S1 Substances and Uses summary notes
The pH scale
LI
1 – To develop our understanding of the pH scale
Acids and alkalis are very common substances in a science lab and at
home. The pH scale measures the acidity and alkalinity of a substance
in a solution. (The substances must be dissolved in water)
There are different pieces of equipment for measuring the pH of a
substance.
Universal indicator
pH meter
pH paper
The pH meter gives a digital value between 0 and 14 which tells us how
acidic or alkaline a substance is.
The pH paper and universal indicator will show a colour when tested
with a substance. This can then be compared with the pH scale to
determine how acidic or alkaline the substance is.
Measuring pH of a substance
2 – To explore the uses of acids and alkalis in everyday life
LI
The diagram below shows how we can easily measure the pH of any
substance with a piece of pH paper.
Substance to be
tested
small piece of
pH paper
dimple
tile
Below are some examples of everyday acids and alkalis and their pH:
Name of Substance
Laboratory or
Household
Colour of pH
paper
pH
number
Acid, Alkali
or Neutral
hydrochloric acid
lab
red
1
acid
sodium hydroxide
lab
blue
14
alkali
vinegar
house
red
3
acid
baking soda
house
blue
9
alkali
nitric acid
lab
red
1
acid
ammonia solution
lab
blue
11
alkali
pure water
house
green
7
neutral
soda water
house
red
5
acid
dishwasher powder
house
blue
10
alkali
lime water
lab
blue
9
alkali
cola
house
red
5
acid
lemon juice
house
red
4
acid
sulphuric acid
lab
red
1
acid
oven cleaner
house
blue
13
alkali
lemonade
house
red
4
acid
salt water
house
green
7
neutral
bleach
house
blue
13
alkali
Common Household Acids
Common Household Alkalis
vinegar
dishwasher powder
lemon juice
oven cleaner
cola
bleach
Laboratory Acids
Laboratory Alkalis
hydrochloric acid
sodium hydroxide
sulphuric acid
ammonia solution
nitric acid
calcium hydroxide
Dilution of acids
LI
3 – To investigate the effect of dilution on acids and alkalis
A concentrated acidic solution is one that contains a lot of acid
compared with water. A dilute solution contains more water than acid.
Think of diluting orange juice:More orange and less water = concentrated solution
Less orange and more water = diluted solution
Diluting an acid by adding more water increases the pH of the solution
towards 7 making it less acidic.
As the acid is diluted the
pH increases towards 7
Diluting an alkali by adding more water decreases the pH of the
solution towards 7 making it less alkaline
As the alkali is diluted the
pH decreases towards 7
It is not possible to dilute an acid so that its pH increases to greater than
7 nor an alkali so that its pH decreases below 7. This is because we are
diluting them with water which is a neutral substance (with a pH of 7) so
adding more and more water will simply maintain the pH of the diluted
acid or alkali at 7.
Neutralisation
LI
4 – To investigate the reactions of acids and why these
reactions are useful
Acids and alkalis neutralise each other to form a salt and water.
Remember that the acid has a low pH and the alkali a high pH, but when
they are mixed in precise quantities they form water (a neutral
substance) plus salt.
Alkali
When performing a neutralisation experiment it is important not to add
too much alkali to the acid as the pH will increase beyond 7 creating an
alkaline solution.
Remember that neutralisation is not the same as dilution. In dilution
water is added to the acid so the pH of the solution cannot increase
above 7 but in neutralisation it is an alkali that is being added so it is
quite easy to go beyond 7 and produce a high pH alkali.
Everyday examples of neutralisation:
Indigestion tablets help with
heartburn by neutralising
stomach acid
Toothpaste neutralises acid
from food and so helps to
prevent tooth decay
Naming Salts made by Neutralising Acids and Alkalis
LI
5 – To apply knowledge to name the products of a
neutralisation reaction
Alkali
The first part of the name is always the metal that is part of the alkali and
the second part comes from the acid.



If the acid is hydrochloric acid, the salt will be a chloride.
If the acid is nitric acid, the salt will be a nitrate.
If the acid is sulphuric acid, the salt will be a sulphate.
In the example below the alkali sodium hydroxide is added to
hydrochloric acid to produce sodium chloride.
eg.
alkali
sodium hydroxide
salt name
sodium chloride
acid
hydrochloric acid
In this example the alkali lithium hydroxide is added to sulphuric acid
and the salt lithium sulphate is produced.
eg.
alkali
acid
lithium hydroxide
salt name
sulphuric acid
lithium sulphate
A general word equation can be used to describe this reaction:
ACID
+
ALKALI
SALT +
WATER
More examples:
hydrochloric acid + copper oxide
→ copper chloride + water
sulphuric acid
+ sodium hydroxide → sodium sulphate + water
nitric acid
+ calcium hydroxide → calcium nitrate + water
Acid Rain
Non-metal elements can react with air or oxygen to form compounds
called oxides.
carbon + oxygen
carbon dioxide
sulphur + oxygen
sulphur dioxide
nitrogen + oxygen
nitrogen dioxide
Non-metal oxides which can dissolve in water (like those above)
produce acids.
e.g. Sulphur dioxide dissolves in water to produce sulphuric acid
Nitrogen dioxide dissolves in water to produce nitric acid
Acid Rain is an environmental problem which can:
 damage buildings made from carbonate rocks (e.g. marble)
 corrode metals structures made of iron and steel (e.g.
bridges)
 affect plant life by changing the pH of soil (e.g. less crops
grown)
 affect animal life by changing the pH of rivers/lakes (e.g. kills
fish)
Acid rain could be reduced if the gases that cause acid rain were
reduced:
 Carbon dioxide is released when fossil fuels like coal, petrol and
gas are burned
 If we burn less fossil fuels, then less carbon dioxide will
be released
 Sulphur dioxide is released when fossil fuels like coal are burned
 Use low-sulphur fuels instead of high sulphur fuels
 Remove sulphur dioxide fumes before release into
atmosphere
 Nitrogen dioxide is produced in car engines by the sparking of air
with petrol to set the petrol on fire.
 Fit catalytic converters to cars to remove nitrogen
dioxide from car exhaust fumes.
Neutralisation with indigestion powder
LI
6 – To investigate how changing certain variables affects the rate
of the reaction between an acid and calcium carbonate
Acids and alkalis neutralise each other to form a salt and water.
Other substances also neutralise acids to form a salt.
Indigestion
powder
stirring rod
spatula
3
10cm hydrochloric acid + 3
drops of universal indicator
In the experiment,
indigestion powder is
used to neutralise
hydrochloric acid just like
an alkali neutralises an
acid. Only this time it is
not just salt and water that
is produced.
This time we see that the red hydrochloric acid turns green as it is
neutralised but we also notice that a gas is given off.
Metal
Acid
Carbon
→
+
Salt
+ Water +
Carbonate
Dioxide
Indigestion powder is a form of metal carbonate which means it will give
off carbon dioxide gas as well as salt and water during neutralisation.
e.g.
sulphuric
lithium
→
+
acid
lithium
carbonate
carbon
+ water
sulphate
+
dioxide