4CH1

Elements, Compounds and Mixtures

Principles of Chemistry · 2 question types

Practise

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 5% of your exam marks.

stable
Rare
Stable5%

Definitions and separation techniques tested consistently across both papers.

  • A chromatogram is read by comparing the spots of an unknown sample against spots from known reference substances
  • → a single spot in its lane
  • Impure substance → two or more spots in the same lane
  • Two samples of the same substance → produce identical chromatograms (same number of spots at the same heights)
  • A separates on the paper, with each of its constituent substances showing up as a distinct spot in the lane
  • Standard practice is to run known reference samples alongside the unknown in adjacent lanes
    • The references give you fixed positions to compare against
    • Any spot in the unknown lane that sits at the same height as a reference spot is the same substance
A developed chromatogram: brown ink separates into three spots (red, blue, yellow) that line up with the pure red, blue and yellow reference substances, all measured from the pencil line
Source: Interpreting chromatograms by Save My Exams

Rf values

  • Rf (retention factor) is used to identify a component quantitatively
  • Defined by:

Rf=distance moved by the substancedistance moved by the solventR_f = \frac{\text{distance moved by the substance}}{\text{distance moved by the solvent}}

  • Distance moved by the substance is measured from the baseline up to the centre of the spot
  • Distance moved by the solvent is measured from the baseline up to the front
  • Properties:
    • Has no units (it's a ratio)
    • Always less than 1 (a spot can't travel further than the solvent that carries it)
    • Constant for a given solute, solvent and paper at a given temperature
    • Changes if you change the solvent or the paper
Worked example

Calculating an Rf value from a chromatogram

A dye spot travels 33 mm from the baseline. In the same time the solvent front travels 55 mm from the baseline. Find the Rf value.

Solution:

  • Rf = distance moved by substance ÷ distance moved by solvent
  • Rf = 33 ÷ 55
  • Rf = 0.60 (Rf has no units — it is a ratio, and is always less than 1)
Exam tip

Spotting an impossible Rf value

What comes up: A table of Rf values from two students is shown and you are asked to identify which value cannot be correct and explain why (2 marks).

Write (two marks): (1) Identify the value that is greater than 1. (2) An Rf value must always be less than 1, because a spot cannot travel further up the paper than the solvent that carries it.

Watch out: The mark scheme also accepts "the spot cannot move further than the solvent front" as the reason. Simply saying "it looks wrong" or "the number is too big" scores zero for M2 — you must state why it is physically impossible.