A student purifies a sample of benzoic acid (literature melting point 122.4 ∘C122.4\ ^\circ\text{C}122.4 ∘C) by recrystallisation from hot water. To assess the purity of the recrystallised product, the student performs a melting point determination.
Which statement correctly evaluates the purity of the sample based on the melting point analysis?
A melting range of 115.5 ∘C115.5\ ^\circ\text{C}115.5 ∘C to 118.5 ∘C118.5\ ^\circ\text{C}118.5 ∘C indicates a highly pure sample, because impurities elevate the melting point of a solid.
A sharp melting range of 121.5 ∘C121.5\ ^\circ\text{C}121.5 ∘C to 122.3 ∘C122.3\ ^\circ\text{C}122.3 ∘C indicates a high degree of purity, as impurities depress the melting point and broaden the melting range.
A melting range of 122.4 ∘C122.4\ ^\circ\text{C}122.4 ∘C to 125.0 ∘C125.0\ ^\circ\text{C}125.0 ∘C indicates the presence of soluble impurities, which typically raise the melting temperature.
If the crystals are pure benzoic acid, mixing them with an authentic, pure standard of benzoic acid will significantly depress the mixture's melting point.
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.