Because chemical change is a change in which an object changes and becomes a new object .
for ex: burning of paper changes paper into Ash

Introduction:
A change is considered a chemical change when it is accompanied by certain characteristics and outcomes. These characteristics include an energy change, the formation of new substances, and a change in chemical properties. These factors distinguish a chemical change from a physical change, where the substances involved may undergo a change in state or form without any alteration in their chemical composition.

Energy Change:
An energy change is one of the key indicators of a chemical change. During a chemical reaction, energy is either released or absorbed. This energy change can manifest in various forms such as heat, light, sound, or electrical energy. For example, when wood burns, it undergoes a chemical reaction called combustion. In this process, energy in the form of heat and light is released.

Formation of New Substances:
Another characteristic of a chemical change is the formation of new substances. In a chemical reaction, the original substances, known as reactants, undergo a rearrangement of their atoms to form new substances, known as products. The chemical bonds between atoms are broken and new bonds are formed, resulting in the creation of different compounds or elements. For instance, when hydrogen gas (H2) combines with oxygen gas (O2), water (H2O) is formed.

Change in Chemical Properties:
Chemical changes also involve a change in the chemical properties of the substances involved. Chemical properties refer to the characteristics that describe how a substance interacts with other substances or undergoes chemical reactions. These properties include flammability, reactivity, and stability. During a chemical change, the reactants and products may exhibit different chemical properties. For example, iron rusting is a chemical change where iron reacts with oxygen in the presence of moisture to form iron oxide. The chemical property of iron changes from being unreactive to reactive with oxygen.

Conclusion:
In conclusion, a change is considered a chemical change when it is accompanied by an energy change, the formation of new substances, and a change in chemical properties. These characteristics distinguish a chemical change from a physical change and are fundamental to understanding the nature of chemical reactions and their outcomes.