Among the provided options, the presence of bubbles in the mixture is the only indicator that a chemical reaction has occurred, though it might not always be the case. Bubbles can indicate the production of gas, which typically suggests a chemical change.
Here's a brief evaluation of the other options:
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The powder dissolves in the liquid: This process is usually a physical change, not necessarily a chemical reaction. When a substance dissolves, it typically retains its chemical identity.
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The liquid or the powder breaks down: This could indicate a chemical change, but breaking down can also refer to physical processes (like melting or dissolving) depending on the context.
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The liquid is at its boiling point: This again can be a physical change (a change of state from liquid to gas) and does not necessarily indicate that a chemical reaction has occurred.
Therefore, observing bubbles in the mixture is the best indicator from the options provided that a chemical reaction has occurred.