Acids and alkali

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