The simple and definitive answer is that the mass of water does not change when it freezes, despite the dramatic change in physical appearance. Mass is a fundamental property representing the amount of matter contained within an object. Freezing is a physical process that alters the state from liquid to solid, but it does not change the total quantity of \(\text{H}_2\text{O}\) molecules present. The mass of a block of ice is exactly the same as the mass of the liquid water from which it was formed.
The Principle of Mass Conservation
This constancy of mass is governed by the Law of Conservation of Mass, a foundational principle in physical science. This law states that matter can neither be created nor destroyed in a closed system, even if it undergoes a physical change or chemical reaction. Freezing water is classified as a phase change, meaning the substance transitions from one physical state to another while its chemical composition (\(\text{H}_2\text{O}\)) remains the same.
When liquid water cools and solidifies, the individual water molecules remain intact. The process involves a rearrangement of the existing molecules, not a modification of their total number or internal structure. Measuring the mass before and after freezing confirms that the total amount of matter is conserved. This principle holds true for virtually all phase changes.
What Changes During Freezing: Volume and Density
The confusion about mass change often stems from the visible changes in volume and density that occur when water freezes. Mass is distinct from volume, which is the amount of space a substance occupies. Density is the relationship between these two properties, calculated as mass divided by volume. Unlike most substances, water expands significantly as it solidifies, meaning its volume increases.
Because the mass remains constant while the volume increases, the density of the resulting ice must decrease. Ice is approximately nine percent less dense than liquid water at its freezing point. This lower density explains the common observation that ice floats on water, a phenomenon unique to very few materials.
This decrease in density has profound implications for the natural world. If ice were denser than water, it would sink, causing bodies of water to freeze solid from the bottom up. Instead, floating ice acts as an insulating layer, protecting aquatic life below from freezing air temperatures. This expansion is also responsible for burst pipes and damaged pavement, as the increasing volume exerts tremendous force.
Water’s Unique Molecular Structure
The reason for water’s unusual expansion upon freezing lies in the unique geometry and bonding behavior of the \(\text{H}_2\text{O}\) molecule. Water is a polar molecule: the oxygen atom attracts electrons more strongly than the hydrogen atoms, creating partial negative and positive charges. These opposite charges lead to strong attractions between neighboring molecules known as hydrogen bonds.
In liquid water, these hydrogen bonds constantly form, break, and reform as the molecules move chaotically and pack closely together. As liquid water cools below \(4^\circ\text{C}\), where it reaches maximum density, the molecules slow down. This reduced thermal energy allows the directional nature of the hydrogen bonds to become dominant.
When water reaches its freezing point, hydrogen bonds lock the molecules into a highly ordered, three-dimensional, crystalline structure. The most common form of ice, known as Ice \(\text{I}_{\text{h}}\), forms an open, hexagonal lattice pattern. This arrangement forces the water molecules to be held farther apart than they were in the disordered liquid state. The resulting open structure accounts for the overall volume increase and subsequent drop in density of the ice.