Scientific enquiry

7E.1 What acids and alkalis are like (HSW)

You should already know Outcomes Keywords

Acids are all around us

Many things around us contain acids. Some acids are in
the food we eat. The acids in food produce a sharp, sour
taste. Many acids like those in food are harmless but
others are very dangerous.

Fruit or drinks made from fruit often

contain acids. They have a tangy,
sharp taste. We say acids taste sour.

Question 1

In the 18th century sailors could be at sea for several years at a

time. If they did not eat fresh fruit for a long time, they became
ill with a disease called scurvy. Scurvy is caused by a lack of
vitamin C in the diet. It can be fatal.
Captain Cook, who discovered Australia, made his sailors eat limes
and lemons. This stopped them getting scurvy. In his three-and-
a-half year trip to Australia, only one of his crew died of scurvy.
This was amazing at the time. From then on, the British Navy made
all its sailors drink lime juice to prevent scurvy. Even today, the
nickname for British sailors is ‘Limeys’.
Lime juice prevents scurvy because it contains vitamin C.
Vitamin C is a weak acid so it tastes sour.

Lime juice tastes sour.

Limes and lemons taste sour because they contain

citric acid. They prevent scurvy because they contain
vitamin C. This is also an acid, but it is very weak.

Question 2

66 7E Acids and alkalis

 7E.1 What acids and alkalis are like
Some acids are dangerous
Fruit juice is not dangerous but there are some acids that are risky to use. This is the
Substances like hydrochloric acid have their own hazard warning sign. warning sign
Hazard warning signs let people know that they need to be careful. for corrosive.

Hydrochloric acid is a strong acid that is corrosive. It will attack your skin
and start eating it away.
CORROSIVE
Question 3

When you mix acid with water, we say that you dilute it. This makes the
acid less dangerous. If you spill an acid, you should wash the area with lots
of water to dilute the acid.
Dilute acids are still harmful. They will harm or irritate your skin even
though they are not as dangerous as undiluted acids. This is the sign This is the sign
for harmful. for irritant.
Many foods contain weak acids. Fruit juices and vinegar are examples. The
weak acids in food can attack the surface of your teeth.
We use a black cross to warn people about them.

Question 4 5

About alkalis
The group of substances shown in the picture contain
alkalis. When acids and alkalis react, their properties
are cancelled out.

All these substances contain alkalis.

Some alkalis are safe to use. Others, such as sodium hydroxide, are just as
dangerous as the strongest acids. They are also described as corrosive.
When you get alkali on your skin it dissolves your skin away. Your skin
feels soapy and you get a chemical burn.
Many kitchen cleaners contain alkalis that attack grease.

These burns were caused by an

Question 6 7 alkali called sodium hydroxide.

7E Acids and alkalis 67

 7E.2 Telling acids and alkalis apart

You should already know Outcomes Keywords

Sodium hydroxide, hydrochloric acid, lemonade and water are all

colourless liquids. They look the same, but they have a different effect on
red cabbage juice, which shows that they are actually quite different.

Before adding red cabbage juice

After adding red cabbage juice

Chemical being tested hydrochloric acid lemonade sodium hydroxide water

Adding red cabbage juice to four

Question 1 colourless liquids.

We can use the colour change of red cabbage juice to show whether a
substance is an acid or an alkali. We can use juices from some other plants
too. We call plant juices extracts.

• Red cabbage juice and beetroot juice are vegetable extracts.

• Blackcurrant juice is a fruit extract.
• Litmus is extracted from a lichen.
These extracts show a different colour in acid to the one they show
in alkali.
We call them indicators.

Question 2 3

68 7E Acids and alkalis

 7E.2 Telling acids and alkalis apart

Beakers containing hydrochloric acid

Beakers containing hydrochloric acid

Beakers containing sodium hydroxide

Beakers containing sodium hydroxide

Indicator added litmus blackcurrant red cabbage

Indicator added litmus blackcurrant red cabbage

The acid and alkali in the diagram have not been diluted very much. The colours of three indicators in
The colours show what happens when different substances are added to acid and in alkali.
strong acid and strong alkali.

