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Why Does Water Expand When It Freezes?

If you’ve ever left a water bottle in your freezer for too long, you understand all too well that water expands when it freezes into ice. But have you ever paused to wonder why? As it turns out, this expansion runs contrary to some of the most fundamental natural laws in the world of physics.

If you search the annals of your brain for some basic facts from your junior high science class, you might remember that molecules tend to move closer and closer together as they are cooled. This generally causes the overall volume of a substance to decrease under frigid conditions.

Therefore, most liquids shrink, rather than expand, as intermolecular forces draw their compositional molecules together in the cold. Furthermore, when a liquid reaches its particular freezing temperature, it tends to contract even further. This is because solidified liquids (otherwise known as “crystalline solids”) are typically tightly packed and extremely dense.

Water, however, has a unique crystalline structure that places its molecules in a remarkably uncompressed shape. This is the fundamental reason that water expands when it freezes.

Few molecules are more well-known than water. Both water and ice molecules are comprised of one oxygen atom (O) and two hydrogen molecules (H) to create H20.

The negative charge of water’s oxygen atom and the positive charges of its hydrogen atoms are predominately responsible for the expansive properties of ice. These charges create a phenomenon that causes neighboring molecules to repel one another during ice’s crystalline formation.

Like all liquids, water continually contracts as it cools until it approaches its freezing point of 32 degrees Fahrenheit (0 degrees Celsius). At this point, it reacts in a counterintuitive manner by suddenly beginning to expand.

As water solidifies into ice, it assumes a defined molecular shape that is considerably less dense than its molecular shape as a liquid. The sheer amount of space between the molecules in this shape separates each molecule with vast structural gaps. At the macro level, this results in a significant volume increase as water transitions from its liquid to its solid state.

Water stops compacting and begins expanding at around 40 degrees Fahrenheit (4 degrees Celsius). This is when it reaches its maximum level of molecular density. As it continues to cool to its freezing point, the overall volume of any specified amount of water will grow by roughly 9 percent.

This common chemical change might wreak havoc on our roads and cause other seasonal problems, but we should actually be quite grateful that water expands as it freezes. Because ice is less dense than water, it floats at the top of any freezing lake, sea, or ocean. If it behaved like most liquids, these bodies of water would freeze solid throughout at extremely cold temperatures, killing most of the plant and animal life within them.