Liquid water is denser than ice due to

Liquid water is denser than ice primarily due to the unique behavior of hydrogen bonding. In liquid water, molecules are in constant motion, and hydrogen bonds frequently break and reform, allowing molecules to pack closely together. However, as water freezes, the molecules slow down and arrange themselves into a stable, rigid, and open hexagonal lattice structure [1]. In this crystalline arrangement, each water molecule forms hydrogen bonds with four others, pushing the molecules further apart compared to their liquid state [1]. This expansion increases the volume for a given mass, thereby decreasing the density of ice [1]. While other liquids typically become denser upon freezing as molecules pack more tightly, the specific orientation and strength of hydrogen bonds in water create an anomalous expansion, making the solid phase less dense than the liquid phase [1].

Sources

1. [1] https://www.swc.nd.gov/arb/news/atmospheric_reservoir/pdfs/2016_12%20-%20Ice%20Vs%20Water%20--%20Density%20Matters.pdf