Question 4

Litmus is a dye. It is a very common indicator. It is purple when it is in a

substance that is neither alkali nor acid. This type of substance is called a
neutral substance.
Litmus turns red in acids. Litmus turns blue in alkalis. Water is neutral.
It is neither acid nor alkali. Litmus turns purple in water.

Substance Litmus colour Type of substance

sodium hydroxide blue alkali
acid
vitamin C red acid
water purple neutral
calcium hydroxide blue alkali
carbon dioxide red acid
hydrochloric acid red acid
alkali
potassium hydroxide blue alkali
Some examples of the colour of litmus in different substances.

Question 5 6

7E Acids and alkalis 69

 7E.3 Universal indicator and the pH scale

You should already know Outcomes Keywords

We can use litmus to show if something is acidic, strongly acidic

alkaline or neutral. We need a better indicator to show
the difference between a strong acid like concentrated weakly acidic
hydrochloric acid and a weak acid like lemon juice.
Universal indicator is a special type of indicator.
You can use it to tell how strong or weak an acid or
an alkali is. It is made by combining other indicators
neutral
together.
Concentrated hydrochloric acid is a strong acid – it will
weakly alkaline
attack your skin.
Lemonade is a weak acid – it does not damage skin.
Universal indicator shows different colours in these
substances. We can also use it to tell whether an alkali
is weak or strong.
strongly alkaline
Type of substance Colour of litmus Colour of universal indicator

strong acid red red

weak acid red yellow

neutral purple green

weak alkali blue blue

strong alkali blue purple

Question 1 2

The strengths of acids and alkalis are measured on a scale of numbers.

This scale is called the pH scale. The scale goes from 0 to 14.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

more acidic more alkaline

neutral The pH scale.

Question 3

70 7E Acids and alkalis

 7E.3 Universal indicator and the pH scale
The different colours of universal indicator are matched to the numbers on
the pH scale. When you buy universal indicator, you can buy a colour chart
like this.

pH numbers

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

strongly acidic weakly acidic weakly alkaline strongly alkaline

neutral

A colour chart for universal indicator that shows the pH scale.

On the pH scale, numbers between 0 and 3 tell us that

the liquid is a strong acid. Hydrochloric acid is a strong
acid. Acids with very low pH numbers carry the corrosive
hazard sign.
The numbers 4 to 6 tell us that the liquid is a weak acid.
Lemon juice is a weak acid.
Number 7 tells us that the liquid is neutral. Water has a pH
of 7. It is neither acid nor alkali.
Numbers from 8 to 14 are for alkalis. Sodium hydroxide
is a strong alkali. It has a pH of 14. Even dilute alkalis
can be corrosive.
This chart shows what happens when you test some
different substances with universal indicator.

Liquid Colour of universal pH Type of

indicator liquid
nitric acid red 1 strong acid

vinegar yellow 5 weak acid

Hydrochloric acid, nitric acid and
sulfuric acid are all acids.
ammonia solution purple 14 strong alkali
Sodium hydroxide and potassium
sodium bicarbonate solution blue 9 weak alkali hydroxide are alkalis.
All of them are corrosive.
salt water green 7 neutral

Question 4 5 6

7E Acids and alkalis 71

 7E.4 Neutralisation

You should already know Outcomes Keywords

An acid reacts with an alkali to cancel it out. This reaction is

called neutralisation.
When neutralisation happens, it produces a neutral substance
called a salt. Water is also produced. The pH of the new
substances is 7.

acid + alkali → salt + water

As you add acid to an alkali, the pH falls from a high number.

You have to add just the right amount of acid to the alkali to get
a neutral solution. Too much acid will make the pH fall below 7
and give an acidic solution. Not enough acid will leave an alkali
solution with a pH higher than 7.

Question 1 2

Never do this to check whether

A neutralisation experiment the acid has gone!

