A student is separating the photosynthetic pigments from a spinach leaf extract using thin-layer chromatography (TLC). After spotting the extract on the silica gel plate, they place it inside a beaker containing a mobile phase of petroleum ether and acetone. The student ensures that the beaker is sealed with a watch glass on top.
Explain why placing a lid on the beaker is necessary for the chromatography to run successfully.
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.