Changes in matter are categorized as either physical or chemical. A physical change alters the form or appearance of a substance, such as melting ice, but does not create a new substance. A chemical change, or chemical reaction, results in the formation of entirely new substances with different properties. Heating a mixture of iron and sulfur initiates a chemical change that permanently alters the elements at a molecular level, producing a compound unlike the starting materials.
Properties of Iron and Sulfur Separately
Before heating, iron and sulfur exist together as a simple mixture, meaning they retain their individual, recognizable properties. Iron is a dark gray, malleable metal that is strongly attracted to a magnet. Sulfur, conversely, is a distinctive bright yellow, non-metallic powder.
The individual properties of these two elements allow them to be easily separated by simple physical methods. Since only the iron is magnetic, a magnet can pull the filings away from the sulfur powder. Sulfur is also insoluble in water, offering another way to separate the components of the mixture.
Observable Evidence During Heating
Heating the mixture of iron and sulfur provides clear, immediate evidence that a chemical reaction is taking place. The reaction requires a certain amount of heat energy to get started, but once that threshold is reached, the process becomes self-sustaining. The heat causes the sulfur to melt and react with the iron to form a new substance.
A rapid and intense release of energy, known as an exothermic reaction, is a sign of this chemical change. A bright red or orange glow spreads rapidly throughout the mixture even after the external heat source is removed. This spreading glow indicates the reaction is releasing enough heat to fuel the process in the surrounding material. The visible light and intense heat are indicators that the atoms are rearranging themselves into a new chemical structure.
Characteristics of the Resulting Compound
The black solid that remains after the mixture cools is a new compound, iron sulfide, which confirms the chemical change. The composition of this product is fixed, meaning iron and sulfur are chemically bonded together in a precise ratio. This new compound possesses properties that are completely different from those of the starting materials.
The most telling difference is that the resulting substance is not magnetic, unlike the original iron filings. Bringing a magnet near the iron sulfide results in no attraction, demonstrating that the iron’s metallic property has been lost during chemical bonding. Furthermore, the product is a black solid, lacking the bright yellow color of the sulfur and the metallic sheen of the iron. This change in color, texture, and magnetic property proves that a new substance has been created.