The diagram shows an experiment with hydrochloric acid, water and

washing soda.
The washing soda crystal dissolves in the water around it, making it
alkaline. A few drops of universal indicator has been added to the
water. Then, some hydrochloric acid is carefully added to the top.

hydrochloric acid

2 days

water

washing soda crystal

Question 3 4

72 7E Acids and alkalis

 7E.4 Neutralisation
The photograph shows the experiment with hydrochloric acid, washing
soda and water.
After a few days:

• the hydrochloric acid has moved slowly down the test tube;
• the washing soda crystal has dissolved in the water near it;
• the alkaline solution has moved up the test tube.
The universal indicator colour shows the change from acid to alkali as
you go down the tube. There is a green band in the centre. The green band
shows where the pH is 7 (in the photo, this is just under the yellow band).
This is where neutralisation happens.
The experiment
Question 5 6 after a few days.

What happens to other acids and alkalis?

When an acid and an alkali neutralise each other, they produce a salt.
The name of the salt depends on which acid and alkali are used.

hydrochloric acid + sodium hydroxide → sodium chloride + water

The salt’s name in this example is sodium

chloride. Its first name is the same as the first alkali
water
name of the alkali. Its second name comes from evaporates
the name of the acid. acid salty
water
• Sodium hydroxide gives sodium.
• Hydrochloric acid gives chloride. salt crystals

This gives the salt the name sodium chloride.

very gentle
Different acids produce salts with different second names. warming

Acid used Second name of salt If you add just the right amount
of alkali to an acid, you can
hydrochloric acid chloride make the acid disappear.

nitric acid nitrate

sulfuric acid sulfate

Question 7 Check your progress

7E Acids and alkalis 73

 7E.5 Where neutralisation is important

You should already know Outcomes Keywords

Curing indigestion
Indigestion is caused by too much acid in the stomach.
You can take medicine to neutralise this acid. Some
indigestion medicines contains a weak alkali called
magnesium hydroxide. This will neutralise excess
stomach acid.
It is important that the alkali in the medicine is weak.
If it is too strong, it will corrode your insides.
Some indigestion cures contain magnesium carbonate
or sodium bicarbonate. Because these are carbonates,
they react with the acid in your stomach to produce
carbon dioxide gas. This is another way of neutralising
the excess acid. Unfortunately, the gas produced can
make you burp!
One of these is a medicine
for indigestion.
Question 1 2

Here are the results of some tests with indigestion tablets.

Tablet Cost per tablet Amount of acid Amount of gas Time taken to neutralise
neutralised (cm3) produced (cm3) the acid (minutes)

Brand A 3p 25 0 3

Brand B 4p 20 0 10

Brand C 5p 30 15 2

Brand D 1p 10 28 1

Brand E 8p 40 15 2

Question 3

74 7E Acids and alkalis

 7E.5 Where neutralisation is important
Toothpaste
Your mouth is full of bacteria. These feed
on bits of food left in your mouth. When
bacteria feed they produce an acid. The acid
can attack your teeth, making them decay.
When you brush your teeth you remove the
bits of food and some of the bacteria. The
toothpaste is also a weak alkali. The alkali
in toothpaste neutralises the acid. This helps
to protect your teeth. Sodium bicarbonate is
a weak alkali that is commonly used Bacteria on a human tongue. The pH of this toothpaste
in toothpaste. is 8 because it contains
sodium bicarbonate.
Question 4

Making cakes rise part of cake, magnified

Baking powder is an ingredient in some cake recipes. It contains carbon dioxide gas
an acid and sodium bicarbonate (which is sometimes called
bicarbonate of soda). The acid is called tartaric acid. When baking powder
the cake is mixed, the liquid dissolves the acid and the sodium in the cake mix

bicarbonate, and they react. Carbon dioxide gas is produced. The

carbon dioxide gas makes the bubbles in sponge cakes. This is
another type of neutralisation reaction because the products from bake
the acid and the sodium bicarbonate reacting are neutral.

