Chemical changes refer to the changes that occur when two or more substances combine to form a new substance with different physical and chemical properties. These changes are usually irreversible, meaning that it is difficult or impossible to reverse the process and obtain the original substances.

Why are chemical changes mostly irreversible?

1. Formation of new substances: In a chemical change, new substances are formed by the combination of two or more substances. These substances usually have different physical and chemical properties than the original substances, making it difficult to revert back to the original.

2. Energy changes: Chemical changes involve the release or absorption of energy in the form of heat, light, or electricity. This energy change can alter the chemical structure of the substances and make it difficult to reverse the process.

3. Bond formation: During a chemical change, new bonds are formed between atoms, and old bonds are broken. These bonds are usually strong and require a lot of energy to break, making it difficult to reverse the process.

4. Chemical reactions: Chemical changes involve chemical reactions, which are irreversible processes. Once a reaction occurs, it cannot be undone, and the substances formed cannot be separated back into their original components.

Examples of irreversible chemical changes:

1. Burning of wood: When wood is burned, it undergoes a chemical change that cannot be reversed, and new substances such as carbon dioxide and water vapor are formed.

2. Rusting of iron: When iron is exposed to water and air, it undergoes a chemical change that cannot be reversed, and new substances such as iron oxide are formed.

3. Digestion of food: When food is digested, it undergoes a chemical change that cannot be reversed, and new substances such as glucose and amino acids are formed.

In conclusion, chemical changes are mostly irreversible because they involve the formation of new substances, energy changes, bond formation, and chemical reactions that cannot be undone.

Chemical changes mostly be irreversible because take a example : if we put a spoon of curd in a milk it turns into a curd because of bacteria . this is a chemical change.then again we cannot bring the curd into milk .