06. Indicators of acids and bases

In pure water, the concentration of hydrogen and hydroxide ions is very low. Water is amphiprotic, its ions able to act as either an acid or base, and it can self-ionise:

H₂O(l) + H₂O(l) → H₃O⁺(aq) + OH^-(aq)

This equation shows the ratio of these ions is equal and therefore pure water is considered neutral. Various substances, including the pigments found in many plants and fungi, respond to the presence of hydrogen ions by changing their shape and structure and this can affect their colour. These 'indicator' molecules provide a useful visual sign of a solution's acidity or alkalinity.

The colour range of 'universal indicator' superimposed with examples of household products of a range of pH values. Note that the pH scale has no units; the pH is a measure of the concentration of hydrogen ions in the solution.

ACTIVITY: Observing indicators of acids and bases

ATL

• Organisation skills: Keep organised and logical system of information files/notebooks; understand and use sensory learning preferences.

Safety: Avoid contact with skin. Solvents used for indicators may be flammable.

Materials and equipment

• • examples of several acid-base indicators, as solutions in dropper bottles or as indicator papers (e.g. red and blue litmus papers, phenolphthalein, methyl orange, bromothymol blue, and universal indicator solutions; natural examples such as brewed 'black' tea, juices from crushed plants, e.g. red cabbage leaves, grapes and petals from various flowers)

  • one or more spotting tiles

  • rinse bottle with water

  • beaker for waste

  • a dropper bottle of an acidic solution, e.g. 0.1 M HCl

  • a dropper bottle of a basic solution, e.g. 0.1 M NaOH

Method

• 1. Design a table to enable you to record the results of systematic tests with each type of indicator and a few drops of HCl, water or NaOH.

  2. Develop the habit of using small quantities of solutions to avoid waste. A single drop of indicator solution, or a small piece of indicator test paper should be sufficient to determine any colour change of up to 10 drops of any test solution.

  3. After testing a set of solutions, rinse the waste into the beaker. Later, it can be washed down the sink.

  4. What happens to the colour of an indicator in a basic solution as you add up to 10 drops of the acid solution, drop by drop: or vice versa? What is happening?

Analysing results

• 1. State general trends in colours of indicators in response to acids and bases.

  2. List properties you value most in an indicator.

  3. Suggest circumstances in which some of the indicators you have used may have an advantage.

  4. State what happened to the colour of an indicator as you added acids to a basic solution, or vice versa.

Assessment opportunities

• In this activity you have practiced the skills that are assessed using Criterion A: Knowledge and understanding.

“PH Scale.” EdPlace, www.edplace.com/worksheet_info/science/keystage3/year7/topic/747/2589/ph-scale.