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ACIDS (acidic solutions) Acids have the following properties:
examples of common acids:
Weak acids
Strong acids
NOTE: strong acids are a lot more dangerous than weak acids.
Acids are CORROSIVE which means they will attack and weaken many things including metals, paper, clothing and skin.
A concentrated acid or alkali is more dangerous than a dilute one.
To make something more dilute water needs to be added.
CONCENTRATED means without much or without any water added.
DILUTE means a lot of water added.
ALKALIS ( alkaline solutions) Alkalis have the following properties:
Examples of common alkalis:
Alkalis are caustic which means they will burn skin and eyes.
A strong alkali (like caustic soda) is VERY dangerous.
A neutral liquid is one with a pH value equal to pH 7.
examples: water, salt (sodium chloride) solution, sugar solution
A scale of numbers ranging from 1 to 14
neutral = pH7
acid = less than pH 7
alkali = more than pH7
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Indicators
An indicator is a liquid that is used to show whether we have an acid or
an alkali
It is one colour in an acid and another in an alkali
There are two indicators you need to know the names of:
Litmus indicator and Universal indicatorLitmus Indicator
Litmus can only be two colours
In ACIDS it is RED
In ALKALIS it is BLUE
We use litmus indicator when we want to tell
precisely when a liquid is neutral but it can't tell us how strong an acid or an
alkali is
Universal Indicator
Universal indicator is a mixture and has a whole range of colours
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We use universal indicator when we want to know how strong an acid or alkali is but it is difficult to know when a liquid is precisely neutral
A table to show the pH values of some common substances
| Substance | Colour with universal indicator | Ph number | Description
(acid, weak acid, alkali, weak alkali or neutral) |
| Hydrochloric acid (N) | Red | 1 | Strong acid |
| Oven cleaner (N) | Purple | 14 | Strong alkali |
| Vinegar | Orange | 5 | Acid |
| Water | Green | 7 | Neutral |
| Lime-water | Blue | 9 | Alkali |
| Caustic soda (N) | Purple | 13 | Strong Alkali |
| Indigestion powder | Bluey green | 8 | Mild alkali |
| Tooth paste | Bluey green | 7.5 | Mild alkali |
| Lemon juice | Orange | 4 | Acid |
(N) NOTE: These substances are very dangerous
When an alkali is added to an acid a chemical reaction takes place.
This reaction is called NEUTRALIZATION and makes the pH number rise.
The alkali is 'opposite' to the acid and cancels it out.
An acid always reacts with an alkali to produce a salt and water
ACID + ALKALI = SALT + WATER
eg Hydrochloric acid will react with sodium hydroxide to produce a solution of
sodium chloride (salty water)
If the solution is warmed so the water evaporates crystals of common salt will
be left
Word equation Hydrochloric acid + sodium hydroxide ----> sodium chloride + water
Some useful applications of neutralization:
The soil becomes too acid due to the bacteria which help plants and animals decompose.
Why do we need to neutralise soil?
2. In medicine:
Indigestion is often caused by too much acid in your stomach.
Indigestion mixture contains a mild alkali which neutralises excess acid in your stomach.
1. Acids react with most metals to release hydrogen gas:
eg zinc + sulphuric acid zinc sulphate + hydrogen
2. Acids react with any metal carbonate (eg copper carbonate or calcium carbonate) to produce carbon dioxide gas
eg calcium carbonate + hydrochloric acid calcium chloride + water + carbon dioxide
3. An acid reacts with an alkali to form a salt plus water
eg i. hydrochloric acid + sodium hydroxide sodium chloride + water
NOTE: A base is any chemical that can neutralise an acid.
An alkali is a base that is soluble in water.
A salt is formed whenever a base neutralises an alkali
eg. sulphuric acid + copper oxide copper sulphate + water
[base + acid salt + water]