During a chemical change, the end substance has a different chemical composition than the starting materials. Any substance that changes form, produces gas bubbles or changes color has undergone a chemical change. A chemical change cannot be reversed. For example, once an egg has been fried, it cannot ever go back to its original form.
A physical change happens when a substance changes form but keeps the same chemical composition. The change can be reversed by taking away the cause of the physical change. Salt water is a good example of physical change. Once the salt and water have been separated, both objects go back to their original states. Folding paper is another example of a physical change. The paper changes form, but its chemical composition stays the same.
Sometimes, it may be hard to tell if the change that has taken place is physical or chemical. Melting ice cream is a physical change because the chemicals used in the ice cream are the same melted or frozen. Food that's gone bad is a chemical change because the food can never go back to being "good."Learn more about Organic Chemistry