In your question you also add that this fact is counterintuitive, since the logical thing would be for it to expand with heat and shrink with cold. And you are absolutely right, that would be to be expected. Your question is the one we all ask ourselves when we put the full bottle of water in the freezer.
The answer to your question is that water has a special quality. Most liquids when cooled take up less space and when heated they take up more. But water is special and behaves differently from other substances. And this is so because of the molecules that compose it. As you know, water is H2O. Each water molecule has two hydrogen atoms and one oxygen atoms. How hydrogen atoms bind together is what explains this special quality of water. When it is in a liquid state, the bonding of these hydrogen atoms is loose and that is why liquid water moves easily. But just at 0ºC, the bond between the hydrogen atoms becomes stable and the water organizes itself, which we call crystallization. What is an orderly grouping, which in the case of water is precisely what makes it occupy more space.
Water is the only substance that has this behavior
Normally substances when frozen take up less space , but water takes up more because its structure is hexagonal, which means that there are fewer molecules in the same volume. And that's why it expands when it freezes. It is also true that from a temperature of 4ºC, water behaves like other substances. As it warms up, it increases in volume. But between 0 and 4 ºC it behaves in that special way because of hydrogen bonds. And it is the only substance that has this behavior.
The increase in volume that it undergoes when it is at these temperatures, between 0 and 4 ºC, is between 9 and 10% of its volume. And this has environmental consequences. The most important is that when it freezes, the water loses density and for that reason the icebergs float in the sea. Because frozen water floats, that ice sheet prevents the entire body of water in the oceans at the Poles from freezing. Floating surface ice acts as an insulator. For this reason, at the Poles, despite the very low temperatures, the entire sea is not frozen, only the top layer. And that is key to the environmental functioning of the planet.
Carolina Guardiola is a doctor in hydraulic engineering and the environment, a senior scientist at the National Geological and Mining Institute of Spain (CSIC), an expert in hydrogeology and geomathematics.
Question sent via email by Emilio
Coordination and writing: Victoria Toro
We respond is a weekly scientific clinic, sponsored by the Dr. Antoni Esteve Foundation and the L'Oréal-Unesco 'For Women in Science' program , which answers readers' questions about science and technology. They are scientists and technologists, members of AMIT (Association of Women Researchers and Technologists) , who answer these questions. Send your questions to firstname.lastname@example.org or via Twitter #nosotrasrespondemos.
You can follow MATERIA on Facebook , Twitter e Instagram , or sign up here to receive our weekly newsletter .