8.4 Formation of Salts
8.4.1 pH SCALE
When a substance dissolves in water, it forms an aqueous solution, which may be
acidic, alkaline or neutral. At home, you can find many examples of substances
whose pH varies from 0 to 14.
Household items containing acids
Household items containing alkalis
Indicators
Indicators, for example litmus, can be used to show whether a solution is acidic,
alkaline or neutral by the way their colours change. Universal indicator can show,
in addition, how acidic or alkaline a solution is.
The acidity or alkalinity of a solution is expressed as its pH number:
pH
0
1
2
3
4
increasing acidity
5
6
7
8
neutral
9
10
11 12
13 14
increasing alkalinity
Acids have a pH less than 7 whereas alkalis/bases have a pH greater than 7.
Acids and alkalis can be used to neutralise each other (cancel each other out) as
they react to produce water (pH7). This process is called neutralisation. We can
follow the process of neutralisation using an indicator such as universal indicator
so we know when the acid or alkali has reached pH7.
You might think that neutralisation only happens in a chemistry lab or on a
chemical plant, but some medicines are alkaline and are used to neutralise excess
acid in the stomach. If you have ever been stung by a bee (which has an acidic
sting) you can neutralise it with an alkali such as baking soda. Wasp stings are
alkaline and can be neutralised using an acid like vinegar.
TOPIC 8.4: FORMATION OF SALTS
1
8.4.2 Reactions of acids
Acid and alkali
When an acid is added to an alkali a neutralisation reaction occurs and a salt and
water are produced. The general word equation for this is:
acid + alkali  salt
+ water
The name of the salt produced depends on the acid and the alkali used. The
metal part of a salt comes from the metal in the alkali (most hydroxides are
alkalis) whilst the non-metal part of the salt depends on the acid.
Hydrochloric acid produces salts called chlorides
Sulphuric acid produces salts called sulphates
Nitric acid produces salts called nitrates
For example:
sodium hydroxide + nitric acid 
NaOH (aq)
+ HNO3 (aq) 
potassium hydroxide + hydrochloric acid
KOH (aq)
+
HCl (aq)


sodium nitrate + water
NaNO3 (aq)
+ H2O
(l)
potassium chloride + water
KCl (aq)
+ H2O (l)
Note that ‘(aq)’ and ‘(l)’ are state symbols, whereby (aq) means ‘aqueous’, i.e. the
substance is dissolved in water, and (l) means ‘liquid’.
We can obtain samples of salt made by neutralisation.
1) Acid is measured out into a flask using a pipette.
2) Alkali is added from a burette.
3) Just enough acid and alkali are added to form a salt and water.
4) The neutral solution is
poured into an
evaporating dish and
heated to remove most
of the water.
5) Finally the last of the
water is allowed to
evaporate more slowly
and crystals eventually
form.
TOPIC 8.4: FORMATION OF SALTS
2
Acid and base
Most metal oxides are bases. Bases do not dissolve in water.
When an acid is added to a base a neutralisation reaction occurs and a salt and
water are produced. The general word equation for this is:
acid + base  salt
+ water
To obtain a sample of a salt made in this way we need a slightly different
approach.
1) Acid and base are mixed in a beaker.
2) The mixture is heated and an excess of
base is added to the acid to make sure all
of the acid reacts.
3) The excess base is filtered off to
remove it.
4) The water is allowed to evaporate off
and the crystals form.
For example, copper(II) sulphate, CuSO4, can be made by the reaction of
copper(II) oxide with sulphuric acid:
copper oxide + sulphuric acid
CuO (s) + H2SO4 (aq)
copper sulphate + water
CuSO4 (aq)
+ H2O
(l)
Acid and carbonate
Most metal carbonates are bases too. They also react with acids to produce a
salt and water but also produce carbon dioxide gas. Samples of salts can be
made by reacting acids with carbonates in exactly the same way as acids and
bases. However, the carbonate must be added slowly so as not to produce too
much gas too quickly and cause frothing over.
acid + carbonate  salt
+ water + carbon dioxide
For example:
sulphuric acid + copper carbonate  copper sulphate + water + carbon dioxide
H2SO4 (aq)
+
CuCO3 (s)
 CuSO4(aq)
+ H2O (l) + CO2 (g)
We can confirm that this gas is carbon dioxide as it turns limewater cloudy.
TOPIC 8.4: FORMATION OF SALTS
3
Acid and metal
Acids react with some metals to produce salts and hydrogen gas. Some metals
(like potassium, sodium, and lithium) react violently with water and should not be
allowed to mix with acids for safety reasons. Some metals (like copper, silver
and gold) are very unreactive and do not react with acids. Many metals in the
middle of the reactivity series do react. The general equation for this is:
acid + metal  salt
For example:
Magnesium + sulphuric acid
Mg (s)
+ H2SO4 (aq)
+
hydrogen
magnesium sulphate + hydrogen
MgSO4 (aq)
+ H2 (g)
We can confirm that this gas is hydrogen as it ignites with a squeaky pop.
Precipitation
When a salt is insoluble in water, it is prepared by precipitation.
1) A solution of a soluble salt containing the desired metal ion (usually the
metal nitrate) is added to a solution containing the desired negative ion
(usually as its sodium salt). The two ions combine immediately to give a
precipitate (a solid).
2) The precipitate is filtered off.
3) The precipitate is washed with water.
4) The precipitate can then be dried in an oven.
For example, to make barium sulphate, a solution of barium nitrate is added to a
solution of sodium sulphate.
Ba(NO3)2 (aq) + Na2SO4 (aq)
Ba2+ (aq) + SO42- (aq)
TOPIC 8.4: FORMATION OF SALTS
BaSO4 (s) + 2NaNO3 (aq)
BaSO4 (s)
4
TOPIC SUMMARY
The pH scale measures how ___________ or ___________ a solution is. The
scale runs from 0 to 14. A neutral solution has a pH of___.
We can identify acidic, alkaline and neutral solutions by observing colour changes
in a chemical called an _____________.
When an acid and an alkali are mixed, they can cancel each other out in a process
known as _____________. If universal indicator is added, the neutral solution
will be a __________ colour.
When an acid and an alkali are mixed, the reaction products are always a salt and
_________. Hydrochloric acid salts are _________; sulphuric acid salts are
___________; nitric acid salts are__________.
There are many examples of acids and alkalis in everyday life: _______ stings
are acidic, but ________ stings are alkaline. Lemon juice and __________ are
acidic whereas _________ ________ is alkaline.
The word equation for the reaction of an acid and a base is as follows:
Acid
+
Base

_________
+
water
Most metal _________, metal hydroxides and metal ___________ are bases.
When an acid reacts with a metal carbonate, a salt, water and _________
_________ are formed. We test for this gas by bubbling it through a solution
of __________ which becomes _________.
When an acid reacts with a metal, a salt and _________ are formed. We test
for this gas by using the ___________ ________ test.
A salt that is insoluble in water is made by a process called _____________.
green
chlorides
oxides
baking powder
indicator
precipitation
water
carbon dioxide
cloudy
TOPIC 8.4: FORMATION OF SALTS
wasp
sulphates
hydrogen
salt
alkaline
bee
5
vinegar
‘squeaky pop’
lime-water
carbonates
7
neutralisation
acidic
nitrates