Lower Secondary School Chemistry- Indicators

2. After this lesson, you should be able to:
Recognise
the pH scale
Understand
what indicators are

3. PH SCALE

To measure the acidity or alkalinity of a substance

It is a range of values between pH 0 to pH 14.
Acids
pH < 7
Neutral
Alkalis
pH > 7
0
High acidity
High alkalinity

4. 0

5. HOW TO FIND OUT PH VALUES OF
SOLUTIONS?

pH papers
pH meter
-Electronic device to measure the acidity or
alkalinity of solutions accurately

Datalogger
- Comes with a pH sensor which will measure the
change in pH value of solutions over time.
- Connected to the computer to transfer the data
collected.


6. RECAP…

Acids turn blue litmus paper red

Alkalis turn red litmus paper blue

7. WHAT IS AN INDICATOR?

A substance which changes colour depending on
whether the solution tested is acidic or alkaline

They are usually solutions

Can come from natural sources or synthetic
sources
Red Cabbage
Phenolphthalein

8. COMMON INDICATORS
Litmus
paper
Universal
Methyl
Indicator
orange
Phenolphthalein

9. pH 0 to pH 3: red
pH 4 to pH6: orange/yellow
pH 7: green
pH 8 to pH 10: dark green/blue
pH 11 to pH 14: purple

10. COLOUR CHANGES OF OTHER
INDICATORS
Methyl Orange
pH 0 to pH 3: Orange to red
pH 4 to pH 14: Orange to Yellow

Phenolphthalein
pH 1 to pH 9: Remain Colourless
pH 10 to pH 14: Colourless to Pink


11. QUESTIONS!
What is the function of a litmus paper?
To test if a solution is an acid or alkaline.

When would it be more suitable to use the
Universal Indicator pH paper?
When you need to find out the exact pH of a
solution.
