TAE: Indicators

Question 1

What are some properties of acids?

Answer 1

Sour taste; corrosive

In solution, acids conduct
electricity.

Question 2

What are some properties of bases?

Answer 2

Bitter taste; soapy feel

In solution, alkalis are good
conductors of electricity.

Mainly insoluble in water
(aqueous bases are alkalis)

Question 3

How do indicators work?

Answer 3

An indicator is a solution containing a weak acid in equilibrium with its conjugate base:

It has one colour in acidic form and colour 2 in its conjugate base form.

The colour will change depending on the concentration of H+ ions.

Question 4

Litmus paper

Color in acid, base and pH range?

Answer 4

Acid: Red
Base: Blue

pH range: 5.0 - 8.0

Question 5

Phenolphtalein

Color in acid, base and pH range?

Answer 5

Acid: Colourless
Base: Pink

pH range: 8.3-10.0 (basic)

Question 6

Methyl orange

Color in acid, base and pH range?

Answer 6

Acid: red
Base: yellow

pH range: 3.1-4.4 (acidic)

Question 7

Bromothymol Blue

Color in acid, base and pH range?

Answer 7

Acid: yellow
Base: blue

pH range: 6.0-7.6 (neutral)

Question 8

Identify and describe some everyday uses of indicators including the testing of soil acidity/basicity

Answer 8

Testing of soils:
–Soil acidity is tested to ensure that the
plants intended for the area have suitable in which to live.
– Measured using electronic instruments or universal indicator.

– If using indicator, the soil can hide any colour change, so neutral, white barium sulfate is
added to damp soil then indicator is added so that any colour change is visible.

Testing pH of swimming pools:
– Ideal is near neutral (close to 7.4) to prevent skin irritation.

• The presence of ions makes the water basic, so is added to return to neutrality. This must be done to protect the eyes and throats of swimmers from becoming irritated.

A pool test kit is used to test samples of the pool water, or paper soaked in indicator can
be used.