The correct answer is option 2, hydrogen bonding.
Hydrogen bonding is a type of intermolecular force that occurs between molecules containing hydrogen atoms bonded to electronegative atoms such as oxygen, nitrogen, or fluorine. In the case of water, each water molecule is composed of two hydrogen atoms bonded to one oxygen atom, resulting in a bent shape.
In the solid state, water molecules are arranged in a hexagonal lattice structure, which gives ice its characteristic shape. The hydrogen bonds between water molecules in ice form a stable network that holds the molecules apart, resulting in a lower density compared to liquid water.
When ice melts into liquid water, the hydrogen bonds between water molecules break, allowing the molecules to move more freely. As a result, the water molecules in the liquid state pack closely together, leading to a higher density than ice.
Options 1, 3, and 4 are incorrect explanations for the higher density of liquid water compared to ice. Surface tension (option 1) refers to the cohesive forces between the molecules at the surface of a liquid and does not directly affect the overall density. Van der Waals forces (option 3) and covalent bonding (option 4) are not directly responsible for the density difference between ice