Matter possesses distinct characteristics, or properties, that allow us to identify and differentiate materials. How matter behaves when these properties change helps classify alterations as either physical or chemical. A key question is whether density represents a physical or chemical change.
Understanding Density
Density is a property of matter that quantifies how much mass is contained within a given volume. It measures how tightly packed the particles of a substance are. The formula for density is mass divided by volume. For instance, a rock sinks in water because it has a greater density, meaning more mass is packed into the same volume compared to water. Conversely, wood floats because it is less dense than water. Density is an intrinsic characteristic, meaning it does not depend on the amount of the substance present.
Physical and Chemical Changes
Changes in matter are categorized as physical or chemical. A physical change alters the form or appearance of a substance but does not change its chemical composition. Examples include melting ice into water, remaining H₂O, or cutting paper into smaller pieces. These changes involve a change in physical properties like shape, size, or state, and are often reversible.
In contrast, a chemical change, or chemical reaction, results in the formation of one or more new substances with different chemical compositions and properties. This involves breaking and forming chemical bonds, rearranging atoms. Burning wood, transforming wood into ash and smoke, or iron rusting, creating a new compound (iron oxide), are examples of chemical changes. These changes are more difficult to reverse than physical changes.
Density: A Fundamental Physical Property
Density is a physical property of matter. It can be measured and observed without altering the chemical identity of the substance. Determining the density of aluminum, for example, does not change it into a new substance.
While density is a physical property, its value can change due to physical alterations. Temperature significantly affects density because most substances expand when heated and contract when cooled. As temperature increases, particles move faster and spread out, increasing the volume and thus decreasing the density. Conversely, cooling causes particles to slow down and move closer, decreasing volume and increasing density. This explains why hot water is less dense than cold water.
While chemical reactions produce new substances with distinct densities, measuring their density remains a physical process. Density is a characteristic of the substance formed, indicating how matter is arranged rather than how it reacts or transforms chemically.