Density is an intrinsic physical property of a substance, not a physical change itself. Understanding the distinction between properties and changes is fundamental to grasping how matter behaves. This distinction helps clarify how substances are characterized and how they transform.
Defining Density
Density quantifies how much mass is contained within a given volume of a substance. It is calculated using the formula: density equals mass divided by volume (D = m/V). Common units include grams per cubic centimeter (g/cm³) or kilograms per cubic meter (kg/m³). For instance, water has a density of approximately 1 gram per cubic centimeter, while wood typically has a lower density, which is why it floats. Different materials possess unique densities due to the varying sizes, masses, and arrangements of their atoms or molecules.
Exploring Physical Changes
A physical change alters a substance’s form or appearance without changing its fundamental chemical composition. The identity of the matter remains the same, even if its state, shape, or size is modified. Melting ice into water is a physical change because the substance is still H₂O, just in a different state. Boiling water also represents a physical change, as liquid water transforms into water vapor, but its chemical makeup remains unchanged.
Tearing paper changes its size and shape. Dissolving sugar in water is another example; the sugar can be recovered by evaporating the water. Crushing an aluminum can alters its form, yet the aluminum remains chemically identical. Many physical changes are reversible.
Exploring Chemical Changes
A chemical change, also known as a chemical reaction, results in the formation of new substances with different chemical properties than the original materials. This process involves the breaking and forming of chemical bonds between atoms or molecules. Unlike physical changes, chemical changes are often difficult to reverse and typically involve a significant absorption or release of energy.
Burning wood is a chemical change because it transforms into ash, smoke, and gases like carbon dioxide, which are entirely new substances. Rusting iron involves iron reacting with oxygen and water to form iron oxide, a new compound with distinct properties. Baking a cake is also a chemical change, as ingredients react under heat to form a new product. Cooking an egg changes its protein structure permanently.
Density’s Role in Changes
While density is a property, its measured value can change during both physical and chemical processes. During a physical change, such as a change in state, the arrangement of molecules shifts, which can affect volume and thus density, even though the substance’s chemical identity remains the same. For instance, when liquid water freezes into ice, its volume increases due to the unique hexagonal arrangement of water molecules, making ice less dense than liquid water, which is why ice floats. Conversely, most other substances become denser when transitioning from liquid to solid.
In chemical changes, the formation of entirely new substances means their densities will inherently differ from the original reactants. When wood burns, the resulting ash, carbon dioxide gas, and water vapor have densities significantly different from the solid wood. A change in density can serve as an indicator that a chemical or physical transformation has occurred, but density remains a characteristic property of the substance, not the change itself.