Chemical changes and physical changes differ primarily in the formation of new substances and reversibility.
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New Substances:
- Chemical Changes: They form new substances with different properties from the original materials. For example, when iron rusts, it reacts with oxygen to form iron oxide (rust), which is a new substance.
- Physical Changes: These do not form new substances. Instead, they may alter the state or appearance of a substance without changing its chemical composition. For example, melting ice into water is a physical change but it remains H2O.
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Reversibility:
- Chemical Changes: Often not reversible, or can be very difficult to reverse. For instance, burning wood creates ash and gases that can't be turned back into wood.
- Physical Changes: Generally reversible. An example would be freezing and melting water, where the substance can return to its original state.
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Heat:
- Chemical Changes: Often involve heat changes, either releasing energy (exothermic) or absorbing energy (endothermic).
- Physical Changes: Can also involve heat changes (like melting, freezing, boiling), but this is not definitive to a physical change.
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Phase of Matter:
- Chemical Changes: Can involve changes in the phase of matter but the defining characteristic is the formation of new substances.
- Physical Changes: Often involve shifts in the phase of matter (solid, liquid, gas) without altering the chemical structure of the substance.
In summary, while both types of changes may involve heat and can include phase shifts, the key determinant of a chemical change is the formation of new substances, whereas physical changes do not create new substances and are usually reversible.