Combining iron filings and sulfur powder raises a common question about the nature of chemical change. When combined at room temperature, the elements do not cause a fundamental transformation. The key distinction is whether the elements have merely been intermingled or if they have undergone a complete atomic rearrangement to create a new material with unique properties. Understanding the difference between physical and chemical changes provides the full answer.
Understanding Changes: Physical Versus Chemical
Matter can change in two distinct ways: physical or chemical. A physical change involves an alteration in the form or appearance of a substance, but its underlying chemical identity remains unchanged. Examples include melting ice or shredding paper, where the material is still water or cellulose, respectively.
A chemical change, conversely, is a chemical reaction that results in the formation of an entirely new substance with a different chemical composition and unique properties. Indicators that a chemical change has occurred include the production of a gas, an unexpected color change, or the emission of light or heat. Unlike physical changes, chemical changes are generally not reversible through simple physical means. The formation of rust or the process of cooking are common examples of this type of transformation.
The Initial Combination: A Simple Mixture
When dark gray, magnetic iron filings are mixed with bright yellow sulfur powder, the initial result is a physical change, forming a simple mixture. This combination is not a chemical reaction because the individual properties of iron and sulfur are retained. The ratio of iron to sulfur can be varied, and the two substances are simply intermingled together.
A key proof that no new substance has formed is the ability to separate the components using a physical method. If a magnet is passed over the mixture, the iron filings will be attracted and pulled out, leaving the sulfur behind. This demonstrates that the original elements retain their properties, defining the combination as a simple mixture.
Triggering the Transformation: Applying Heat
The situation changes dramatically when the iron and sulfur mixture is heated. Applying heat provides the necessary activation energy to overcome the forces holding the elements apart and initiates a powerful chemical reaction. The atoms of iron and sulfur begin to rearrange, forming new chemical bonds between them.
This process is highly exothermic, meaning it releases a significant amount of heat and light once the reaction is underway.
The chemical transformation is represented by the equation: Fe + S → FeS. In this synthesis reaction, one atom of iron (Fe) combines with one atom of sulfur (S) to form the new compound, iron(II) sulfide (FeS). This reaction marks the definitive shift from a physical change to a chemical change, as the original elements fuse into a single substance with a fixed composition.
Characterizing the New Substance
The product of the heating process is iron(II) sulfide, which is a new compound with properties entirely different from the starting materials. Instead of a mixture of gray and yellow powders, the result is a dark gray or black solid mass. This new substance is no longer attracted to a magnet, proving that the iron has lost its characteristic magnetic property through chemical bonding.
Separating the iron(II) sulfide back into its constituent elements is extremely difficult, requiring another chemical reaction rather than simple physical means. The loss of magnetism and the change in color provide clear evidence of the transformation. Because the starting elements have been chemically joined into a new material with a new set of properties, the heating of the iron and sulfur mixture is classified as a chemical change.