All of the scenarios you provided could be signs of a chemical reaction. However, some are more definitive than others. Here’s an evaluation of each:
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A blue liquid is mixed into a yellow liquid, and the mixture turns green.
- This indicates a possible chemical reaction due to the change in color, suggesting new substances may have formed.
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A piece of metal is placed in water and produces sparks and gas.
- The production of sparks and gas indicates a vigorous reaction, which is a strong sign that a chemical change is occurring.
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A liquid is heated on a hotplate, and bubbles form.
- Bubbles can indicate the formation of gas, which can occur in both chemical and physical processes (like boiling). However, if the gas is a product of a chemical reaction, this would be a sign.
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A solid powder is mixed into a liquid and disappears.
- If the solid completely dissolves, this could indicate a physical change (if it dissolves in a solvent) or a chemical reaction (if it's a reaction product).
Among these options, the second scenario (a piece of metal placed in water producing sparks and gas) is the most clear and indicative of a chemical reaction.