Matter exists in three primary states: solid, liquid, and gas. A change of state, also known as a phase transition, describes the process where a substance shifts from one of these forms to another, like melting, freezing, or condensation. These transitions are driven by the addition or removal of energy, usually heat, which affects the movement of the substance’s constituent particles. Although the appearance of a substance changes dramatically—a rigid solid becomes a flowing liquid or an invisible gas—the total amount of mass remains the same. The question of what happens to the mass during this transformation is a fundamental concept in chemistry and physics.
The Law of Conservation and Phase Changes
The total mass of a system does not change when matter undergoes a change of state, a principle known as the Law of Conservation of Mass. This law states that mass can neither be created nor destroyed during a physical or chemical change. To demonstrate this conservation, scientists rely on a “closed system,” a defined space where matter cannot enter or escape. For example, if ice is placed in a completely sealed container and allowed to melt, measuring the container’s mass before and after the process will show no difference. The mass remains constant because a phase change is classified as a physical change, not a chemical one, altering the form but not the underlying chemical identity.
Mass and Molecular Arrangement
The conservation of mass during a phase change is best understood by looking at the substance on a microscopic level. Mass is determined by the quantity of atoms and molecules present, and these particles do not change or disappear when the substance changes state. For instance, a water molecule (H2O) is always composed of two hydrogen atoms and one oxygen atom, whether it is ice, liquid water, or steam. When ice melts, the added energy causes the water molecules to vibrate faster and move farther apart. The phase change alters only the spacing, movement, and energy of the molecules, not their composition or total number, ensuring the total mass remains the same.
Why Mass Appears to Change in Open Systems
In everyday observations, mass often appears to be lost or gained during phase changes, creating a common misconception. This apparent discrepancy arises because most real-world scenarios occur in an “open system,” which allows matter to freely enter or exit the defined space, contrasting with the closed system required to observe the Law of Conservation of Mass. When a pot of water is boiled on a stove, the liquid level drops because gaseous water vapor (steam) escapes the pot and mixes with the surrounding air. The mass of the liquid water decreases because matter has been transferred out of the system boundary. Conversely, mass can appear to be gained when water vapor condenses to form frost on a cold surface, as matter from the environment enters the system and changes state from gas to solid.