Acidity, pH,
and
Indicators
At this point, you should be familiar with the
general properties of acids and bases.
At this point, you should be familiar with the
general properties of acids and bases. If not,
make sure you review them at this time.
pH of Acidic, Basic,
and Neutral
Solutions
The degree of acidity,
The degree of acidity, or how acidic a
solution is,
The degree of acidity, or how acidic a
solution is, is often represented by a quantity
called pH.
The degree of acidity, or how acidic a
solution is, is often represented by a quantity
called pH.
“pH” has a small “p” and a capital “H”.
pH is just a number,
The degree of acidity, or how acidic a
solution is, is often represented by a quantity
called pH.
“pH” has a small “p” and a capital “H”.
pH is just a number, there is no unit for it.
You should know what the general range in
pH is for acidic solutions,
You should know what the general range in
pH is for acidic solutions, basic solutions,
You should know what the general range in
pH is for acidic solutions, basic solutions,
and neutral solutions.
You should know what the general range in
pH is for acidic solutions, basic solutions,
and neutral solutions. The pH scale diagram
in the Science 10 data booklet will help you
with this:
You should know what the general range in
pH is for acidic solutions, basic solutions,
and neutral solutions. The pH scale diagram
in the Science 10 data booklet will help you
with this:
Let’s have a look at the information in this
scale:
Let’s have a look at the information in this
scale:
Let’s have a look at the information in this
scale:
A Neutral solution has
a pH = 7.0
Let’s have a look at the information in this
scale:
A Basic solution has a
pH >7.0
Let’s have a look at the information in this
scale:
Let’s have a look at the information in this
scale:
Getting more acidic
Getting more basic
Getting more corrosive
Getting more corrosive
Quite
corrosive
Getting more corrosive
Getting more corrosive
Quite
corrosive
Getting more corrosive
Getting more corrosive
Non - corrosive
Getting more corrosive
Getting more corrosive
A More Detailed
Look at pH
Like the Richter Scale used for comparing
strength’s of earthquakes, the pH scale is
a logarithmic scale.
All you are required to know about this
here, is that a change of one whole
number in pH value represents a ten-fold
change in acidity.
A solution with a pH
of 3 is 10 times more
acidic than a solution
with a pH of 4
10 × more
acidic
A solution
with a pH of
2 is 10 times
more acidic
than a
solution with
a pH of 3
10 × more
acidic
How many
times more
acidic is a pH
of 2 than a pH
of 4?
? × more acidic
100 × more acidic
10 × more acidic
10 × more acidic
100 × more acidic
10 × more acidic
10 × more acidic
10
102 × more acidic
101 × more acidic
101 × more acidic
pH
Change in Acidity  10
Where ΔpH means the change in the value of pH
pH
Change in Acidity  10
Where ΔpH means the change in the value of pH
pH
Change in Acidity  10
Where ΔpH means the change in the value of pH
pH
Change in Acidity  10
Where ΔpH means the change in the value of pH
Change in Acidity  10 pH
Where ΔpH means the change in the value of pH
A solution with a pH of 9 is
times (more/less)
acidic than a solution with a pH of 3.
Change in Acidity  10 pH
Where ΔpH means the change in the value of pH
A solution with a pH of 9 is
times (more/less)
acidic than a solution with a pH of 3.
pH  9  3  6
Change in Acidity  10 pH
 10
6
Change in Acidity  10 pH
Where ΔpH means the change in the value of pH
A solution with a pH of 9 is
times (more/less)
acidic than a solution with a pH of 3.
pH  9  3  6
Change in Acidity  10 pH
 10
6
Change in Acidity  10 pH
Where ΔpH means the change in the value of pH
A solution with a pH of 9 is
times (more/less)
acidic than a solution with a pH of 3.
pH  9  3  6
Change in Acidity  10 pH
 10
6
Change in Acidity  10 pH
Where ΔpH means the change in the value of pH
A solution with a pH of 9 is
times (more/less)
acidic than a solution with a pH of 3.
pH  9  3  6
Change in Acidity  10 pH
 10
6
Change in Acidity  10 pH
Where ΔpH means the change in the value of pH
A solution with a pH of 9 is
times (more/less)
acidic than a solution with a pH of 3.
pH  9  3  6
Change in Acidity  10 pH
 10
6
Change in Acidity  10 pH
Where ΔpH means the change in the value of pH
A solution with a pH of 9 is
times (more/less)
acidic than a solution with a pH of 3.
pH  9  3  6
Change in Acidity  10 pH
 10
6
or 1 million
A solution with a pH of 9 is106 times (more/less)
acidic than a solution with a pH of 3.
pH  9  3  6
Change in Acidity  10 pH
 10
6
or 1 million
?
