The purity of a solid substance can be checked by various methods, including:

1. Melting Point: Pure substances have a specific and well-defined melting point. By comparing the observed melting point of a sample to the known melting point of the pure substance, you can determine its purity. Impurities tend to lower the melting point and broaden the melting range.

2. Boiling Point: Similar to the melting point, pure substances have a specific boiling point. The observed boiling point of a sample can be compared to the known boiling point of the pure substance to assess its purity. Impurities can cause the boiling point to increase and the boiling range to broaden.

3. Chromatography: Chromatographic techniques, such as thin-layer chromatography (TLC) or high-performance liquid chromatography (HPLC), can separate different components in a mixture. By analyzing the resulting chromatogram, you can determine the presence and relative amounts of impurities in the sample.

4. Spectroscopy: Spectroscopic techniques, such as infrared spectroscopy (IR), nuclear magnetic resonance (NMR), or ultraviolet-visible spectroscopy (UV-Vis), can provide information about the molecular structure and functional groups present in a substance. By comparing the obtained spectrum with the reference data for the pure substance, you can assess its purity.

5. Elemental Analysis: Elemental analysis, such as using techniques like elemental combustion analysis or X-ray fluorescence (XRF), can determine the elemental composition of a substance. Deviations from the expected composition can indicate the presence of impurities.

6. Solubility: The solubility of a substance in a particular solvent can be indicative of its purity. If a substance is expected to be highly soluble in a specific solvent, but it exhibits poor solubility or leaves behind a residue, it suggests the presence of impurities.

These are just a few common methods used to check the purity of a solid substance. The choice of method depends on the specific substance and the available equipment and techniques.