ACIDS, BASES & SALTS

CHEMISTRY
 SALTS
• A salt is a compound that has been formed when all the hydrogen ions
of an acid have been replaced by metal ions or by the ammonium ion
(NH4+).
• The name of salt has two parts
• The first part comes from the metal, metal oxide or metal carbonate
used in the reaction
• The second part comes from the acid
• The name of the salt can be determined by looking at the reactants
• For example hydrochloric acid always produces salts that end in
chloride and contain the chloride ion, Cl-
 SALTS
• Salts can be classified as those which are soluble in water or those
which are insoluble in water.
• The general solubility rules for salts are:
✓ all sodium, potassium and ammonium salts are soluble
✓ all nitrates are soluble
✓ all chlorides are soluble, except lead and silver
✓ all sulfates are soluble, except barium, calcium and lead
✓ all carbonates are insoluble, except sodium, potassium and
ammonium.
✓ hydroxides are insoluble, except sodium, potassium, ammonium
and calcium (partially)
 Salts Soluble Insoluble
Sodium, potassium and
All None
ammonium
Nitrates All None
Chlorides Most are soluble Silver and lead(II)

Sulfates Most are soluble Barium, calcium and lead(II)

Carbonates of sodium,
Carbonates Most are insoluble
potassium and ammonium

Hydroxides of sodium
potassium and ammonium
Hydroxides Most are insoluble
(calcium hydroxide is
sparingly soluble)
TYPE OF SALTS
METHOD OF PREPARING
SOLUBLE SALTS
 PREPARATION OF SOLUBLE SALTS

There are four general methods of preparing soluble salts.

These involve the reaction of an acid with:

• Excess metal

• Excess insoluble carbonate

• Excess insoluble base

• An alkali by titration.
 REACTION OF ACID WITH INSOLUBLE SUBSTANCE

acid + metal → salt + hydrogen

acid + insoluble base → salt + water
acid + insoluble carbonate → salt + water + carbon dioxide
• In this method, the metal, base or carbonate must be in excess so
that all the acid is used up. Otherwise, the salt produced will be
contaminated with the acid
• Metal, base or carbonate are insoluble in water. Thus, the excess
starting materials can be removed from the salt solution by
filtration.
 REACTION OF ACID WITH A METAL

• This method can only be used with the less reactive metals
such as aluminium, zinc.
• This method cannot be used with sodium, potassium, and
calcium as they react violently with acids, Lead reacts too
slowly, and copper, silver, and gold do not react at all.
• The metals usually used in this method of salt preparation are
the MAZIT metals, that is, magnesium, aluminium, zinc, iron
and tin.
 REACTION OF ACID WITH A METAL
• Excess metal is added to warm dilute acid with constant stirring.
• Excess metal is added to make sure that all of the acid has
reacted and some metal is left at the end of the reaction.
• During this addition an effervescence is observed due to the
production of hydrogen gas.
• Filter the mixture into an evaporating basin to remove the excess solid.
• Heat the solution to evaporate water and to make the solution
saturated.
• The hot concentrated magnesium chloride solution produced is tested
by dipping a cold glass rod into it. If salt crystals form at the end of the
rod the solution is ready to crystallise and is left to cool.
• Any crystals produced on cooling are filtered and dried between clean
tissues.
Method A: Adding acid to a solid metal, insoluble base or insoluble carbonate
Method A: Adding acid to a solid metal, insoluble base or insoluble carbonate
REACTION OF DILUTE HYDROCHLORIC ACID WITH MAGNESIUM
• Add dilute acid into a beaker and heat using a Bunsen burner flame
• Excess magnesium is added to warm hydrochloric acid.
• With constant stirring, add magnesium until no more magnesium will
dissolve In the acid or no more effervescence Is observed.
• The excess magnesium is removed by filtration.
• The magnesium chloride solution is evaporated slowly to form a saturated
solution of the salt.
• In this reaction the hydrogen ions from the hydrochloric acid gain electrons
from the metal atoms as the reaction proceeds.
hydrogen ions + electrons → hydrogen gas
2H+ + 2e− → H2 (g)
Magnesium + hydrochloric acid → magnesium chloride + hydrogen
Mg(s) + 2HCl(aq) → MgCl2 (aq) + H2 (g)
 REACTION OF ACID WITH CARBONATE
• This method can be used with any metal carbonate and any acid, providing
the salt produced is soluble.
• The procedure is similar to that carried out for an acid and a metal.
REACTION OF NITRIC ACID WITH COPPER (II) CARBONATE
• Copper(II) carbonate would be added in excess to dilute nitric acid.
Effervescence would be observed due to the production of carbon dioxide.
copper(II) carbonate + nitric acid → copper(II) nitrate + carbon dioxide + water
CuCO3 (s) + 2HNO3 (aq) → Cu(NO3)2 (aq) + CO2 (g) + H2O (l)
• Metal carbonates contain carbonate ions, CO32−.
• In this reaction the carbonate ions react with the hydrogen ions in the acid.
Carbonate ions + hydrogen ion → carbon dioxide + water
CO32−(aq) + 2H+(aq) → CO2 (g) + H2O (l)
 REACTION OF NITRIC ACID WITH COPPER (II) CARBONATE

• When excess carbonate has been added you will see it collect at the
bottom of the beaker and the effervescence will stop.
• The excess copper(II) carbonate can be filtered off and the copper(II)
nitrate solution is evaporated slowly to form a saturated solution of the
salt.
• The hot concentrated copper(II) nitrate solution produced is tested by
dipping a cold glass rod into it.
• If salt crystals form at the end of the rod, the solution is ready to
crystallise and is left to cool.
• Any crystals produced on cooling are filtered and dried between clean
tissues.
 REACTION OF ACID WITH INSOLUBLE BASE

acid + base → salt + water

• This method can be used to prepare a salt of an unreactive metal,
such as lead or copper.
• In these cases it is not possible to use a direct reaction of the metal
with an acid so the acid is neutralised using the particular metal oxide.
• The method is generally the same as that for a metal carbonate and
an acid.
REACTION OF SULFURIC ACID WITH COPPER (II) OXIDE

