Two colourless substances give the same Rf R_f\,Rf value in one solvent. What is the safest conclusion?
They are probably the same, so no further tests are needed
They may be different, because colourless substances have no Rf value
They may be the same, but a test in another solvent is needed
They are probably different, because the spots cannot be seen
107 exam-style questions on Edexcel GCSE Chemistry 3.2 Methods of separating and purifying substances, covering 3.2.1 Pure substances and mixtures, 3.2.2 Techniques for separating mixtures, 3.2.3 Paper chromatography and Rf values, and 3.2.4 Making water potable. Each one has a worked solution and a mark scheme showing where the marks go.