7.5 Formation of Salts
7.5.1 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 sulfates
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 7.5: FORMATION OF SALTS
1
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) sulfate, CuSO4, can be made by the reaction of
copper(II) oxide with sulfuric acid:
copper oxide + sulphuric acid
CuO (s) + H2SO4 (aq)
copper sulfate + 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:
sulfuric acid + copper carbonate  copper sulfate + 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 7.5: FORMATION OF SALTS
2
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 sulfate + 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 sulfate, a solution of barium nitrate is added to a
solution of sodium sulfate.
Barium nitrate + sodium sulfate
Ba(NO3)2 (aq) + Na2SO4 (aq)
TOPIC 7.5: FORMATION OF SALTS
barium sulfate + sodium nitrate
BaSO4 (s) + 2NaNO3 (aq)
3
TOPIC SUMMARY
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 _________; sulfuric 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
precipitation
water
cloudy
sulfates
hydrogen
salt
lime-water
carbon dioxide
TOPIC 7.5: FORMATION OF SALTS
vinegar
carbonates
bee
‘squeaky pop’
nitrates
neutralisation
4
wasp