During a lab investigation, a group of students combined four different sets of substances in four separate beakers. They recorded their observations in the table below.

The observation that is least likely to indicate a chemical reaction is:

**C - 2** (One substance floated on top of the other)

This observation suggests a physical phenomenon rather than a chemical change. The other observations (1, 3, and 4) typically indicate chemical reactions, such as color change, gas production (bubbles), and the release of heat and smoke. Floating indicates a difference in density rather than a transformation of substances.