The term 'TEST THE PURITY OF' can be summarized by the crossword solution 'ASSAY, ' which is a five-letter word. An ASSAY is a type of test used to determine the concentration or purity of a substance. It is commonly used in chemistry and biology to measure the amount of a particular element or compound in a sample. Assays are also used to detect the presence of contaminants or impurities in a sample.