Sulfuric acid + copper(II) oxide → copper(II) sulfate + water

H2 SO4(aq) + CuO(s) → CuSO4 (aq) + H2 O(l)
• Metal oxides contain the oxide ion, O2−.
• The ionic equation for this reaction is therefore:
2H+(aq) + O2−(s) → H2O(l) or
CuO(s) + 2H+(aq) → Cu2+(aq) + H2O(l)
REACTION OF ACID WITH SOLUBLE BASE (ALKALI) BY TITRATION
acid + alkali → salt + water
• Titration is generally used for preparing the salts of very reactive
metals, such as potassium or sodium.
• In this case, we solve the problem indirectly and use an alkali which
contains the reactive metal whose salt we wish to prepare.
• In the titration method, both titration used are soluble.
• If either reactant is in excess, it cannot be removed from the salt
solution by filtration, and the salt produced will be impure. Therefore,
the volume of each starting material used must be exact.
• If the metal oxide or hydroxide does not dissolve in water, it is known
as an insoluble base.
• A few metal oxides and hydroxides that do dissolve in water to
produce OH–(aq) ions are known as alkalis, or soluble bases.
EXAMPLES OF SOLUBLE AND INOSLUBLE BASES
Method B: Reacting a dilute acid and alkali (soluble base)
Method B: Reacting a dilute acid and alkali (soluble base)
 REACTING A DILUTE ACID AND ALKALI (SOLUBLE BASE)
• Use a pipette to measure the alkali into a conical flask and add a few drops of
indicator (thymolphthalein or methyl orange)
• Add the acid into the burette
• Record the starting volume of acid in the burette
• Add the acid very slowly from the burette to the conical flask until the indicator
changes to the appropriate colour. This is the end-point of titration.
• Record the final volume of acid in the burette
• Calculate the volume of acid added (Final volume - initial volume)
• Add this same volume of acid to the same volume of alkali without the indicator
• Heat the resulting solution in an evaporating basin to partially evaporate,
leaving a saturated solution (crystals just forming on the sides of the basin or
on a glass rod dipped in and then removed)
• Leave to crystallise, decant excess solution and allow crystals to dry
 REACTION OF BASE WITH AMMONIUM SALTS

• Small quantities of ammonia gas, NH3 , can be produced by heating

any ammonium salt, such as ammonium chloride, with a base, such as
calcium hydroxide.

• This reaction can be used to make ammonia in a laboratory.

Calcium hydroxide + ammonium chloride → calcium chloride + water + Ammonia

Ca(OH)2 (s) + 2NH4Cl → CaCl2 + H2O + 2NH3

• The ammonia produced can be detected as being formed by its

pungent odour and by turning damp red litmus blue.
 CHOOSING REACTANTS

The table shows some examples of the salts produced by different

combinations of insoluble reactants and acids.
METHOD OF PREPARING
INSOLUBLE SALTS
 PREPARATION OF INSOLUBLE SALTS
• Insoluble salts can be prepared using a Precipitation reaction
• The solid salt obtained is the precipitate, thus in order to successfully
use this method the solid salt being formed must be insoluble in water,
and the reactants must be soluble
Method
• Dissolve soluble salts in water and mix together using a stirring rod in a
beaker
• Filter to remove precipitate from mixture
• Wash the residue with distilled water to remove traces of other solutions
• Leave in an oven to dry
This method may be summarised as follows:
Soluble salt + soluble salt → insoluble salt + soluble salt
(AX) + (BY) → (BX) + (AY)
Preparation of pure, dry lead(II) sulfate crystals using a precipitation
reaction

• Dissolve lead(II) nitrate and potassium sulfate in water and mix

together using a stirring rod in a beaker
• Filter to remove precipitate from mixture
• Wash precipitate with distilled water to remove traces of potassium
nitrate solution
• Leave in an oven to dry
lead(II) nitrate + potassium sulfate → lead(II) sulfate + potassium
nitrate
Pb(NO3)2 (aq) + K2SO4 (aq) → PbSO4 (s) + 2KNO3 (aq)
 HYDRATED AND ANHYDROUS SALTS
• When salts are being prepared, some water can be retained within the structure
of the salt during the crystallisation process
• A hydrated substance, or hydrate is one that is chemically combined with water.
• An anhydrous substance is one containing no water.
• Water of crystallisation is the water molecules present in crystals, e.g.
CuSO4.5H2 O and CoCl2.6H2 O.
• The shape of the crystal hydrate is very much dependent on the presence of
water of crystallisation.
• The formula shows the number of moles of water contained within one mole of
the hydrated salt:
Hydrated copper(II) sulfate, CuSO4∙5H2O, contains 5 moles of water in 1 mole
of hydrated salt.
Q. Complete the word equations and write balanced chemical
equations for the following soluble salt preparations:
a. magnesium + sulfuric acid →
b. calcium carbonate + hydrochloric acid →
c. zinc oxide + hydrochloric acid →
d. potassium hydroxide + nitric acid →
also write ionic equations for each of the reactions.
Q. Lead carbonate and lead iodide are insoluble.

Which two soluble salts could you use in the preparation of each
substance?

Write:

1. a word equation

2. a symbol equation

3. an ionic equation to represent the reactions taking place.

THANK YOU