One pound of ice, when completely melted, yields exactly one pound of liquid water. This answer is rooted in a fundamental law of physics, but confusion often arises because the resulting water looks like less than the ice it came from. The collection of water molecules remains the same throughout the melting process, which is why the weight stays constant. The apparent loss of size is a change in volume, not a loss of mass.
Mass Conservation: Why a Pound Stays a Pound
The principle governing this constant weight is the Law of Conservation of Mass, which states that matter cannot be created or destroyed in a closed system. Melting is a physical change, not a chemical reaction, meaning the identity and total quantity of the substance’s molecules are preserved. When one pound of ice melts, the total number of H₂O molecules remains the same as when it was solid. The mass, defined as the amount of matter within an object, must therefore be conserved.
The change from solid to liquid involves a rearrangement of the existing molecules, not their destruction or creation. Although the state of matter changes, the mass of the ice equals the mass of the resulting water. This conservation holds true for all phase changes, including freezing, boiling, or melting. Therefore, regardless of the starting temperature or melting speed, one pound of ice produces exactly one pound of liquid water.
The Role of Density: Why the Volume Changes
While the mass remains constant, the volume of the water decreases upon melting, which causes the common misconception. This volume difference is explained by density, the measure of mass per unit of volume. For most substances, the solid form is denser than the liquid form. Water is a notable exception because its solid form, ice, is less dense than its liquid form.
When water freezes, the polar nature of the H₂O molecules causes them to form strong hydrogen bonds. These bonds force the molecules to align into a rigid, open crystalline lattice structure. This ordered arrangement creates significant empty space or “voids” within the structure. Consequently, the same mass of water occupies a larger volume when it is ice.
As the ice melts, the energy input breaks the rigid hydrogen bonds, allowing the water molecules to move closer together. In the liquid state, the molecules are more randomly packed and fill the spaces that were previously voids in the crystalline structure. This closer packing results in liquid water being denser than ice, causing a noticeable reduction in volume. This unique property allows ice to float.
The Energy Required for Phase Change
The melting process is driven by the transfer of energy needed to overcome the molecular forces holding the ice together. This required energy is termed the Latent Heat of Fusion. The term “latent” indicates that this energy is absorbed without causing a change in temperature. Instead of increasing the kinetic energy of the molecules, the energy is used entirely to change the physical state.
The energy input goes directly into breaking the stable hydrogen bonds that maintain the ordered structure of the ice. For water, the specific latent heat of fusion is approximately 334 kilojoules per kilogram. This energy must be absorbed by the ice before the temperature of the resulting water can begin to rise above 0°C. Once all the bonds are broken and the solid is converted entirely to liquid, the added heat then begins to warm the water.