A physical change can't reverse a chemical change. This is because a chemical change results in a new substance being formed. Chemical bonds are broken and and new bonds are formed between different atoms. Another chemical change would be needed to break the new bonds and reform the original bonds.
When a physical change occurs the chemical composition of the substance doesn't change. Some examples of physical changes are melting, freezing, vaporization, and condensation. Physical changes can be reversed, usually by heating or cooling the substance, without any chemical bonds being broken or formed.
Here are examples of both a chemical and a physical change involving water:
When hydrogen gas reacts with oxgen gas, water vapor is formed:
`2H_2 + O_2 -> 2H_2O`
The water formed in this reaction can't be turned back into hydrogen and oxygen gases unless enough energy is provided to break the chemical bonds, which would change the chemical formula.
Here's a description of ice melting:
`H_2O_((solid)) + heat -> H_2O_((liquid))`
This is a physical reaction. There was no change in the chemical formula of the substance. The liquid water can be changed back to solid by cooling it, and in doing so the chemical formula still doesn't change.
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