Formation of new substance:
Chemical changes are characterized by the formation of new substances. During a chemical change, the original substances react with each other to form different substances with new chemical properties. This is different from a physical change, where the original substances remain the same even though their physical appearance may change.

Example:
For example, when iron rusts, it undergoes a chemical change. The iron reacts with oxygen in the presence of water to form iron oxide, which is a new substance with different properties than the original iron.

Evidence of chemical change:
There are several signs that can indicate a chemical change is taking place. These include the formation of a precipitate, a change in color, the release of gas, or a change in temperature. These changes indicate that a new substance is being formed through a chemical reaction.

Irreversibility:
Unlike physical changes, which can often be reversed, chemical changes are typically irreversible. Once a chemical reaction has taken place and new substances are formed, it is usually not possible to revert back to the original substances. This is because the chemical bonds between atoms have been broken and rearranged to form the new substances.
In conclusion, chemical changes are characterized by the formation of new substances with different chemical properties. This process is irreversible, unlike physical changes, and can be identified through various signs such as changes in color, the formation of a precipitate, or the release of gas.