Chemical and physical properties of a substance are related to chemical and physical changes, respectively. For example, melting point is a physical property and melting is the related physical change.
A chemical change takes place when the chemical composition of the substance changes. It can be represented with the help of a chemical reaction. For example, during cellular respiration, glucose is broken down into water, carbon dioxide and energy molecules (ATP). This chemical reaction can be written as:
`C_6H_12O_6 + 6O_2 -> 6CO_2 + 6H_2O +ATP`
Chemical changes are, thus, accompanied by the formation of new compounds.
Physical changes do not cause any change in the chemical composition of a substance. In other words, no new substance is formed; only the state of the matter may change. However, at the molecular level, the same molecules are present before and after a physical change. For example, when we melt a piece of iron, it coverts to liquid iron; however, in both cases, the substance is still iron.
Ethan can best defend his conclusion by demonstrating that the chemical composition of the substance has changed during heating. In other words, he needs to show that the substance contains different molecules than it did before the heating. This would be his best defense for claiming a chemical reaction (or change).
Hope this helps.
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