Exercises: Indicators and the pH scale

The pH scale runs from 0 to 14. Copy and complete the table by stating whether each substance is acidic, neutral or basic.

In a school laboratory in Galway, a student adds a few drops of litmus solution to a sample of vinegar.

1. a) What colour does litmus turn in an acid?

1. b) What colour does litmus turn in a base?

1. c) What is an indicator?

Name one acid and one base that are found in the home or in a kitchen. For each one, state where it is used and give an approximate pH value (above or below 7).

A student from Cork tests four unknown liquids with universal indicator and records these colours:

• Liquid P: red

• Liquid Q: green

• Liquid R: blue

• Liquid S: orange

1. a) Put the liquids in order from most acidic to most basic.

1. b) Which liquid is most likely to be pure water?

1. c) Suggest a pH value for liquid R.

A farmer in County Kerry finds that the soil in a field has a pH of 5. Grass grows best at a pH close to 6.5. The farmer spreads lime (a base) on the field.

1. a) Is the soil acidic, neutral or basic before the lime is added?

1. b) Explain what you expect to happen to the pH of the soil after the lime is added.

1. c) Why is it not a good idea to add so much lime that the pH rises to 12?

Indigestion is caused by too much acid in the stomach. An indigestion tablet contains a base.

1. a) What type of reaction takes place between the acid in the stomach and the base in the tablet?

1. b) The stomach contents have a pH of 1.5. What pH would you expect after a suitable tablet has been taken? Explain your answer.

1. c) Explain why a student should not use a very strong base such as sodium hydroxide for this purpose.

A student claims: "A solution of pH 3 is just a little more acidic than a solution of pH 4." The pH scale is logarithmic, so each step of one pH unit represents a tenfold change in acidity.

1. a) How many times more acidic is a solution of pH 3 than a solution of pH 5? Show your reasoning.

1. b) Evaluate the student's claim.

1. c) A river in Wicklow has a pH of 6 after a spill, compared with a normal pH of 8. How many times more acidic is it than normal?

Red cabbage juice is purple in neutral solutions. It turns pink or red in acids and green or yellow in bases. A student tests three colourless liquids X, Y and Z with red cabbage juice. X turns pink, Y stays purple and Z turns green.

1. a) Classify each liquid as acidic, neutral or basic.

1. b) Mixing equal amounts of X and Z gives a purple mixture. Explain what has happened.

1. c) Explain why red cabbage juice would be a less precise indicator than universal indicator for finding the exact pH of a liquid.

A water treatment plant in Dublin must keep the pH of drinking water between 6.5 and 9.5. Four samples were measured: A pH 5.8, B pH 7.2, C pH 9.0, D pH 10.1.

1. a) State which samples meet the standard.

1. b) For each sample that does not meet the standard, state whether acid or base should be added to correct it.

1. c) Explain why a plant would use a pH meter rather than litmus paper to monitor the water.