The option that always indicates a chemical reaction has occurred when mixing a powder into a liquid is:
D) you observe bubbles in the mixture.
The presence of bubbles often suggests that a gas is being produced as a result of a chemical reaction. In contrast, the other options may involve physical processes (such as dissolving or boiling) that do not necessarily indicate a chemical change. For example, the powder dissolving in a liquid (option B) is a physical change, and the liquid being at its boiling point (option C) does not imply a chemical reaction, only a phase change.