Organic substances are primarily defined as compounds containing covalent bonds, typically characterized by low melting points and limited solubility in water. They are not stable in hot concentrated sulfuric acid, which is often used to identify organic matter through oxidation, resulting in coal-like carbon particles. The discussion clarifies that organic compounds generally exclude carbon oxides, carbonates, and cyanides, with a focus on carbon compounds that have a formal valence of less than 4. This includes hydrocarbons like methane and its homologs, as well as carboxylic acids. Overall, the conversation aims to provide a foundational understanding of what constitutes organic substances.