The pH scale
1
Universal indicator and pH paper can be used to identify solutions as being acid or alkali.
They can also be used to measure just how much acid or alkali is in a solution.
The scale that is used is called the pH scale . This is a continuous scale that runs from
below 0 (acid) to above 14 (alkali) with water and neutral solutions having a pH of 7. Acidity
and alkalinity increase as the pH moves away from 7, i.e. the lower the pH of a solution
(below 7), the greater the acidity and the higher the pH of a solution (above 7), the greater
the alkalinity.
The indicator turns a different colour at each pH number and so the pH of acids and alkalis
can be found by colour-matching with a chart.
Red
Orange / Yellow
Green
Green/blue
Blue
What is the effect of citric acid powder on dry pH paper?
Some compounds called acids or alkalis do not behave as acids and alkalis unless water
is present
they only show on the pH scale when in solution, e.g. dry solids and gases do
not change the colour of dry pH paper.
Universal indicator and pH paper can only measure the pH to the nearest whole number. A
pH meter can be used to give a more accurate measurement of acidity and alkalinity.
Chemical Changes and Structure
Acids and Alkalis
The pH scale
2
Describe how to measure the pH of an aqueous solution.
What kind of solutions have a pH of … less than 7?... equal to 7 … greater than 7?
Complete the table to show whether each of the following statements is TRUE or FALSE
True or False
An acid can have a pH of 3.7
An alkali can have a pH of 14.2
An acid can have a pH of 0.
Solutions with a pH between 6 and 8 are neutral.
An acid can have a pH of -1.
A solution with a pH of 4 is less acidic than a
solution with a pH of 6.
A solution with a pH of 11 is less alkaline than a
solution with a pH of 9.
Hydrogen chloride gas turns Universal indicator red.
Why does hydrogen chloride gas have no effect on dry pH paper?
Chemical Changes and Structure
Acids and Alkalis