Skip to notes
wise otter.Chemistry
Contents
K223 / K225 / 2027
Data and Experimental Skills

Lesson 6 of 7

Reason from qualitative tests

Keep the observation separate from the inferred identity.

SEC G2 Science K223 / K225 (2027)

  • I can take readings and record observations.Syllabus

    Syllabus K223 / K225, C2. Take readings and record observations

  • I can interpret and evaluate data and observations.Syllabus

    Syllabus K223 / K225, C3. Interpret and evaluate data and observations

Reason from an unknown test without guessing

A valid inference needs the right reagent, a clear observation and an appropriate comparison.

Use clean apparatus and a fresh small portion for each test. Add reagent gradually with mixing; observe a few drops, excess reagent and warming separately when instructed. Write the actual colour and physical change. A blue solution and a pale-blue precipitate are different observations. Name a gas only after the appropriate confirmatory test.

For a simple organic test, record the reagent and what changes. Bromine water changing from orange to colourless is consistent with an alkene under the relevant conditions, but decolourisation alone is not a universal proof of C=C because other substances can also react. Use the stated candidate structures and any other evidence to narrow the conclusion.

Check your understanding

A precipitate dissolves when excess reagent is added, leaving a clear coloured liquid. How should this be recorded?

  1. Give the precipitate colour, that it dissolves in excess, and the solution colour.
  2. Write "no precipitate", since none is left once the excess reagent is in.
  3. Write "the colour disappears", since the solid is no longer seen in the tube.
Show the answer

Give the precipitate colour, that it dissolves in excess, and the solution colour.

Record each stage: the precipitate and its colour, that it dissolves in excess, and the colour of the final solution. Saying the colour disappeared is wrong if the solution is still coloured.

In G2 the gas tests you must know are hydrogen (lighted splint, squeaky pop), oxygen (glowing splint relights) and carbon dioxide (limewater turns milky). A question may describe another test and ask you to record or interpret what you would see: describe the change first, then state what it suggests.