Why does a chemical change alter the substance's identity?

A chemical change alters a substance's identity because it involves a reaction that forms new substances with different properties.

In a chemical change, also known as a chemical reaction, the atoms of the original substances are rearranged to form new substances. This is different from a physical change, where the substance may change form or state (like from solid to liquid), but its fundamental identity remains the same. In a chemical change, the original substances, or reactants, lose their identity and new substances, or products, are formed.

The reason why a chemical change alters a substance's identity lies in the nature of chemical bonds. Atoms are held together by chemical bonds to form molecules. During a chemical reaction, these bonds are broken and new ones are formed, leading to the creation of new molecules. These new molecules have different properties from the original substances because they are composed of different arrangements of atoms.

For example, when hydrogen gas reacts with oxygen gas, water is formed. The hydrogen and oxygen gases have their own distinct properties. However, once they undergo a chemical reaction, they form water, which has entirely different properties. The hydrogen and oxygen gases have lost their original identities and have become a new substance - water.

In essence, a chemical change is a transformative process that results in the creation of new substances with different properties. This is why it alters the substance's identity. It's important to note that this process is usually irreversible, meaning the new substances cannot easily be changed back into the original substances. This further emphasises the fact that the original substance's identity has been fundamentally altered.