Chemistry
Acids, bases and salts
Acids have a pH of less than 7 and bases have a pH of more than 7. When bases
are dissolved in water they are known as alkalis.
Salts are made when an acid reacts with a base, carbonate or metal. The name of
the salt formed depends on the metal in the base and the acid used. For example
salts made using hydrochloric acid are called chlorides.
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Acids and bases
Reactions of acids
Acids, alkalis and neutralisation (Higher Tier)
Salt preparation
Acids and bases
You'll already have learnt quite a bit about acids and bases in Key Stage 3
Science. If you feel a bit rusty on this topic, have a quick look at KS3 Bitesize
Chemistry Acids and Alkalis. Here are the bare bones you need to know:
Acids turn blue litmus paper red.
Alkalis turn red litmus paper blue.
Acids
Substances with a pH of less than 7 are acids. The stronger the
acid, the lower the pH number. Acids turn blue litmus paper red.
They turn universal indicator red if they are strong, and orange
or yellow if they are weak.
Bases
Substances that can react with acids and neutralise them to
make a salt and water are called bases. They are usually
metal oxides or metal hydroxides. For example, copper oxide
and sodium hydroxide are bases.
copper oxide
Alkalis
Bases that dissolve in water are called alkalis. Copper oxide is
not an alkali because it does not dissolve in water, but sodium
hydroxide is an alkali because it does dissolve in water.
Alkaline solutions have a pH of more than 7. The stronger the
alkali, the higher the pH number. Alkalis turn red litmus paper
blue. They turn universal indicator dark blue or purple if they are
strong, and blue-green if they are weak.
Neutral solutions
Neutral solutions have a pH of 7. They do not change the colour
of litmus paper, but they turn universal indicator green. Water is
neutral.
Reactions of acids
You need to be able to describe the reactions of acids with bases, carbonates
and metals. You should be able to work out the particular salt formed in the
reaction.
Acids and bases
When acids react with bases, a salt and water are made. This reaction is called
neutralisation. In general,
acid + metal oxide
salt + water
acid + metal hydroxide
salt + water
Remember that most bases do not dissolve in water. But if a base can dissolve
in water it is also called an alkali.
Carbonates
When acids react with carbonates, such as calcium carbonate (found in chalk,
limestone and marble), a salt, water and carbon dioxide are made. In general,
acid + metal carbonate
salt + water + carbon dioxide
Notice that compared to the reaction with metal oxides and metal hydroxides,
an extra product, carbon dioxide, is made. It causes bubbling during the
reaction and can be detected using limewater. You usually meet this reaction
when you study the effects of acid rain on rocks and building materials.
Reactive metals
Acids will react with reactive metals, such as magnesium and zinc, to make a
salt and hydrogen. In general,
acid + metal
salt + hydrogen
The hydrogen causes bubbling during the reaction and can be detected using a
lighted splint. You usually meet this reaction when you study the reactivity
series of metals.
Acids, alkalis and neutralisation (Higher Tier)
When atoms or groups of atoms lose or gain electrons, charged particles called
ions are formed. Ions can be either positively or negatively charged.
For the Higher Tier you need to know which ions are produced by acids and
which are produced by alkalis. You'll also need to know the ionic equation for
neutralisation.
Acids
When acids dissolve in water they produce hydrogen ions, H+. For example,
looking at hydrochloric acid:
HCl(aq)
H+(aq) + Cl-(aq)
Alkalis
When alkalis dissolve in water they produce hydroxide ions, OH-. For example,
looking at sodium hydroxide:
NaOH(aq)
Na+(aq) + OH-(aq)
Ammonia is slightly different. This is the equation for ammonia in solution:
NH3(aq) + H2O(l)
(aq) + OH-(aq)
Be careful to write OH- and not Oh-.
Neutralisation reaction
When the H+ ions from an acid react with the OH- ions from an alkali, a
neutralisation reaction happens to form water. This is the equation for the
reaction:
H+(aq) + OH-(aq)
H2O(l)
If you look at the equations above for sodium hydroxide and hydrochloric acid
you will see that there are Na+ ions and Cl- ions left over. These form sodium
chloride, NaCl.
Salt preparation
You need to be able to work out which particular salt is made in a reaction. You may
be asked to describe how to make a salt.
Naming salts
The name of a salt is in two parts. The first part of the name comes from the metal in
the base or carbonate, or the metal itself if a reactive metal like magnesium or zinc is
used.
The second part of the name comes from the acid used to make it. The names of
salts made from hydrochloric acid end in -chloride, while the names of salts made
from sulphuric acid end in -sulphate.
metal
acid
salt
sodium hydroxide reacts with hydrochloric acid to make sodium chloride
copper oxide
reacts with hydrochloric acid to make copper chloride
sodium hydroxide reacts with sulphuric acid
to make sodium sulphate
zinc oxide
to make zinc sulphate
reacts with sulphuric acid
Ammonia forms ammonium salts when it reacts with acids. Thus
ammonia
reacts with hydrochloric acid to make ammonium chloride
Making salts
If the base dissolves in water, you need to add just enough acid to make a neutral
solution (check a small sample with universal indicator paper), then evaporate the
water. You get larger crystals if you evaporate the water slowly.
Copper oxide and other transition metal oxides or hydroxides do not dissolve in
water. If the base does not dissolve in water, you need an extra step. You add the
base to the acid until no more will dissolve and you have some base left over (called
an excess). You filter the mixture to remove the excess base then evaporate the
water in the filtrate to leave the salt behind.
Acids, bases and salts
Test:
1. A solution has a pH of 4 - what does this mean?
a)
It is acidic.
b)
It is neutral.
c)
It is alkaline.
2. Which of the statements below is correct?
a)
Bases are acids that dissolve in water.
b)
Bases are alkalis that dissolve in water.
c)
Alkalis are bases that dissolve in water.
3. A liquid has a pH of 7. What does this tell you about the liquid?
a)
It is water.
b)
It is sodium chloride solution.
c)
It is neutral.
4. Which salt is made when calcium carbonate reacts with hydrochloric acid?
a)
sodium chloride
b)
calcium chloride
c)
calcium sulphate
5. Which pair of substances will react together to make copper sulphate?
a)
copper and sulphuric acid
b)
copper oxide and sulphuric acid
c)
copper oxide and hydrochloric acid
6. Which is the correct order of methods for making a salt from an acid and an
insoluble base?
a)
filtration ==> evaporation ==> neutralisation
b)
neutralisation ==> evaporation ==> filtration
c)
neutralisation ==> filtration ==> evaporation