1. Natural Sciences Grade 7
Term 2: Matter and Materials
Acids, bases and neutrals
(PPT 3)

2. Topic 3 Acids, bases and neutrals
Acid-base indicators
Natural Sciences - Grade 7

3. Using acid-base indicators [1]
Indicators are substances used in laboratories to test if a substance is a base or an acid.
Dyes in the indicators change colour as soon as it mixes with bases or acids.
Testing a substance:
Must be in liquid form.
Solid substance must first be dissolved in water to make a solution.
Use red and blue litmus for testing.
Use a new piece of litmus paper for each test
Litmus is an indicator, extracted from lichens [type of fungus].
Litmus has a purple [mauve] colour in distilled water.
Red and blue litmus paper must be used when a substance is tested.
Images of blue and red litmus paper:
Red litmus paper stays red when it comes into contact with an acid or a neutral substance.
Acids will turn blue litmus paper red if the substance is tested.
Natural Sciences - Grade 7

4. Using acid-base indicators [2]
How litmus paper reacts to acids, bases and neutrals:
Acid:
Blue litmus paper – turns red
Red litmus paper – stays red
Base:
Blue litmus paper - stays blue
Red litmus paper - turns blue
Neutral:
Red litmus paper - stays red
Images of blue and red litmus paper:
Natural Sciences - Grade 7

5. Using acid-base indicators [3]
An acid-base indicator is a weak acid.
Most detergents and soaps turn red litmus blue, indicating that they are bases.
Certain detergents and soaps may test as acids or neutrals (e.g. lemon juice added to some dishwashing liquids; citrus fruit extracts added to some shampoos; etc.).
Natural Sciences - Grade 7

6. Using acid-base indicators
The flowers of the Hydrangea macrophylla (Christmas flower) changes colour, depending on the soil pH value.
Hydrangea macrophylla [Christmas flowers]
Chemical reactions occur in acid soil that make aluminium available to these plants and its flowers turn blue.
There is no reactions in alkaline soils, therefore the flowers remain pink.
It is a deciduous shrub that grows 2 m tall and 2.5 m broad with large heads of pink or blue flowers in summer and autumn.
The blooms can be red, pink, blue, dark purple or light purple.
The pigment colour of the flower is affected by the pH of the soil.
Acidic soil (pH below 7) usually produce a flower colour that is closer to blue.
Alkaline soil (pH above 7) produce pink flowers.
Recent tests of the leaves reveal that it can be used as a possible source of:
- a new chemical compounds with antimalarial activity.
- a possible anti-diabetic drug as it significantly lowered blood glucose, triglyceride, and free fatty acid levels in laboratory animals.
Natural Sciences - Grade 7