The graph below shows the heating curves obtained when two different solid samples, Sample P and Sample Q, are heated at a constant rate until they have completely melted.
Which of the following statements correctly identifies the purity of the samples and explains the observed behaviour?
Sample Q is a chemically pure substance because it maintains a temperature gradient during melting, whereas Sample P is an impure mixture because it exhibits a constant temperature plateau.
Sample P is an impure mixture because the plateau shows its individual components are melting sequentially, whereas Sample Q is a pure substance.
Sample P is a chemically pure substance because it melts at a sharp, defined temperature, whereas Sample Q is an impure mixture because it melts over a range of temperatures.
Both Sample P and Sample Q are pure substances, but Sample Q is undergoing a chemical decomposition rather than a simple physical state change.