What is a hydrogen bond, and how does it affect the properties of water?

A hydrogen bond is a strong type of intermolecular (between molecules) force that occurs when a hydrogen atom is bonded to a highly electronegative atom (like nitrogen, oxygen, or fluorine) and is then attracted to another electronegative atom in a different molecule. In water (H2O), the oxygen atom is more electronegative than the hydrogen atoms, resulting in a polar covalent bond. This means that the oxygen atom carries a slight negative charge, and the hydrogen atoms carry a slight positive charge. The slightly positive hydrogen atom in one water molecule will then be attracted to the slightly negative oxygen atom in another water molecule, creating a hydrogen bond. Hydrogen bonding significantly impacts the properties of water. It increases water's boiling and melting points compared to what they would be without hydrogen bonding. It also contributes to water's high heat capacity (it can absorb a lot of heat before changing temperature), its high surface tension, and its unusual behaviour when freezing (it becomes less dense as a solid than as a liquid).