This is an observation. An observation is the act of noting or perceiving an occurrence or phenomenon, and in this case, the chemist has witnessed a reaction between sodium and water. The observation is that the reaction is violent, which is an objective and measurable phenomenon. It is not a theory, which is a well-substantiated explanation for a set of observations or experimental results. It is also not a hypothesis, which is a proposed explanation for a phenomenon that can be tested through further observation or experimentation. Finally, it is not a law, which is a concise statement that describes a fundamental relationship or regularity observed in nature.