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Several substances expand on freezing. Gallium expands 3.1% and bismuth 3.3%. However, both of these are much less than water which expands 9%. Is there any substance that expands more than 9% on freezing?

We can generalize this question to any system at thermodynamic equilibrium and constant pressure. Density normally decreases with temperature, but sometimes increases. For example, heating plutonium increases it's density in three places: a solid-solid phase transition, a negative thermal expansion, and melting. These effects combine to make the density increase about 5% between ~320C to 640C, which is still less than cooling 4C water to -0.01C ice.

If we allow substances that dissolve/precipitate and reversible chemical reactions can we beat water? We do not allow super-cooling or super-saturation since that isn't thermodynamic equilibrium.

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Silicon beats it.

Silicon has the familiar diamond-type semiconductor only when solid, and like the hydrogen-bonded structure of water ice the directional bonding adds volume to the solid phase. Melted silicon collapses to a structure more typical of a liquid metal, including its electrical conductivity.

Wikipedia reports the density of solid silicon at room temperature and the liquid at the melting point:

Solid at room temperature: 2329 kg/m³

Liquid at melting point: 2570 kg/m³

Thus the solid at room temperature has 10.3% more volume than the liquid at the melting point ... with the thermal contraction from melting point to room temperature included.

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