Acids and Alkalis

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Acids  Alkalis   Neutral liquids   The PH scale   Indicators  Some pH values  Neutralization  Applications of neutralization:  Reactions involving acids

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 

Table of pH numbers

pH 1 2 3 4 5 6 7 8 9 10 11 12 13 14

 weak acid


Weak alkali

Getting more acid

Getting more alkaline

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 indicator

Litmus Indicator
Litmus can only be two colours
In ACIDS it is RED
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

pH pH 4

or less

pH 5 pH 6 pH 7 pH 8 pH 9 pH10 pH 11

or more





stronger acid weak acid Neutral weak alkali stronger alkali

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

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 Blue-green 8 Mild alkali
Tooth paste Blue-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:

In agriculture:
 Lime (or quicklime), which is an alkali, is placed on the soil to neutralise acid soil and so raise its pH.

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.


Reactions involving acids

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]

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