Hydrogen Bonds: Why Ice Floats

Also called

hydrogen bond · hydrogen bonding · intermolecular hydrogen bond · why ice floats · structure of ice · hexagonal ice lattice · open cage structure of ice · density of ice · anomalous expansion of water · water densest at 4 °C · pond freezes from the top · DNA base pairing · A–T and G–C hydrogen bonds · हाइड्रोजन आबंध · बर्फ़ · हाइड्रोजन बंध

Topics

• A hydrogen bond is a weak attraction between an H bonded to N, O or F and a lone pair on another N, O or F.
• It is far weaker than a covalent bond, so in liquid water hydrogen bonds keep breaking and re-forming.
• On freezing, each water molecule holds four others by hydrogen bonds in an open hexagonal lattice.
• The open lattice has empty space, so ice (0.92 g cm⁻³) is less dense than water (1.00) and floats.
• A floating block sinks until it displaces its own weight: about nine-tenths of the ice is under water.
• Water is densest at 4 °C, so colder water stays on top and a pond freezes from the surface downwards.
• The ice sheet insulates the water below, which stays near 4 °C, so fish survive the winter.
• In DNA, A pairs with T by two hydrogen bonds and G with C by three: strong enough to hold, weak enough to unzip.

About this model

A GLB file of about 624 KB, shown in Lably's 3D viewer and, on a phone, placed in your room with AR. Credit: Model, science and animation: Lably (built procedurally with three.js; no downloaded meshes). | CC BY 4.0. Original geometry and animation made for Lably.

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