Why Does a Sealed Water Bottle Crack in the Freezer?

[Gamma]

This is not a homework question, but a conceptual question that I am trying to understand.

We all have experienced/heard that if you put a completely full water bottle with the cap on in the freezer, the bottle cracks as the water freezers. The explanation is that the water expand as it freezes. But if you look at the V vs. T graph of water, it start to expand only below 4 Celsius. So the bottle has enough space to hold the expanded volume. Since, in real life, the bottle actually cracks, what am I missing?

Thank you! - Gamma.

 

[TSny]

Hello, Gamma.

When the water freezes, ice is formed. What is the density of ice compared to water at 0 ^oC ?

http://en.wikipedia.org/wiki/Ice

 

[Gamma]

got it. thanks! feel stupid.

 

[TSny]

It's a good question. For most substances, the solid state is more dense than the liquid state. Water is one of the exceptions.

 

[Nickie]

I can explain this phenomenon by first understanding the concept of thermal expansion. When a substance is heated, its molecules gain energy and move farther apart, causing the substance to expand. The opposite happens when a substance is cooled - its molecules lose energy and move closer together, causing the substance to contract.

In the case of water, it follows a unique behavior known as the "anomalous expansion of water." This means that as water is cooled from room temperature, it will contract and become denser until it reaches 4 degrees Celsius. However, as it continues to cool below 4 degrees, it will start to expand again. This is due to the formation of hydrogen bonds between water molecules, which cause them to arrange in a lattice structure and take up more space.

Now, let's consider the scenario of a full water bottle being placed in the freezer. As the water cools, it will initially contract until it reaches 4 degrees Celsius. At this point, the water will start to expand again, but since the bottle is sealed, it has no room to accommodate this expansion. This leads to an increase in pressure inside the bottle, which can eventually cause it to crack or burst.

It is also worth noting that the expansion of water is not uniform - it is greatest near its freezing point and decreases as the temperature decreases further. This means that even if the bottle has some room to accommodate the initial expansion at 4 degrees Celsius, it may not be enough to prevent cracking as the temperature continues to decrease.

In conclusion, the cracking of a water bottle in the freezer is due to the unique behavior of water's thermal expansion, where it expands upon further cooling below 4 degrees Celsius. This can lead to an increase in pressure inside a sealed container, causing it to crack or burst. I hope this explanation helps you understand this concept better.