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Water is a simple molecule, yet it has unique properties that have profound effects on our world. At its most basic level, a water molecule consists of one oxygen atom bonded to two hydrogen atoms, forming a 'V' shape. The molecular structure is polar, meaning there is an uneven distribution of electron density. Oxygen is more electronegative, drawing electrons towards itself and away from the hydrogen atoms, which results in partial charges 鈥� the oxygen becomes slightly negative, and the hydrogens become slightly positive. This polarity is crucial for the water's behavior and is responsible for the many unique properties of water, including its behavior as a liquid and its ability to expand as it freezes into ice.

The polarity of water leads to hydrogen bonding, where the positive hydrogen side of one water molecule is attracted to the negative oxygen side of another.
Hydrogen bonds are weak compared to covalent bonds, but they are strong enough to have significant effects on water's properties. These bonds are constantly forming and breaking in liquid water, which allows for fluidity.
However, as water cools, the hydrogen bonds last longer, and when it freezes, a stable network of hydrogen bonds holds the water molecules in a rigid structure.

The states of matter鈥攕olid, liquid, and gas鈥攁re heavily influenced by the interactions between molecules. In liquids like water, molecules are in contact but can move relative to each other, allowing the liquid to flow and take the shape of its container.
As water cools to its freezing point, the motion of the water molecules decreases and they settle into a fixed pattern. Solid water鈥擨ce鈥攆orms because the extended hydrogen-bonding network locks the water molecules into place, which contrasts with the random, more compact arrangement in the liquid state.

When water freezes, the relatively open hexagonal structure of ice forms. Because of the angle of the hydrogen bonds, water molecules in ice are spread further apart than in liquid water, creating a crystal lattice that maximizes the distance between the negatively charged oxygen atoms.
This arrangement is less dense than the random, closer arrangement of molecules in liquid water. Consequently, ice is less dense than liquid water, a unique and important characteristic that has implications for aquatic life and the environment as it allows ice to float, providing insulation for bodies of water during cold weather.