Bases and Alkalis
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.
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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.
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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.
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Like acids, alkalis can be classified by the extent
of their ionization in water. Strong alkalis like
sodium hydroxide and calcium hydroxide are
completely ionized in water.
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Weak alkalis like ammonia are only partially
ionized in water.
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More about acids, bases &
neutralisation
Neutralisation is when the right amounts of
acid and base/alkali react together. After
neutralisation, the solution(what is left after
the reaction) is neutral (pH7) and the acid
and base/alkali have both lost their
properties. A salt dissolved in water is
formed.
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Farmers spread an alkali called calcium
hydroxide on their fields. If the soil is acidic, it
neutralises it, so that plants, that need neutral
soil can grow more easily.
This is the formula for neutralisation:
Acid + Alkali --> Salt + Water
Here is an example of a neutralisation reaction:
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This reaction leaves behind a liquid. This is the
Magnesium Nitrate dissolved in water. If you
evaporate the water away (by heating the liquid
on a bunsen burner) you get crystals of
Magnesium Nitrate.
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Another salt is Sodium chloride.
This is the salt that you put on your chips. The
formula for this reaction is:
Hydrochloric acid + Sodium hydroxide -->
Sodium chloride + Water
Like in the reaction above, the products of the
reaction will be common salt dissolved in water.
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Alkalis in industry
Alkalis are good at dissolving oil and grease, so
they are widely used in the manufacture of soaps
and detergents. Most dirt is bound to skin,
clothes or eating utensils by grease. The grease
makes it difficult to remove the dirt with water
alone, because water and grease do not mix. A
soap or detergent, such as washing up liquid,
breaks the greases up into tiny drops and allows
the water to wash away the dirt.
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The main raw material in the alkali industry is
BRINE (Salt water). Sodium hydroxide, which
is used to make soaps and paper, is produced
from brine by electrolysis (passing electricity
through it). Brine will also absorb carbon
dioxide to make sodium carbonate, which is
used in textile treatment, photography and glass
making.
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Neutralizing Acids
An alkali and an acid react together to give a
neutral salt. In addition, hydroxide ions (OH-) in
the alkali combine with the acid’s hydrogen ions
(H+) to produce water (H2O). In daily life,
problems of unwanted acidity are solved by
adding an alkali of the appropriate strength.
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Q&A
1. Sodium hydroxide is used in oven
cleaners to remove fat and grease.
Explain.
2. Limestone, slaked lime and quick lime
are all bases but only slaked lime is an
alkali.
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