The two responses that could provide definitive evidence of a chemical reaction are:
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A new substance, rust, has formed. - Rust is a chemical compound that forms when iron reacts with oxygen and moisture, indicating a chemical change.
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A new substance, oxygen gas, has formed. - The formation of oxygen gas typically indicates a chemical reaction, as it usually involves the breaking and forming of chemical bonds.
The other two options refer to physical changes rather than chemical reactions:
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A solid has dissolved in a liquid. - This is usually a physical change, as the substances can often be recovered by evaporation and do not change their chemical identities.
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Liquid water has frozen to form ice. - This is also a physical change, as freezing and melting are phase changes, not chemical changes.