When paper is set alight, the visual change from a solid sheet to smoke and ash is a common observation. This act of burning explores a fundamental concept in science: the difference between a physical change and a chemical transformation. Understanding this distinction requires examining whether new substances are created at the molecular level.
Understanding Chemical vs. Physical Changes
A physical change involves an alteration to the form, appearance, or state of matter, but the substance’s chemical identity remains the same. Tearing a sheet of paper or melting an ice cube are examples of physical changes, as the paper is still paper and the water molecules are still \(H_2O\). Physical changes can often be reversed by simple means, such as refreezing liquid water back into ice.
A chemical change, conversely, is a process that results in the formation of one or more new substances with entirely different chemical properties than the starting materials. This transformation involves the breaking and forming of chemical bonds between atoms. Signs of a chemical change often include an unexpected color change, the release of gas or light, or a significant temperature shift.
The Chemical Reaction of Combustion
The burning of paper is a process known as combustion, which is a specific type of chemical reaction. Paper is primarily composed of cellulose, a complex carbohydrate made up of carbon, hydrogen, and oxygen atoms. Combustion requires the cellulose in the paper to react vigorously with an oxidizer, which in this case is the oxygen gas present in the air.
This reaction is highly exothermic, meaning it releases a substantial amount of energy in the form of heat and light, which we observe as the flame. As the paper burns, the chemical bonds holding the cellulose molecules together are broken, and the atoms rearrange themselves to form new, simpler compounds.
The main gaseous products are carbon dioxide (\(CO_2\)) and water vapor (\(H_2O\)). The residual material is ash, which is the non-combustible, inorganic mineral content of the paper. This process permanently changes the chemical composition, altering cellulose into gaseous molecules and solid ash.
Classifying the Change
Burning paper is definitively classified as a chemical change because new substances are formed that cannot be converted back into paper under normal conditions. The entire mass of the paper does not simply vanish; rather, it is transformed into the solid ash and the gaseous products like carbon dioxide and water vapor. The dramatic change in composition from fibrous cellulose to black ash and invisible gases is the ultimate proof of a chemical transformation.
The irreversibility of the change distinguishes it from a physical change. While you can shred paper and tape it back together (a physical change), you cannot recombine the smoke and ash to recreate the original sheet of paper. This confirms the substance’s molecular structure has been fundamentally altered, making combustion a chemical reaction.