A solution with a pH of 9 is106 times (more/less)
acidic than a solution with a pH of 3.
pH  9  3  6
Change in Acidity  10 pH
 10
6
or 1 million
The higher the pH of a solution is, the
less acidic it is.
A solution with a pH of 9 is106 times (more/less)
acidic than a solution with a pH of 3.
pH  9  3  6
Change in Acidity  10 pH
 10
6
or 1 million
The higher the pH of a solution is, the
more basic, or less acidic it is.
A solution with a pH of 9 is106 times less
acidic than a solution with a pH of 3.
pH  9  3  6
Change in Acidity  10 pH
 10
6
or 1 million
The higher the pH of a solution is, the
more basic, or less acidic it is.
A solution with a pH of 9 is106 times less
acidic than a solution with a pH of 3.
The higher the pH of a solution is, the
more basic, or less acidic it is.
A solution with a pH of 9 is106 times less
acidic than a solution with a pH of 3.
A solution with a pH of 9 is one million times less
acidic than a solution with a pH of 3.
pH
Indicators
A pH indicator is a substance that changes
colour when a certain change in pH
occurs.
A pH indicator is a substance that changes
colour when a certain change in pH
occurs.
For example, litmus is a familiar indicator.
A pH indicator is a substance that changes
colour when a certain change in pH
occurs.
For example, litmus is a familiar indicator.
Paper soaked in litmus is called litmus
paper and it is used to test substances to
see if they are acidic, basic, or neutral.
There are two main types of litmus paper:
There are two main types of litmus paper:
 red litmus paper
There are two main types of litmus paper:
 red litmus paper
 blue litmus paper
There are two main types of litmus paper:
 red litmus paper
 blue litmus paper
Type
red litmus
blue litmus
Add to an Acid
Add to a Base
There are two main types of litmus paper:
 red litmus paper
 blue litmus paper
Type
red litmus
blue litmus
Add to an Acid
Add to a Base
There are two main types of litmus paper:
 red litmus paper
 blue litmus paper
Type
Add to an Acid
red litmus
stays red
blue litmus
Add to a Base
There are two main types of litmus paper:
 red litmus paper
 blue litmus paper
Type
Add to an Acid
red litmus
stays red
blue litmus
Add to a Base
There are two main types of litmus paper:
 red litmus paper
 blue litmus paper
Type
Add to an Acid
Add to a Base
red litmus
stays red
turns blue
blue litmus
There are two main types of litmus paper:
 red litmus paper
 blue litmus paper
Type
Add to an Acid
Add to a Base
red litmus
stays red
turns blue
blue litmus
There are two main types of litmus paper:
 red litmus paper
 blue litmus paper
Type
Add to an Acid
Add to a Base
red litmus
stays red
turns blue
blue litmus
There are two main types of litmus paper:
 red litmus paper
 blue litmus paper
Type
Add to an Acid
Add to a Base
red litmus
stays red
turns blue
blue litmus
turns red
There are two main types of litmus paper:
 red litmus paper
 blue litmus paper
Type
Add to an Acid
Add to a Base
red litmus
stays red
turns blue
blue litmus
turns red
There are two main types of litmus paper:
 red litmus paper
 blue litmus paper
Type
Add to an Acid
Add to a Base
red litmus
stays red
turns blue
blue litmus
turns red
stays blue
There are two main types of litmus paper:
 red litmus paper
 blue litmus paper
Type
red litmus
blue litmus
Add to an Neutral water
There are two main types of litmus paper:
 red litmus paper
 blue litmus paper
Type
red litmus
blue litmus
Add to an Neutral water
There are two main types of litmus paper:
 red litmus paper
 blue litmus paper
Type
Add to an Neutral water
red litmus
stays red
blue litmus
There are two main types of litmus paper:
 red litmus paper
 blue litmus paper
Type
Add to an Neutral water
red litmus
stays red
blue litmus
There are two main types of litmus paper:
 red litmus paper
 blue litmus paper
Type
Add to an Neutral water
red litmus
stays red
blue litmus
There are two main types of litmus paper:
 red litmus paper
 blue litmus paper
Type
Add to an Neutral water
red litmus
stays red
blue litmus
stays blue
There are two main types of litmus paper:
 red litmus paper
 blue litmus paper
Type
Add to an Neutral water
red litmus
stays red
blue litmus
stays blue
Adding either type of litmus paper to
neutral water will not change its colour.
