This document discusses acids and alkalis and how they can be detected and measured. It explains that Robert Boyle discovered plant juices like red cabbage juice and litmus could detect acids and alkalis through color changes. The juice would turn red in acids and green in alkalis. This led to the development of pH indicators that scientists now use to measure the strength of acids and alkalis on the pH scale, from 0-14, invented by Soren Sorensen. Common indicators and their color changes in acids and alkalis are also described.

1. Acids and Alkalis

2. Alkalis
• Examples: Sodium hydroxide (NaOH)and potassium
hydroxide(KOH).
• Calcium hydroxide, also called :Slaked lime.
(Used in many industries to make products as: Bleach
and white wash)
-A weak solution (called: limewater)
can be used in lab to test carbon dioxide.

3. Detecting acids and alkalis
- Robert Boyle was an Irish scientist who lived 300 years
ago.
- He decided to try and find and easy way to identify acids
and alkalis.
- He knew that workers in France used plant juices to dye
silk clothes.
- So he began testing plant juices to see if they would solve
his problem.

4. - When he used red cabbage juice he found that:
Red cabbage juice
(purple)
+ Acid Juice turns red
Red cabbage juice
(purple)
+ Alkali Juice turns green

5. - When he used violets he found that:
+ Acid Juice turns Purple
+ Alkali Juice turns
greenish yellow

6. - He discovered the colour change in litmus
(juice from lichen)
- Litmus was used in laboratories around
the world.
- It can be used as a solution of absorbed
into paper strips.

8. - There are over 20 indicators that
scientists use.
- 2 examples:
- Methyl orange:
Turns pink in acids.
Turns Yellow in Alkalis.
- Phenolphthalein:
Turns colourless in acid solutions.
Turns Pink in alkaline solutions.
Acid Alkali
AlkaliAcid

9. Hydrangea plant:
• The colour of the flower
indicates the alkalinity of the
soil.
• If the soil contain lime water
the flower will be pink and if
the soil is free of any alkali the
flower will be blue.

10. pH scale
- After indicators where found to identify acids and alkalis,
scientists wanted compare the strength of the acids and
alkalis.
- In 1909, a Danish scientist called Soren Sorensen
invented a scale called pH scale to compare the strengths.
- pH stand for ‘power of hydrogen’ because all acids
hydrogen (active part in the chemical reactions)
- The pH scale starts from 0 – 14.
- The strongest acid is 0 and the strongest alkali is 14.

11. pH Classification
0 - 2 Strong acid
3 - 6 Weak acid
7 Neutral
8 - 11 Weak alkali
12 - 14 Strong alkali
- N.B. An electrical instrument known as pH meter is used to measure pH
accurately.

12. Universal indicator
- The pH indicator is made of a mixture of indicators.
- Each indicator changes colour in the range shown in the scale.
- By combining the indicators a solution is made that gives
different colours over the whole range of the scale