The question is an illustration of physical and chemical properties.
To distinguish A from B, you simply observe their physical and chemical properties.
When taking note of physical changes, you simply observe how the form of the substance changes.
For instance, when substance A changes from liquid to solid, then substance A has undergone a physical change.
However, the chemical change to observe includes the formation of a new substance.
For instance, when a new substance (say B1) is formed from substance B, then you know that substance B has undergone a chemical change.
Hence, to distinguish between A and B, you have to find out their physical and chemical properties.
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