If we have an unknown solution and we want to find out
whether it is acidic, basic, or neutral,
Type
Add to an Acid
Add to a Base
red litmus
stays red
turns blue
blue litmus
turns red
stays blue
Adding either type of litmus paper to
neutral water will not change its colour.
If we have an unknown solution and we want to find out
whether it is acidic, basic, or neutral, we could dip red and
blue litmus paper into it
Type
Add to an Acid
Add to a Base
red litmus
stays red
turns blue
blue litmus
turns red
stays blue
Adding either type of litmus paper to
neutral water will not change its colour.
If we have an unknown solution and we want to find out
whether it is acidic, basic, or neutral, we could dip red and
blue litmus paper into it and from the results and the
information below,
Type
Add to an Acid
Add to a Base
red litmus
stays red
turns blue
blue litmus
turns red
stays blue
Adding either type of litmus paper to
neutral water will not change its colour.
If we have an unknown solution and we want to find out
whether it is acidic, basic, or neutral, we could dip red and
blue litmus paper into it and from the results and the
information below, we could find out whether it is acidic,
basic, or neutral.
Type
Add to an Acid
Add to a Base
red litmus
stays red
turns blue
blue litmus
turns red
stays blue
Adding either type of litmus paper to
neutral water will not change its colour.
For example, you have a solution called Unknown 1 and:
Type
Add to an Acid
Add to a Base
red litmus
stays red
turns blue
blue litmus
turns red
stays blue
Adding either type of litmus paper to
neutral water will not change its colour.
For example, you have a solution called Unknown 1 and:
 Red litmus paper stays red
Type
Add to an Acid
Add to a Base
red litmus
stays red
turns blue
blue litmus
turns red
stays blue
Adding either type of litmus paper to
neutral water will not change its colour.
For example, you have a solution called Unknown 1 and:
 Red litmus paper stays red
 Blue litmus paper stays blue
Type
Add to an Acid
Add to a Base
red litmus
stays red
turns blue
blue litmus
turns red
stays blue
Adding either type of litmus paper to
neutral water will not change its colour.
For example, you have a solution called Unknown 1 and:
 Red litmus paper stays red
 Blue litmus paper stays blue
Then Unknown 1 is
Type
Add to an Acid
Add to a Base
red litmus
stays red
turns blue
blue litmus
turns red
stays blue
Adding either type of litmus paper to
neutral water will not change its colour.
For example, you have a solution called Unknown 1 and:
 Red litmus paper stays red
 Blue litmus paper stays blue
Then Unknown 1 is neutral
Type
Add to an Acid
Add to a Base
red litmus
stays red
turns blue
blue litmus
turns red
stays blue
Adding either type of litmus paper to
neutral water will not change its colour.
For example, you have a solution called Unknown 2 and:
Type
Add to an Acid
Add to a Base
red litmus
stays red
turns blue
blue litmus
turns red
stays blue
Adding either type of litmus paper to
neutral water will not change its colour.
For example, you have a solution called Unknown 2 and:
 Red litmus paper stays red
Type
Add to an Acid
Add to a Base
red litmus
stays red
turns blue
blue litmus
turns red
stays blue
Adding either type of litmus paper to
neutral water will not change its colour.
