Acids   Alkalis   Neutral liquids   The PH scale   Indicators

Neutralization   Reactions involving acids

ACIDS (acidic solutions)

Acids have the following properties:

  •  Have a pH value less than pH 7
  • Turn blue litmus indicator red
  • Can neutralise an alkali
  • Have a sour taste (WARNING: never taste any chemicals)

examples of common acids:

Weak acids

  • citric acid (found in lemons and other citrus fruits) tannic acid (found in tea)
  • acetic acid (found in vinegar) formic acid (found in ant stings)
    [ Acetic acid is correctly called ethanoic acid  and formic acid is correctly called called methanoic acid ]

Strong acids

  • sulphuric acid (accumulator acid, found in car batteries)
  • hydrochloric acid (in our stomach to kill germs)
  • nitric acid (used in the manufacture of explosives and fertilizers)

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:

  • Have a pH value more than pH 7
  • Turn red litmus indicator blue
  • Can neutralise an acid
  • Feel soapy to touch (reacts with fat to form soap)

Examples of common alkalis:

  • caustic soda (sodium hydroxide): used to clean ovens and drains
  • washing soda (sodium carbonate)
  • ammonia solution (often used in household cleaners)
  • garden lime (calcium oxide): used to neutralise acid in the soil
  • indigestion powder (often magnesium hydroxide)

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
(Just because a liquid is neutral does not mean that it is safe to drink)


  • A scale of numbers ranging from 1 to 14
  • The pH number tells you if a liquid is a liquid is an acid or an alkali.
  • pH 7 is neutral.
  • Less than pH7 is acid.
  • More than pH7 is an alkali

Table of pH numbers

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 liquid that changes colour depending on the pH of the liquid it is added to.
universal indicator
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
ph of substances
(N) NOTE: These substances are very dangerous because they are very strong acids or alaklis and must be used with great care.

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:

1. 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?

  • Some plants grow better in less acid soils
  • Bacteria, which cause plants to decompose and so fertilize the soil, grow better in less acid soils.

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  hydrochloric acid + sodium hydroxide →sodium chloride + water

This is an example of a neutralization reaction


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 acid

eg. sulphuric acid + copper oxide copper sulphate + water

[acid + base    salt + water]