When we say that mass is conserved during a physical change, we mean that the total mass of a closed system remains constant throughout the process, regardless of any changes in the state or form of the substances involved. This principle is rooted in the law of conservation of mass, which states that mass cannot be created or destroyed in a chemical reaction or physical change.
In practical terms, during a physical change—such as melting, freezing, boiling, or dissolving—the substances involved may change their physical state (for example, ice melting into water), but the total amount of matter, or mass, remains the same. Any mass that appears to be lost or gained in such transformations is accounted for by changes in the arrangement of particles or the state of the substance, rather than an actual change in the amount of matter.
For instance:
- When ice melts into water, the mass of the ice before melting is equal to the mass of the water formed after melting.
- If you evaporate water, the mass of the water vapor released into the air will equal the mass of water you started with, although the mass is now distributed in a different form.
This concept is fundamental in both physics and chemistry, underpinning the understanding of reactions, changes of state, and the behavior of matter in various contexts.