Why Does Ice Float?

Water is different. With most everything on Earth except freshwater, the colder it gets, the more dense it becomes. Take alcohol for instance. If we were to fill up a 1 liter container with pure alcohol at 30 degrees Celsius (86 Fahrenheit) and then take another 1 liter container and fill it with pure alcohol at 10 degrees Celsius (50 Fahrenheit) the cooler container of alcohol would weigh more. This is because the cooler alcohol is more dense, so more alcohol molecules can fit in the same container. This is true with freshwater too, up to a point. But at about 4 degrees Celsius (40 Fahrenheit) water reaches its densest point. Amazingly, as water cools further, it actually becomes less dense.

Each water molecule is made of 2 hydrogen atoms and 1 oxygen atom. These are connected to one another by very strong chemical bonds called covalent bonds. Water molecules are connected to each other by much weaker chemical bonds called hydrogen bonds between the positively charged hydrogen atoms, and one negatively charged oxygen atom in a neighboring water molecule.

As water gets colder than 4 degrees Celsius (40 Fahrenheit) the hydrogen bonds connecting different water molecules adjust to keep the negatively charged oxygen atoms apart. This results in a crystal latice which begins to form at less than 4 degrees Celsius. This crystal latice is completely formed at freezing, and is commonly known as ice.

So, why does ice float? Like most things that float, ice floats because it is less dense than liquid water. Ice is about 9% less dense. When ice forms, it takes up about 9% more space than it did as a liquid. Thus, a 1 liter container of ice weighs less than a 1 liter container of liquid water, and the lighter material floats to the top. As we said, water is different.

Dragonfly Challenge:
Imagine a world where ice was more dense than liquid water. What would this world be like? What would the consequences be for life in your local freshwater streams and lakes. Write a story or an essay about this, and send it to us at: Dragonfly@MUOhio.Edu

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