In a chemical reaction, the molecules of one substance break apart and join together with those of another substance to create a different compound (combination of molecules). Many chemical reactions are NON-REVERSIBLE CHANGES .You cannot turn a baked cake back into its raw ingredients. Some chemical reactions can be reversed, and re-formed into the original substances. These are REVERSIBLE CHANGES.
A melting ice lolly is an example of a physical change, not a chemical change. The liquid ice lolly is not a new material, just a different form of the old one. Physical changes do not create new substances and no chemical bonds are broken or made. Melting, freezing, tearing, bending, and crushing are all physical changes that alter a substance’s appearance but not its chemical properties.
Many chemical reactions are non-reversible changes. This means they are permanent changes that cannot be undone. You cannot turn the new materials made back into the original materials again. Rusting is a non-reversible change. However, if rust is mixed with magnesium powder another chemical reaction occurs and iron can be extracted from the rust.
A few chemical reactions can be reversed – the original materials can be re-created from the new materials. These reactions are called reversible changes. They have a forward reaction and a backward reaction. Both reactions are actually happening at the same time but, depending on the conditions, one will be stronger than the other.
When the gas nitrogen dioxide is heated, a forward chemical reaction changes the brown nitrogen dioxide gas into two colourless gases – nitrogen monoxide and oxygen. However, if these colourless gases are cooled, they will re-form into brown nitrogen dioxide gas. This is called a backward chemical reaction.