Chemical changes occur on the molecular level. A chemical change results in the formation of a new substance.
Examples of Chemical Changes include:
• Combustion or burning e.g. burning wood
• Dissolution of salt in water
• combination of acid and base
• digestion of food
• cooking of an egg,
• rusting of an iron or metal object
• Combination of hydrochloric acid and sodium hydroxide to produce salt and water.
Physical changes deals with energy and states of matter. A physical change unlike the chemical change does not lead to the formation of a fresh substance. Changes in state such as melting, freezing, vaporization, condensation, sublimation are all physical changes.
Examples of physical changes include:
• crumpling a sheet of paper
• melting an ice cube
• casting silver in a mold
• breaking a bottle
• Crushing a can.
How to know a Chemical Change and a physical change
A chemical change results to the formation of a substance which was not there previously. You may be able to ascertain a chemical change through some indicators like light, heat, color alteration, gas formation, odor, or sound. The reactant and the product of a physical change are the same, although they may appear to be variable.
A physical change may have occurred if the changes that are associated with a chemical change are not found. It can be hard to tell this in some reactions such as when sugar is dissolved in water. In this case the content is still the same chemically although the sugar has dissolved. The sugar is now present in the mixture as molecules of sucrose .Nevertheless, when you dissolve salt in water, the salt dissociates into its ions of Na+ and Cl- resulting to a chemical change. In the two scenarios, a white solid (salt) is dissolved into a clear liquid and in the two scenarios the reactant can be recovered by taking away the water. Irrespective of this, the two reactions are different.