ACIDS AND ALKALIS

Acids are chemicals that have certain properties.

There are weak acids and strong acids.
One property acids have is that they have a
sour, sharp and tangy taste.
Many foods contain weak acids.
 What is a weak acid?
Acids are found in household items such as food, drink
and beauty/skincare products.

It is safe to handle these acids and even taste them, if they

are for eating! These acids are known weak acids.
Lemons and limes contain the weak acid known
as citric acid
Vinegar contains the weak acid known as
ethanoic acid
 What is a strong acid?
Some acids, like those found in the laboratory or a car
battery, are very dangerous (too dangerous to taste or touch).

These acids are said to be corrosive as they can damage

other materials by wearing them away! They are strong acids.
Why must strong acids always be handled very carefully?
Strong acids are corrosive – they eat away at
substances they come in contact with
including your skin
Most acids we use in the laboratory are strong
acids.
These are sulfuric acid, nitric acid and
hydrochloric acid
When you add water to an acid you dilute it.
A dilute acid is less dangerous than a
concentrated acid but it is still harmful and an
irritant.
If you spill or come in contact with acid wash the
area with a lot of water to dilute it.
An alkali is the chemical opposite of an acid.
They cancel each other out when mixed
together.
Alkalis feel soapy
Many cleaning products, including hand soap
and toothpaste, contain alkalis
 What is a weak alkali?
Alkalis are found in soaps and other materials used for cleaning.

It is safe to handle these alkalis, which can feel soapy.

These alkalis are known as weak alkalis.
Strong alkalis are corrosive
Strong alkalis are sodium hydroxide and
potassium hydroxide
A weak alkali is calcium hydroxide
 What is a strong alkali?
Some alkalis, like those in the laboratory or in cleaning
materials such as toilet cleaner, are too dangerous to touch.

These alkalis are said to be caustic because they can burn skin and damage other
materials! They are strong alkalis.
Why must strong alkalis always be handled very carefully?
Alkalis can be diluted to make them less
dangerous.
 What are hazard symbols?
How can you tell which chemicals are safe and which are dangerous?

Special symbols are used on bottles and vehicles that

contain dangerous chemicals.

toxic irritant harmful corrosive highly

flammable
These hazard symbols show why the chemical is dangerous.

Why is it important that these symbols can be recognised by

people from other countries?
Copy notes on working safely with chemicals on
page 91
An indicator is a substance which changes colour
depending on whether it is in an acid or an
alkali.
Red cabbage can be used as a natural indicator
Litmus is one of the most common indicators.
It is a dye.
Litmus turns red in acid
Litmus turns blue in alkali
Litmus turns purple in a neutral substance
 What is an indicator?
How can you test a liquid to find if it is an acid or an alkali?

An indicator is a special chemical that changes

to a different colour in an acid or an alkali.

Litmus paper and litmus solution are examples of

indicators.

Litmus is red Litmus is

in acid. blue in alkali.
Litmus test experiment
Litmus dye is often put on a piece of paper and
called litmus paper.
However litmus can only tell us if something is
acid or alkali
Universal indicator can tell us if something is a
strong or weak acid or alkali – it measures the
strength of the acids
Universal indicator uses the pH scale
The pH scale goes from 0-14
Acids have a pH of less than 7
The lower the pH the stronger the acid
pH of 1 is a very strong acid
pH of 6 is a very weak acid
Alkalis have a pH of more than 7
The higher the pH the stronger the alkali
A ph of 14 is a very strong alkali
A ph of 8 is a very weak alkali
A neutral substance has a pH of 7
Water is a neutral substance
Universal indicator changes colour with the pH
 What is universal indicator?
Litmus is used to show if a solution is acid or alkali.

Litmus does not show if the acid or alkali is weak or strong.

Universal indicator has a range of colours that show how weak or strong the acid or
alkali is.

stronger acid stronger alkali

Strong Weak weak Strong

acid acid alkali alkali
What is the colour for a weak acid?
neutral
What is the colour for a strong alkali?
 What is the pH scale?
The strength of an acid or alkali is measured by the pH scale.

Each universal indicator colour is given a number called the pH value.

Universal indicator can tell you the pH of a solution.

stronger acid stronger alkali

1 2 3 4 5 6 7 8 9 10 11 12 13 14
Strong Weak weak Strong
acid acid neutral alkali alkali
What is the pH of a weak acid?
What is the pH of strong alkali?
What is the pH?
Are these substances acidic or alkaline?
Are they weak or strong?

Substance pH Description of acid/alkali

soda water 6 very weak acid

car battery acid 1 very strong acid

soap 8 very weak alkali

washing soda 10 weak alkali

stomach acid 2 strong acid

oven cleaner 14 very strong alkali

vinegar 4 weak acid

1 1 1 1 1
acid 1 2 3 4 5 6 7 8 9
0 1 2 3 4
alkali
TYPE OF SUBSTANCE COLOUR OF
INDICATOR
STRONG ACID RED
WEAK ACID YELLOW
NEUTRAL GREEN
WEAK ACID BLUE
STRONG ACID PURPLE
 NEUTRALISATION
If you add an acid to an alkali they cancel each
other out and make a neutral solution
If you add too much acid to an alkali you get an
acid solution
If you add too much alkali to an acid you get an
acid solution
You need to add exactly the right amount
To do this you use a burette.
Draw diagram on page 96 showing one of
burettes
Put alkali in a flask and add indicator
Put acid in burette and add acid to flask, drop by
drop. Shake to mix each time.
Stop when the indicator shows the solution is
neutral.
The acid has reacted with the alkali and
neutralised it.
 Neutralizing acid rain
Gases emitted by cars, power stations and
factories mix with rainwater in the
atmosphere.
This rainwater has a pH <5.5.
Why is it called acid rain?

Acid rain can have a harmful effect on the

environment killing trees, polluting lakes
and damaging buildings.

How can the effects of

acid rain be prevented?
 Neutralizing soil
Soils in different places can be acidic,
alkaline or neutral.

Many plants do not grow

well in acidic soil.

Why do farmers test soil

to check the pH?

Farmers often add an alkali to acidic soil to make their plants grow better.

How does the alkali help to treat the acidic soil?

 Neutralizing stings
A bee sting A wasp sting
is acidic. is alkaline.

Why can bicarbonate of soda (pH Why can vinegar (pH 3)

9) be used to treat a bee sting? be used to treat
a wasp sting?
Neutralisation is an important reaction and we
use it all the time.
The stomach produces hydrochloric acid to help
digest food. If it produces too much you can
get indigestion. Antacids are medicine that
contain alkalis and can be used to help
indigestion.
Bacteria in you mouth feed on food pieces in
your teeth and produce acid. This acid
damages your teeth and causes tooth decay.
Toothpaste contains alkali and can neutralise
the acid.
Acid rain can cause the water in lakes to become
acidic. Some countries drop alkalis in their
lakes to neutralise the lakes.
Soil can become acid and stop some crops
growing. Farmers add alkali to the soil to
neutralise the soil.
Do activities 7.6 a and 7.6 b on page 100