Topic 6 of 6
Predict and identify
Combine independent physical and chemical clues.
A-Level 9476 (2026-2027)
Combine the clues before assigning an element
An inference is strongest when configuration, physical properties and reactions agree.
Worked example
Identify a period-3 element
A period-3 element conducts electricity. Its hydroxide precipitate dissolves in both acid and excess aqueous NaOH. Its chloride gives an acidic solution. Which element is most consistent?
- Electrical conduction favours one of the metals, rather than a molecular non-metal.
- Amphoteric hydroxide behaviour identifies aluminium among the period-3 metals.
- Acidity from hydrolysis of hydrated Al3+ is consistent with the chloride clue.
Aluminium. The given period matters: amphoteric behaviour alone would not uniquely identify an element across the entire table.
Worked example
Predict a Group-2 comparison
How should strontium compare with magnesium in ionic radius, first ionisation energy and carbonate thermal stability?
- Sr2+ has more occupied shells, so its ionic radius is larger.
- Greater shielding and distance generally lower the first ionisation energy of Sr.
- The larger Sr2+ ion has lower charge density and polarises carbonate less strongly.
Larger ion, lower first ionisation energy, more thermally stable carbonate. Each prediction uses the mechanism appropriate to that property.
When practical evidence is supplied, distinguish observation from interpretation. "A white precipitate dissolves in excess alkali" is an observation; "the hydroxide is amphoteric" is an interpretation. Use the stated temperature, physical state and reagent excess. If two identities fit, name the additional evidence needed instead of forcing a unique answer.