cake
Question 5

Acid rain
Pollution from factories and power stations produces acid gases in the air.
Rainwater dissolves these acid gases so that its pH is lower than 7. Acid
rain harms the environment. The water in rivers and lakes becomes slightly
acidic. In Scandinavian countries, they add crushed limestone to their lakes
to neutralise the acidity caused by acid rain.
The pH of soil is different in different places. Serious gardeners test their
soil using universal indicator so they know its pH. Different plants grow
best in different soils. In some areas, the soil is too acidic for any plants to
grow well. Calcium carbonate can be added to
Lime is the common name for an alkali called calcium oxide. Farmers lakes to neutralise the water from
acid rain.
spread it on fields to neutralise some of the acid in the soil. Adding lime
means that the pH of the soil is raised, making it less acidic. Plants can now
grow well. Review your work

Question 6 Summary

7E Acids and alkalis 75

 7E.HSW Investigations and safety

You should already know Outcomes Keywords

Investigating the effect of temperature on a reaction

Some pupils are investigating whether the temperature of hydrochloric acid
affects its reaction with a metal.

Group A adds some zinc to cold hydrochloric acid and some iron to some
hot hydrochloric acid. Both test tubes produce bubbles of hydrogen gas at
the same rate.

Group B sees what group A has done and decide that the test is not
fair because they have used different metals – they cannot tell if the
temperature is having an effect because the two metals might not react in Zinc in cold acid and hot acid.
the same way. Group B adds zinc to both cold and hot acid, and repeats the
experiment with iron.

Question 1 2 3

The teacher explains to group A that their method does not give reliable
evidence. This is because they have changed more than one thing at once
when trying to compare the two test tubes.

When you make a judgement about how good or reliable an experiment is

it is called evaluation. Iron in cold acid and hot acid.

Deciding what to do to make your method of working safe is called a

risk assessment. The pupils in the diagram are wearing goggles but the
bottles of acid are too close to the back edge of the desk. The pupil on
the left is holding a test tube very close to his body. Both lab coats are
open and not protecting the students very well at all if there is a spill.
Both pupils are engrossed in their own activity and are not aware of
what the other is doing.
Part of a risk assessment is to state what you would do to reduce the risk.

Question 4 5

76 7E Acids and alkalis

 7E.HSW Investigations and safety
Safety in science
When scientists plan investigations they need to think about:

• the plan of what to do;

• how to make any tests fair;
• the equipment and substances they use;
• safety. CORROSIVE

You can look up hazards for different substances in books This is the This is the sign This is the sign
or on cards that are published about the substances. Some warning sign for harmful. for irritant.
substances have hazard warning signs on their containers. In for corrosive.
a risk assessment, you identify each hazard and decide what Hazardous chemicals have warning labels. You
to do to reduce it. need to be able to recognise them.

A funnel is used to fill a tube A tube is used to put a measured Some indicator is added to
with hydrochloric acid. amount of sodium hydroxide into the flask.
a flask.
The photographs show part of an experiment in which hydrochloric acid is
Risk assessment is one part of
added to sodium hydroxide. There are several safety features.
science that affects the way
• The person is wearing goggles. people think and behave.
• The person is standing up.
• A funnel is used to direct the acid into a narrow tube.
• The substances used are clearly labelled.
Standing up is the result of a simple risk assessment. You can reduce the
risk of spilling harmful chemicals on your legs if you stand up. If there is a
spill on the worktop, it will drip onto the floor and not onto your lap.

Question 6 7

Even when you have done a risk assessment, accidents can happen. This
might be because your risk assessment was not good enough. It might be
because you are not following your risk assessment. It might just be due to
some unexpected event, a pure accident.
Someone distracted this student
during an experiment and she has
Question 8 9 spilled alkali onto her hand.

7E Acids and alkalis 77

 7E Questions

7E.1
1 Give some examples of substances that contain acids.
2 Write down:
a one word that best describes the acid taste of lime juice;
b the name of the acid that gives lime juice this taste;
c the name of the disorder that this acid helps to prevent.
3 What does the word ‘corrosive’ mean?
4 How can you make a strong acid less dangerous?
5 Why must you always wear eye protection when you use hydrochloric acid?
6 What can alkalis and acids do to each other?
7 What do you think you should do if you spill oven cleaner on your skin?
Give a reason for your answer.

