Bases and Alkalis


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Bases and Alkalis

A base is a compound that reacts with and neutralises an acid. The
reaction forms a salt plus water.

Alkalis are a type of base - ones that dissolve in water. Alkalis are
compounds that react and neutralise acids. Like bases, they form a
salt and water in the reaction.

Alkalis turn red litmus indicator paper blue and they cause Universal
Indicator paper to turn blue or purple. Alkaline solutions have a pH
value greater than 7. Alkaline solutions feel soapy and are corrosive.

Examples of bases and alkalis:

NAME

FORMULA

DOES IT DISSOLVE IN WATER

ALKALI OR BASE

FOUND IN/USED FOR

Copper oxide

CuO

No

Base

Sodium hydroxide

NaOH

Yes

Alkali pH 13

Used to remove grease from ovens & drains. Used to make soap

Ammonia

NH3

Yes

Alkali pH 10

The main active ingredient in household cleaning fluids

Calcium hydroxide

Ca(OH)2

No

Base

Slaked lime used in agriculture

Aluminium hydroxide

Al(OH)3

No

Base

Used in some indigestion tablets

Acids and Alkalis

Neutralisation

Neutralisation is the reaction of an acid with a base or alkali until
the solution is neutral (pH7). In neutralisation, the acid and base
(or alkali) both lose their properties and the solution becomes
neutral. A salt and water are formed.

Neutralisation is used in the following everyday situations:

* Farmers use an alkali called calcium hydroxide (slaked lime or
lime). This is spread on the fields or ploughed ground. It is used
to neutralise the slightly acidic soil or even to make the soil
slightly alkaline, because grass and vegetable crops often need
often soils with a specific pH to grow well.

* When you get a burning feeling in your throat after you've been

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sick - that is the hydrochloric acid which has been brought up
from your stomach. If too much is produced you get indigestion.
When people take indigestion tablets they are using a base such as
magnesium hydroxide, calcium carbonate or aluminium hydroxide to
react with the acid and neutralise it.

magnesium hydroxide

+

hydrochloric acid

Becomes

magnesium chloride

+

water

* A lot of the electricity we use comes from the burning of fossil
fuels in power stations. These fossil fuels often have some
sulphur in them. When the fuel is burnt it also produces the acid
gas, sulphur dioxide. Sulphur dioxide gas will turn into sulphuric
acid when it dissolves in water. If this gas escapes into the
atmosphere it will make acid rain. Therefore it is neutralised
using a slurry of calcium carbonate.

calcium carbonate

+

sulphuric acid

Becomes

calcium sulphate

+

water

+

carbon dioxide

Laboratory examples of neutralisation reactions

Try and learn the patterns for the 3 different types of reaction
below.

1 Acids + alkalis:
Here the acid is neutralised by the base, and a salt and water are
formed

nitric acid

+

magnesium oxide

Becomes

magnesium nitrate

+

water

[IMAGE]

hydrochloric acid

+

calcium hydroxide

Becomes

calcium chloride

+

water

2 Acids + metal carbonates:
Notice the difference here: a salt , water and carbon dioxide are
formed..

sulphuric acid

+

sodium carbonate

Becomes

sodium sulphate

+

water

+

carbon dioxide

[IMAGE]

hydrochloric acid

+

copper carbonate

Becomes

copper chloride

+

water

+

carbon dioxide

3 Acids + metals:
A salt is formed, but accompanied this time by hydrogen gas..

hydrochloric acid + iron Becomesiron chloride + hydrogen

sulphuric acid + zinc Becomeszinc sulphate + hydrogen

Acid (a hydrogen compound, turning Universal Indicator yellow, orange
or red)

Alkali (a soluble base, turning Universal Indicator blue or purple)

Base (a metal oxide or hydroxide. They neutralise acids)

Neutralisation (a reaction of an acid with a base (alkali) forming a
salt and water)

Indicator (a dye that has different colours in acid and alkaline
solutions)

pH (a number that shows the strength of the acid or alkali. Also
indicates neutral)

Salt (a compound formed when a metal replaces the hydrogen in an acid)

Acid rain (acidic rain water, often formed as a result of pollution)

PH colour chart


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