For example, you have a solution called Unknown 2 and:
 Red litmus paper stays red
 Blue litmus paper turns red
Type
Add to an Acid
Add to a Base
red litmus
stays red
turns blue
blue litmus
turns red
stays blue
Adding either type of litmus paper to
neutral water will not change its colour.
For example, you have a solution called Unknown 2 and:
 Red litmus paper stays red
 Blue litmus paper turns red
Then Unknown 2 is
Type
Add to an Acid
Add to a Base
red litmus
stays red
turns blue
blue litmus
turns red
stays blue
Adding either type of litmus paper to
neutral water will not change its colour.
For example, you have a solution called Unknown 2 and:
 Red litmus paper stays red
 Blue litmus paper turns red
Then Unknown 2 is acidic
Type
Add to an Acid
Add to a Base
red litmus
stays red
turns blue
blue litmus
turns red
stays blue
Adding either type of litmus paper to
neutral water will not change its colour.
Litmus paper is handy for identifying solutions as either
acidic, basic or neutral.
Litmus paper is handy for identifying solutions as either
acidic, basic or neutral.
Litmus paper is handy for identifying solutions as either
acidic, basic or neutral.
Litmus changes
colour at a pH of 7
Litmus paper is handy for identifying solutions as either
acidic, basic or neutral.
If pH is below 7,
litmus is red
Litmus paper is handy for identifying solutions as either
acidic, basic or neutral.
If pH is above 7,
litmus is blue
Different indicators change colour at different pH values.
Indicators change colour gradually, over a range of pHs.
3.2
4.4
Sloped line between pH = 3.2 and 4.4
Indicators change colour gradually, over a range of pHs.
3.2
red
4.4
yellow
Methyl Orange gradually changes from red
to yellow between pH = 3.2 to 4.4
Indicators change colour gradually, over a range of pHs.
3.2
red
4.4
orange
yellow
Methyl Orange gradually changes from red
to yellow between pH = 3.2 to 4.4
Indicators change colour gradually, over a range of pHs.
3.2
red
4.4
orange
yellow
Methyl Orange gradually changes from red
to yellow between pH = 3.2 to 4.4
Indicators change colour gradually, over a range of pHs.
3.2
red
4.4
orange
yellow
Methyl Orange gradually changes from red
to yellow between pH = 3.2 to 4.4
Indicators change colour gradually, over a range of pHs.
3.2
red
4.4
orange
yellow
Methyl Orange gradually changes from red
to yellow between pH = 3.2 to 4.4
Methyl Red changes from red to yellow from
pH 4.8 to 6.0
Methyl Red changes from red
to yellow from pH 4.8 to 6.0
Bromthymol Blue changes from yellow to blue
from pH 6.0 to 7.6
Bromthymol Blue changes from
yellow to blue from pH 6.0 to 7.6
Using this table, we can see what colours
different indicators are at a given pH.
Colours of Various
Indicators at a pH of 2
At pH =2:
 Methyl Orange is red
 Methyl Red is red
 Bromthymol Blue is yellow
 Litmus is red
 Phenolphthalein is colourless
 Indigo carmine is blue
Colours of Various
Indicators at a pH of 2
At pH =2:
 Methyl Orange is red
 Methyl Red is red
 Bromthymol Blue is yellow
 Litmus is red
 Phenolphthalein is colourless
 Indigo carmine is blue
Colours of Various
Indicators at a pH of 2
At pH =2:
 Methyl Orange is red
 Methyl Red is red
 Bromthymol Blue is yellow
 Litmus is red
 Phenolphthalein is colourless
 Indigo carmine is blue
Colours of Various
Indicators at a pH of 2
At pH =2:
 Methyl Orange is red
 Methyl Red is red
 Bromthymol Blue is yellow
 Litmus is red
 Phenolphthalein is colourless
 Indigo carmine is blue
