A physical change is reversible by physical means.

1. the solution process -- one substance is dissolved in another substance.  Example:  salt dissolved in water -- if the water evaporates, the salt is left unchanged.
2. a phase change -- in which a substance goes from one phase to another; solid, liquid, or gas, by the addition or removal of heat.  Example:  ice (solid water) melts to liquid water -- if the temperature of the liquid is lowered, the ice is restored.

A chemical change -- a substance undergoes a reaction in which the products have different properties than the reactants.  Example:  silver nitrate reacts with sodium chloride to produce silver chloride and sodium nitrate.

A nuclear change -- involves changes in nuclear structure, such as fission (splitting) of a nucleus or an atom, or fusion (combining) of neutrons and protons to form heavier atoms.  This involves about 1,000,000 times as much energy as a chemical change.

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