Residue – IGCSE Chemistry Definition

The term “residue” appears frequently in IGCSE practical questions and in salt preparation descriptions. Getting the terminology right — residue vs filtrate — is essential for clean communication in Paper 4 and Paper 6 answers.

The 0620 definition

The residue is the insoluble solid that remains on the filter paper after filtration.

Residue in context

During filtration, a mixture is poured through filter paper in a funnel. Two things are separated:

Component	What it is	Where it goes
Residue	Insoluble solid	Stays on the filter paper
Filtrate	Liquid (solution)	Passes through into the flask below

Examples in IGCSE Chemistry

Salt preparation (excess base method)

When excess CuO is added to dilute H₂SO₄:

• Residue = unreacted CuO (black solid on the filter paper)
• Filtrate = CuSO₄ solution (blue liquid) → crystallise to get salt

Precipitate collection

When Pb(NO₃)₂(aq) is mixed with NaCl(aq):

• Residue = PbCl₂ (white precipitate on the filter paper)
• Filtrate = NaNO₃ solution → discarded

Purifying a sample

When impure solid is dissolved in water and insoluble impurities are present:

• Residue = insoluble impurities (removed and discarded)
• Filtrate = solution of the desired substance → crystallise

Worked exam question

A student adds excess zinc to copper(II) sulfate solution, then filters the mixture. (a) Name the residue. [1] (b) State the colour of the residue. [1] (c) Name the filtrate. [1]

Mark scheme

(a) Copper (and excess zinc) [1]

(b) Brown / red-brown (copper) with grey (zinc) [1]

(c) Zinc sulfate solution [1] (colourless)

Common exam mistakes

• Confusing residue with filtrate — remember: the residue remains on the paper (both start with “re”).
• Calling the residue “the filter paper” — the residue is the solid on the paper, not the paper itself.
• Forgetting to wash the residue when preparing a pure product — soluble impurities must be washed away.