A physical change is a process where a substance alters its physical form or state without changing its fundamental chemical composition. Examples include melting, freezing, boiling, dissolving, or crushing a material. Scientists rely on the concept of conservation to understand these changes. Identifying what is conserved establishes the principles that govern how matter and energy interact and provides a reliable framework for predicting the outcome of any physical transformation.
The Conservation of Mass
The Law of Conservation of Mass, attributed to Antoine Lavoisier, states that mass is neither created nor destroyed in a closed system. This law applies directly to physical changes because the total quantity of matter remains unchanged, even as its appearance is altered. When an ice cube melts into liquid water, the total mass of the water molecules does not change; it simply transitions from a rigid structure to a fluid one.
This conservation is rooted in the fact that physical changes do not affect the atoms themselves. The total number of atoms of each element remains the same before and after the change. For instance, when salt is dissolved in water, the salt crystals disappear, but the initial and final mass will be identical if weighed in a sealed container.
Even in phase changes like evaporation, where liquid water seems to vanish, the mass is conserved because the water molecules disperse as a gas. In a closed system, the mass of the water vapor and the remaining liquid equals the original mass of the liquid water.
The Conservation of Energy
The Law of Conservation of Energy, or the First Law of Thermodynamics, dictates that the total energy within an isolated system remains constant, though it can be transformed. This principle is evident in physical changes involving heat transfer, such as phase transitions. Energy must be absorbed or released for a substance to change its state.
When water boils, it requires energy (latent heat of vaporization) to overcome intermolecular forces and transition to a gas. This energy is stored as the increased internal energy of the gas molecules. Conversely, when water freezes, energy (latent heat of fusion) is released back into the surroundings.
The total energy of the system does not change overall. Energy simply moves into or out of the substance, or shifts between different forms.
The Conservation of Identity and Composition
The most defining conserved property in a physical change is the substance’s chemical identity and molecular composition. A physical change affects the arrangement and movement between molecules, but not the structure within the molecules. The chemical formula of the substance remains the same from start to finish.
For example, whether water exists as solid ice, liquid water, or gaseous steam, its molecular structure is always two hydrogen atoms bonded to one oxygen atom (H2O). The chemical bonds holding these atoms together are not broken or rearranged during melting or boiling. This stability of the internal molecular structure distinguishes a physical change from a chemical reaction.
Because the substance’s chemical identity is conserved, physical changes are generally reversible by simply reversing the conditions, such as temperature or pressure. This conservation of the molecular blueprint ensures that the substance can always return to its original state.