Water exists in three common states: solid ice, liquid water, and gaseous steam or vapor. When water transitions between these states, it presents an example of a physical change, not a chemical one. This means the fundamental chemical identity of the water molecule, H2O, remains the same whether it is ice or gas. The difference between the states is purely a matter of how the individual molecules are organized and how much energy they possess.
The Molecule Stays H2O
The chemical structure of water is defined by the covalent bonds that link the two hydrogen atoms to the central oxygen atom. These internal bonds, known as intramolecular bonds, require a massive amount of energy to break, about 464 kilojoules per mole.
In contrast, melting ice or boiling water involves adding or removing relatively small amounts of energy. This energy is not enough to break the water molecule itself. If the energy input were sufficient to break the covalent bonds, the result would be a chemical reaction, such as electrolysis, which separates water into hydrogen gas (H2) and oxygen gas (O2). A change in state is therefore an alteration of molecular arrangement, not a transformation into a new substance.
The Mechanism of Change: Hydrogen Bonds
The mechanism governing water’s state changes is the forming and breaking of hydrogen bonds. A hydrogen bond is an electrostatic attraction between a hydrogen atom on one water molecule and the highly electronegative oxygen atom on a neighboring molecule. Water molecules are polar, meaning the oxygen atom carries a slight negative charge and the hydrogen atoms carry slight positive charges, which facilitates this intermolecular attraction.
Hydrogen bonds are weaker than the covalent bonds within the molecule, requiring about 21 kilojoules per mole of energy to break. The addition of heat, which increases the molecules’ kinetic energy, causes these weak intermolecular attractions to stretch, break, and reform. The state of water depends on the balance between the kinetic energy of the molecules and the collective strength of their hydrogen bonds.
Unique Arrangement in the Solid State
When water cools and transitions into the solid state, or ice, the molecules slow down, and hydrogen bonds lock them into a fixed arrangement. This organization forms a crystalline structure known as hexagonal ice, or ice Ih. In this lattice, each water molecule is held in a tetrahedral structure, bonding with four neighboring molecules.
The fixed arrangement creates an open framework within the crystal structure. This framework maximizes the distance between the molecules compared to the liquid state. Consequently, solid ice is about nine percent less dense than liquid water, a rare property that causes ice to float.
Dynamic Organization in Liquid and Gas
As ice melts into liquid water, the increased kinetic energy causes the hydrogen bonds to break and reform on an extremely rapid timescale. This rapid rearrangement allows the water molecules to slide past one another, giving liquid water its fluid property. While a molecule in ice is bonded to four neighbors, a molecule in liquid water is bonded to an average of about 3.4 other molecules.
The temporary nature of the hydrogen bonds in the liquid state allows the molecules to pack more closely together than in the open structure of ice, making liquid water denser. When liquid water is heated further, such as when it boils, the kinetic energy increases and overcomes all the remaining hydrogen bonds. The water molecules become highly energetic and widely separated, moving independently of one another. This complete disruption of the intermolecular forces results in water vapor, a gas with no organized structure and a much lower density than the liquid or solid states.