Colours of Various
Indicators at a pH of 2
At pH =2:
 Methyl Orange is red
 Methyl Red is red
 Bromthymol Blue is yellow
 Litmus is red
 Phenolphthalein is colourless
 Indigo carmine is blue
Colours of Various
Indicators at a pH of 2
Colours of Various Indicators at
a pH of 11
Colours of Various Indicators at
a pH of 11
At pH =11:
 Methyl Orange is yellow
 Methyl Red is yellow
 Bromthymol Blue is blue
 Litmus is blue
 Phenolphthalein is pink
 Indigo carmine is blue
Colours of Various Indicators at
a pH of 11
At pH =11:
 Methyl Orange is yellow
 Methyl Red is yellow
 Bromthymol Blue is blue
 Litmus is blue
 Phenolphthalein is pink
 Indigo carmine is blue
Colours of Various Indicators at
a pH of 11
At pH =11:
 Methyl Orange is yellow
 Methyl Red is yellow
 Bromthymol Blue is blue
 Litmus is blue
 Phenolphthalein is pink
 Indigo carmine is blue
Colours of Various Indicators at
a pH of 11
At pH =11:
 Methyl Orange is yellow
 Methyl Red is yellow
 Bromthymol Blue is blue
 Litmus is blue
 Phenolphthalein is pink
 Indigo carmine is blue
Question: A solution is yellow in bromthymol blue,
yellow in methyl red, and blue in indigo carmine. The
pH is: a) 4.0 b) 6.0 c) 7.0 d) 8.0
Question: A solution is yellow in bromthymol blue,
yellow in methyl red, and blue in indigo carmine. The
pH is: a) 4.0 b) 6.0 c) 7.0 d) 8.0
Question: A solution is yellow in bromthymol blue,
yellow in methyl red, and blue in indigo carmine. The
pH is: a) 4.0 b) 6.0 c) 7.0 d) 8.0
Question: A solution is yellow in bromthymol blue,
yellow in methyl red, and blue in indigo carmine. The
pH is: a) 4.0 b) 6.0 c) 7.0 d) 8.0
Question: A solution is yellow in bromthymol blue,
yellow in methyl red, and blue in indigo carmine. The
pH is: a) 4.0 b) 6.0 c) 7.0 d) 8.0
Question: A solution is yellow in bromthymol blue,
yellow in methyl red, and blue in indigo carmine. The
pH is: a) 4.0 b) 6.0 c) 7.0 d) 8.0
Question: A solution is yellow in bromthymol blue,
yellow in methyl red, and blue in indigo carmine. The
pH is: a) 4.0 b) 6.0 c) 7.0 d) 8.0
Question: A solution is yellow in bromthymol blue,
yellow in methyl red, and blue in indigo carmine. The
pH is: a) 4.0 b) 6.0 c) 7.0 d) 8.0
Question: A solution is yellow in bromthymol blue,
yellow in methyl red, and blue in indigo carmine. The
pH is: a) 4.0 b) 6.0 c) 7.0 d) 8.0
Question: A solution is yellow in bromthymol blue,
yellow in methyl red, and blue in indigo carmine. The
pH is: a) 4.0 b) 6.0 c) 7.0 d) 8.0
Question: A solution is yellow in bromthymol blue,
yellow in methyl red, and blue in indigo carmine. The
pH is: a) 4.0 b) 6.0 c) 7.0 d) 8.0
Question: A solution is yellow in methyl orange, pink
in phenolphthalein, and green in indigo carmine. The
pH is: a) 5.0 b) 6.5 c) 12.5 d) 14.0
Question: A solution is yellow in methyl orange, pink
in phenolphthalein, and green in indigo carmine. The
pH is: a) 5.0 b) 6.5 c) 12.5 d) 14.0
Question: A solution is yellow in methyl orange, pink
in phenolphthalein, and green in indigo carmine. The
pH is: a) 5.0 b) 6.5 c) 12.5 d) 14.0
Question: A solution is yellow in methyl orange, pink
in phenolphthalein, and green in indigo carmine. The
pH is: a) 5.0 b) 6.5 c) 12.5 d) 14.0
Question: A solution is yellow in methyl orange, pink
in phenolphthalein, and green in indigo carmine. The
pH is: a) 5.0 b) 6.5 c) 12.5 d) 14.0
Question: A solution is yellow in methyl orange, pink
in phenolphthalein, and green in indigo carmine. The
pH is: a) 5.0 b) 6.5 c) 12.5 d) 14.0