A Level Chemistry OCR Practice Exam

Which method can be used to identify the presence of water of crystallisation in a sample?

1. Infrared spectroscopy

2. Electrolysis

3. Thermogravimetric analysis

4. Chromatography

The correct answer is: Thermogravimetric analysis

Thermogravimetric analysis (TGA) is a powerful technique employed to identify the presence of water of crystallisation in a sample due to its ability to measure changes in the mass of a material as it is heated over a range of temperatures. When a sample containing water of crystallisation is subjected to TGA, the water will evaporate at a specific temperature, resulting in a distinct mass loss on the TGA curve. By analyzing the temperature at which this mass loss occurs and the amount of mass lost, one can infer the presence and amount of water of crystallisation within the sample. In contrast, infrared spectroscopy, while useful for identifying functional groups and bonds, would not provide direct information about water of crystallisation in terms of quantitative mass loss. Electrolysis pertains more to the decomposition of substances through electrical means, rather than the thermal properties of solids with crystallisation water. Chromatography is primarily a separation technique for analyzing mixtures and is not designed to study thermal properties or mass changes related to water of crystallisation. Thus, thermogravimetric analysis stands out as the most appropriate method for this specific purpose.