7E.2
1 What colour does red cabbage juice turn when added to:
a hydrochloric acid?
b lemonade?
c sodium hydroxide?
d water?
2 What property of red cabbage juice makes it useful as an indicator?
3 Name two indicators that can be made from vegetables.
4 How could you use an indicator to tell whether a bottle of household cleaner
with a missing label is likely to damage your skin?
5 Give an example of a neutral substance.
6 What can litmus tell you about a substance?

7E.3
1 What is the advantage of universal indicator over litmus?
2 How can you use universal indicator to tell if lemonade is a stronger acid
than lemon juice?
3 What is the pH scale used for?
4 What colour will universal indicator go in water? Give a reason for
your answer.
5 What difference would you expect between the labels of two household
cleaners, one of which has a pH of 14 and the other of which has a pH of 8?
6 What is the most alkaline substance shown in the table?
Give a reason for your answer.

78 7E Acids and alkalis

 7E Questions

7E.4
1 What two substances are produced when you add an acid to an alkali?
2 What type of solution is produced if too much acid is added to an alkali?
Give a reason for your answer.
3 What colour is the solution next to the washing soda crystal?
4 How could the person doing the experiment tell what pH a solution of
washing soda is?
5 What is the pH at the top of the test tube? Give a reason for your answer.
6 Both the yellow and the red sections of the solution in the test tube are
acidic. What is the difference between them?
7 If we used potassium hydroxide
instead of sodium hydroxide, 7E.5
what would be the name of the 1 What causes indigestion?
salt produced? 2 What gas is produced when magnesium carbonate neutralises
an acid?
3 a Which tablet neutralises the most acid?
b Which is the cheapest tablet?
c Which tablet would you recommend?
Give your reasons.
4 Your saliva is slightly
7E.HSW alkaline. What effect
1 Why is group B’s method better than group A’s? will saliva have on the
2 Both groups used the same size pieces of metal. Suggest why this might acid in your mouth?
be important to give reliable results. 5 Explain how tartaric
3 What conclusion can you draw from group B’s experiment? acid and sodium
4 What do the pupils in the diagram need to do to work in a safer manner? bicarbonate can make
5 Write down the things you would do to reduce the risks of group B’s a cake rise.
investigation. 6 What might happen
6 Use secondary sources to find out the hazards of some household to soil if you add too
substances, for example oven cleaner and bleach. much lime to it? Give
a reason for your
7 Some hazards are so common that it might be worth putting them in a
answer.
set of laboratory rules so you don’t have to keep putting the same ones
in every risk assessment. Write a set of laboratory rules to cover what
you think are the main points for most experiments.
8 What emergency action would you talk to deal with this accident?
9 Write a risk assessment for the experiment the pupil is doing.
• List the substances used and the actions she is carrying out.
• State the hazard for each one.
• State what she should do to reduce the hazard.
(Hint: there are some things she could do to be safer even though, in the
accident shown, these things are not involved.)

7E Acids and alkalis 79

 7E Summary

Keywords

We make acids and alkalis Some acids and alkalis can We use symbols called
safer by adding water to be dangerous. They can be hazard warning signs to
dilute them. corrosive, harmful or irritant. show what the risk is.

We use acids like

Lemon juice and vinegar
Soap, toothpaste and oven hydrochloric acid and alkalis
contain acids. That is why
cleaner contain alkalis. like sodium hydroxide in the
they taste sour.
laboratory.

We use universal indicator

to find the pH.

An indicator is a dye that is We use the pH scale to tell Acids have a pH

a different colour in an acid us how acidic or alkaline a below 7. Adding an
and an alkali. substance is. alkali raises the pH.

Many indicators are plant If a solution is not an acid Alkalis have a pH

juices. We extract them or an alkali, we say that it above 7. Adding an
from plants. is neutral. Neutral solutions acid lowers the pH.
have a pH of 7.

We use neutralisation in
many everyday situations.
An acid and an alkali react When an acid and an
For example, we use
to cancel each other out. alkali react, they make a
indigestion tablets to
We call this neutralisation. salt and water.
neutralise some of the acid
in our stomach.

Different acids and alkalis

Some indigestion tablets make different salts.
produce carbon dioxide gas
when they neutralise an acid.

Check your progress Review your work Scientific enquiry

140 Summaries