Science Form 2

1. Chapter 6 : Acid &
Alkali
6.1 Properties of Acid and Alkali

2. Learning Outcomes :
• Define operationally acid and alkali.
• Explain with examples acidic and alkaline substances.
• Demonstrate the technique to determine the strength of
acid and alkali based on pH value.
• Identify the uses of acid and alkali in daily life.

3. • The word ‘acid’ comes from the Latin word, acidus
which means sour
• The word ‘alkali’ comes from the Arabic word, alqali
which means ashes of plants.

4. Properties of Acid & Alkali
• pH
• Taste
• Corrosiveness
• Effect on blue litmus paper and red litmus paper
• Reaction with metals

5. pH value
Dilute sodium
hydroxide

7. pH value
Dilute sodium
hydroxide
purple
red

8. pH value
Dilute sodium
hydroxide
purple
red
pH = 1 pH = 14

9. pH 1 – 6 pH 8 - 14

10. Taste
• Taste lime juice followed by bitter gourd juice. Gargle
with water after tasting each substance
• Lime juice : sour (acid)
• Bitter gourd juice : bitter (alkali)

11. Corrosiveness
Sodium
hydroxide
Corrode the paper

12. Effect on blue litmus paper and red litmus
paper
• Place a blue litmus paper and a red litmus paper on a white tile.
• Put one drop of dilute hydrochloric acid on both litmus papers and record
your observations
Substance Blue litmus paper Red litmus paper
Hydrochloric acid Changes from blue to red Remains red
Sodium hydroxide Remains blue Changes from red to blue

13. Reaction with metals
• Clean a magnesium ribbon with sandpaper.
• Put the magnesium ribbon into a test tube filled with 5 ml of dilute
hydrochloric acid.
• Close the test tube with your thumb for one minute.
• Remove your thumb and place a lighted wooden splinter at the mouth of the
test tube

14. Hydrochloric acid : burns with ‘pop’ sound
Sodium hydroxide : no reaction

15. pH value
Taste
corrosiveness
Effect on blue and red
litmus paper
Reaction with metal

17. Operational definition of an acid and an alkali
• Acid is a substance that has pH value between 1 – 6, sour
taste, corrosive, changes blue litmus paper to red and
reacts with metal to produce hydrogen gas
• Alkali is a substance that has pH value between 8 – 14,
bitter taste, corrosive, changes red litmus paper to blue
and no reaction with metal.

18. The Role of Water
in Showing the
Properties of
Acids and Alkalis
You should remember that
acids and alkalis show their
properties only in the
presence of water

20. Acidic and Alkaline Substances
• Substances that contain acids are known as acidic
substances
• Substances that contain alkali are known as
alkaline substances

22. Indicator
• The characteristic of different substances can be
determined using a suitable indicator.
• An indicator is a colouring or a mixture of different
colourings that changes colour based on the substance
tested. The colour change observed can be used to
determine if a substance is neutral, acidic or alkaline.

24. Strength of Acids and Alkalis
• The pH scale is used to show the strength of acids or
alkalis. The range of pH value is between 0 to 14.

25. Uses of Acids and Alkalis in Daily Life
• For instance, we use vinegar which is acidic in cooking,
and detergent which is alkaline for washing clothes.
• Besides, acids and alkalis are also used in various sectors
such as the agricultural, industrial and medical sectors
• For example, fertilisers used in agriculture are produced
from the reaction between acidic and alkaline substances.

28. Homework
•Bring fizzy drinks, dishwashing liquid,
syrup and salt water