Classifying materials

This is an example of a common experiment used to classify material which should help you to understand how to work scientifically.

Aim of the experiment

To classify some mystery substances as acids or bases.

Method

  1. If the substance is a liquid, add three drops of universal indicator and compare the colour to a colour chart.
  2. If the substance is a solid, try to dissolve it in some water and then test its pH.
  3. Test to see what effect the substance has on the pH of an acid or alkaline solution to see if it neutralises it.
A coloured pH scale, ranging from dark red at pH0 and green at pH7, to dark purple at pH14.Colour chart of universal indicator colours at different pH values

Risks

Acids and alkalis are irritants. If they get onto your skin, rinse them off.

Typical results

SubstanceLiquidpH of liquidSolidDissolvepH of solutionAcid effectAlkaline effect
AYes7No changeNo change
BNoYesYes3No changeNeutralises alkali
CNoYesNoNeutralises acidNo change
DYes2No changeNeutralises alkali
ENoYesYes12Neutralises acidNo change
FYes13Neutralises acidNo change

What the results mean

Substance A is neutral and has no effect on acids or alkalis. B is a soluble acid, so it could be a non-metal oxide. C is an insoluble base because it neutralises an acid, so it might be a metal-oxide. D is an acid solution. E is a soluble base, so it dissolves to make an alkali. F is also an alkali.

Evaluation

  • Your measurements are accurate if they are close to their true value.
  • Your measurements are precise if they are similar when completed again.
  • Your experiment is repeatable if you get precise measurements when it is repeated.
  • Your experiment is reproducible if others get precise measurements when they repeat it.