Learning Goals:

To understand how a pH indicator works

To know a variety of pH indicators

INDICATORS

CABBAGE INDICATOR

H+

Cl

Cl

H+

H+

Cl

Hydrochloric Acid

All acids contain (H+) in solution.

WHAT IS pH?

pH is a fancy measure of the number of H+ ions in solution (but backward)

What will sort of number will an acid with lots of H+ give us?

H+ SO4

SO4H+

H+

SO4

Sulphuric Acid

All acids contain (H+) in solution.

WHAT IS THE pH?

Is the pH of this acid higher than HCl?

H+

H+

H+

More H+ ions means a stronger acid?

CABBAGE INDICATOR

H+

Cl

Cl

H+

H+

Cl

Hydrochloric Acid

Indicators detect H+ ions

HOW DOES THE INDICATOR WORK?

H+ ions change the shape of the indicator molecule

A different shaped molecule means a different colour

H+

Cl

Cl

H+

H+

Cl

WHAT IF WE ADD MORE WATER?

Has the number of H+ ions changed?

Has the pH changed?

DILUTE

H+

Cl

Cl

H+

H+

Cl

WHAT IF WE HAVE LESS WATER?

Has the number of H+ ions changed

Has the pH changed?

CONCENTRATED

Litmus

Red in acid

Blue in alkali

Phenolphthalein

Colourless in acid

Pink/Red in alkali

Bromothymol Blue

Yellow in acid

Blue in alkali

Methyl Orange

Red in acid

Orange in alkali

Explain what the following litmus test tells you about the solution in the test tube

?

alkali

LITMUS

What does the following litmus test tells you about the solution in the test tube

?

acid

LITMUS

Explain what the following litmus test tells you about the solution in the test tube

?

acid

LITMUS

What does the following litmus test tells you about the solution in the test tube

?

alkali

LITMUS

Textbook:

Read p220

P222 Q6

INDICATORS