The distinction between physical and chemical changes lies in the nature of the processes involved and the identity of the substances before and after the change.
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Melting Ice (Physical Change):
- When ice (solid water) melts, it undergoes a physical change from a solid state to a liquid state. The chemical composition of the substance remains the same, as the molecules of water (H₂O) are not altered; they simply move more freely as the ice turns into liquid water. This change is reversible; if the water is cooled, it will freeze back into ice.
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Creating Water (Chemical Change):
- The formation of water (H₂O) from its elemental gases—hydrogen (H₂) and oxygen (O₂)—involves a chemical reaction. During this process, the bonds between the atoms in hydrogen and oxygen molecules are broken and new bonds are formed to create water molecules. This results in a new substance with different properties. This change is not easily reversible, as separating water into hydrogen and oxygen requires energy input and specific conditions.
In summary, melting ice is a physical change because it involves a change in state without altering the chemical composition of the substance, while creating water from its elemental gases is a chemical change because it involves breaking and forming bonds to create a different substance.