water

A base is a substance that neutralises an acid by combining with the hydrogen ions in them to produce water.

A base usually means a metal oxide, a metal hydroxide or ammonia.

Alkalis are bases which are soluble in water.

Some metal oxides are soluble in water and react with it to form solutions of metal hydroxides. For example:

Na₂O (s) + H₂O (l) → 2NaOH (aq)

Apart from this and other group 1 oxides (such as potassium oxide) most other metal oxides are not soluble in water.

One exception is calcium oxide which does dissolve slightly in water to form calcium hydroxide (known as limewater):

CaO (s) + H₂O (l) → Ca(OH)₂ (aq)

Ammonia is another base. Ammonia reacts with water to form ammonium ions and hydroxide ions:

NH₃ (aq) + H₂O (l) ⇋ NH₄⁺ (aq) + OH⁻ (aq)

All the solutions produced here contain hydroxide ions (OH⁻) so they are all alkalis.

Hydr0Gen2023-01-10T11:42:29+00:00Categories: (g) Acids, bases and salt preparations, 2 Inorganic chemistry, Edexcel iGCSE Chemistry|Tags: 2:38, Chemistry, Double, GCSE, GCSE_Chemistry_SpecPoint, SpecPoint, Topic: Acids & Salts, Topic: Acids & Salts (D), Topic: Key Reactions|

2:39 describe an experiment to prepare a pure, dry sample of a soluble salt, starting from an insoluble reactant

Excess Solid Method:

Preparing pure dry crystals of copper sulfate (CuSO₄) from copper oxide (CuO) and sulfuric acid (H₂SO₄)

Step	Explanation
Heat acid (H₂SO₄) in a beaker	Speeds up the rate of reaction
Add base (CuO) until in excess (no more copper oxide dissolves) and stir with glass rod	Neutralises all the acid
Filter the mixture using filter paper and funnel	Removes any excess copper oxide
Gently heat the filtered solution (CuSO₄)	To evaporate some of the water
until crystals form on a glass rod	Shows a hot saturated solution formed
Allow the solution to cool so that hydrated crystals form	Copper sulfate less soluble in cold solution
Remove the crystals by filtration	Removes crystals
Dry by leaving in a warm place	Evaporates the water

Hydr0Gen2023-01-10T11:42:40+00:00Categories: (g) Acids, bases and salt preparations, 2 Inorganic chemistry, Edexcel iGCSE Chemistry|Tags: 2:39, Chemistry, Double, GCSE, GCSE_Chemistry_SpecPoint, SpecPoint, Topic: Acids & Salts, Topic: Acids & Salts (D)|

2:40 (Triple only) describe an experiment to prepare a pure, dry sample of a soluble salt, starting from an acid and alkali

Titration Method:

Preparing pure dry crystals of sodium chloride (NaCl) from hydrochloric acid (HCl) and sodium hydroxide (NaOH)

Before the salt preparation is carried out using the below method, the volume of acid that exactly reacts with 25cm³ of the alkali is found by titration using methyl orange indicator.

Step	Explanation
Pipette 25cm³ of alkali (NaOH) into a conical flask	Accurately measures the alkali (NaOH)
Do not add indicator	Prevents contamination of the pure crystals with indicator
Using the titration values, titrate the known volume acid (HCl) into conical flask containing alkali	Exactly neutralises all of the alkali (NaOH)
Transfer to an evaporating basin & heat the solution	Forms a hot saturated solution (NaCl(aq))
Allow the solution to cool so that hydrated crystals form	Sodium chloride is less soluble in cold water
Remove the crystals by filtration and wash with distilled water	Removes any impurities
Dry by leaving in a warm place	Evaporates the water

(Note – This process could be reversed with the acid in the pipette and the alkali in the burette)

How to select the right method for preparing a salt:

Hydr0Gen2019-05-13T15:37:35+00:00Categories: (g) Acids, bases and salt preparations, 2 Inorganic chemistry, Edexcel iGCSE Chemistry|Tags: 2:40, Chemistry, GCSE, GCSE_Chemistry_SpecPoint, SpecPoint, Topic: Acids & Salts, Topic: Specification experiments, Triple|

2:41 (Triple only) describe an experiment to prepare a pure, dry sample of an insoluble salt, starting from two soluble reactants

Precipitation Method:

Preparing pure dry crystals of silver chloride (AgCl) from silver nitrate solution (AgNO₃) and potassium chloride solution (KCl)

Step	Explanation
Mix the two salt solutions together in a beaker	Forms a precipitate of an insoluble salt (AgCl)
Stir with glass rod	Make sure all reactants have reacted
Filter using filter paper and funnel	Collect the precipitate (AgCl)
Wash with distilled water	Removes any the other soluble salts (KNO₃)
Dry by leaving in a warm place	Evaporates the water

Hydr0Gen2019-05-13T15:37:24+00:00Categories: (g) Acids, bases and salt preparations, 2 Inorganic chemistry, Edexcel iGCSE Chemistry|Tags: 2:41, Chemistry, GCSE, GCSE_Chemistry_SpecPoint, SpecPoint, Topic: Acids & Salts, Topic: Specification experiments, Triple|

2:42 practical: prepare a sample of pure, dry hydrated copper(II) sulfate crystals starting from copper(II) oxide

Excess Solid Method:

Preparing pure dry crystals of copper sulfate (CuSO₄) from copper oxide (CuO) and sulfuric acid (H₂SO₄)

Step	Explanation
Heat acid (H₂SO₄) in a beaker	Speeds up the rate of reaction
Add base (CuO) until in excess (no more copper oxide dissolves) and stir with glass rod	Neutralises all the acid
Filter the mixture using filter paper and funnel	Removes any excess copper oxide
Gently heat the filtered solution (CuSO₄)	To evaporate some of the water
until crystals form on a glass rod	Shows a hot saturated solution formed
Allow the solution to cool so that hydrated crystals form	Copper sulfate less soluble in cold solution
Remove the crystals by filtration	Removes crystals
Dry by leaving in a warm place	Evaporates the water

Hydr0Gen2023-09-18T11:07:48+00:00Categories: (g) Acids, bases and salt preparations, 2 Inorganic chemistry, Edexcel iGCSE Chemistry|Tags: 2:42, Chemistry, Double, GCSE, GCSE_Chemistry_SpecPoint, SpecPoint, Topic: Acids & Salts, Topic: Acids & Salts (D), Topic: Key Reactions, Topic: Specification experiments|

2:43 (Triple only) practical: prepare a sample of pure, dry lead(II) sulfate

Objective: prepare a pure, dry sample of lead (II) sulfate (PbSO₄).

Preparing a pure, dry sample of lead (II) sulfate (PbSO₄) from lead (II) nitrate solution (Pb(NO₃)₂) and sodium sulfate solution (Na₂SO₄).

Pb(NO₃)₂ (aq) + Na₂SO₄ (aq) → PbSO₄ (s) + 2NaNO₃ (aq)

Mix similar volumes lead nitrate solution and sodium sulfate solution in a beaker. The precise volumes do not matter since any excess will be removed later.
A white precipitate of lead (II) sulfate will form.
The reaction mixture is filtered.
The residue left on the filter paper is washed with distilled water several times to remove impurities.
The residue is then moved to a warm oven to dry.

Hydr0Gen2019-05-13T15:37:14+00:00Categories: (g) Acids, bases and salt preparations, 2 Inorganic chemistry, Edexcel iGCSE Chemistry|Tags: 2:43, Chemistry, GCSE, GCSE_Chemistry_SpecPoint, SpecPoint, Topic: Acids & Salts, Topic: Specification experiments, Triple|