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Water is one of the most abundant and important substances on Earth, so you think we'd know everything there is to know about it. But it turns out water is way weirder, and it often behaves in ways that leave scientists with more questions than answers.

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Water is among the most abundant compounds in the universe. It makes up about 70 percent of the Earth's surface and about 60 percent of our bodies.

And the odds are pretty good that you've already encountered a bunch of it today. But despite how common water is, it continues to baffle scientists because it behaves unlike anything else out there. Seriously, water is way weirder than you'd think.

First off, liquid water. You know, the super familiar stuff you drink and wash your hands with. Yeah, turns out, liquid water is complicated.

Because it seems to have not one, but two liquid phases that occur at the same time. This likely happens at very low temperatures and high pressures — around -45 degrees Celsius and 2400 times the normal atmospheric pressure. So, it's not something you'd encounter every day.

Regardless, under these conditions, water can spontaneously split into two liquid phases that coexist like the oil and vinegar in your salad dressing: in separate layers, each with its own density. The low-density portion is made of the standard tetrahedral pattern of water molecules, where a central molecule is linked to four neighbors. But the high-density liquid has an extra molecule trying to squeeze into the group.

So far, scientists have only observed this in a computer model, partly because, well, those temperature and pressure conditions are timely and expensive to replicate. But if this idea holds up in physical experiments, it could help explain water's other weird properties. Like how ice has regions of low and high density, which is how it floats on water.

These regions could somehow be frozen remnants of the two liquid phases. And if so, the two densities could help us develop a model that predicts how water will behave from super cold temperatures to the ones we experience all the time. Since water is such an integral part of our world, a model like that could be useful for all sorts of research.

So, not too shabby! Moving out of the liquid phase, we have our next weird thing: Scientists can't figure out when water starts to act like a glass. If that sentence sounded weird... yeah, that's fair.

Because the glass phase is a weird one. It's a sub-state between solid and liquid, where water can exist... well, like glass. In short time scales, it looks like a solid, but in reality, it's very slowly relaxing into a liquid state.

Water isn't the only substance with a glass phase, but how it gets there seems to be unique. Typically, as other substances are heated, they experience a gradual increase in heat capacity, which is the amount of heat needed to raise their temperature one degree Celsius. Their heat capacity continues to rise until it reaches the glass transition temperature, where it suddenly jumps 100% higher.

At which point, it's officially in the glass phase. But as water is heated, its heat capacity barely changes until all of a sudden it crystallizes and becomes a solid. This has made it difficult for scientists to pin down a glass transition temperature.

Right now, they think it happens somewhere around -123 to -53 degrees Celsius. But it's been so hard to figure out anything more specific that they've dubbed this window “no man's land.” At this point, it's not totally clear what's going on, but at a minimum, this tells us that something about water's heat capacity isn't normal. That its temperature doesn't change like we'd expect.

Scientists have been looking into it, though, because understanding water's glass phase could really come in handy. After all, this form of water is actually the most abundant in the universe and appears in a number of places, even in space aboard interstellar dust particles. So, uncovering the secrets of glassy water could help us understand how it forms and shapes our solar system.

Of course, once you move past regular H2O, things start getting even weirder. Like, apparently, you can't explain exactly how water acts in your body without involving quantum mechanics. Scientists reported this in a 2019 paper, where they were studying mixtures of water and charged polymers.

These kinds of solutions are found in your joints, and they're really thick and viscous; much more than you'd expect. Which is helpful for your knees, but overall, kind of confusing. For a while, we thought this viscosity was caused by repulsive interactions between the polymers, where similar electric charges repelled each other.

But in their paper, this team found there's much more to the story. They discovered that the polymers' electric charge also affected how water molecules were interacting with each other. These interactions made water's hydrogen bond network more ordered.

And that made it hard for molecules to move and hindered the flow of the solution, therefore making it more viscous! That by itself was a cool result, because it showed you can't treat water as a neutral background for chemistry like we sometimes tend to do. It's an active molecule you need to pay attention to.

But what's stranger is what this team found next. In their study, they also looked at solutions of charged polymers and heavy water. That's water made of oxygen and deuterium, a form of hydrogen with twice the usual mass.

This solution behaved a lot differently than the one with normal water. The molecules interacted in different ways, and the viscosity was different. In fact, these changes were so significant, they couldn't actually be explained with traditional chemistry models.

Instead, the team concluded you need to consider quantum mechanics to fully understand them. It's hard to say exactly how the quantum world comes into play here. But ultimately, the team suspects these effects influence how hydrogen bonds break in each type of water, because that would affect the viscosity of the solutions.

So, water can't be ignored, and it's way more complicated than it seems on the surface. But learning more about how this all works could help us learn more about applications for polymers solutions and how water behaves in our bodies. Because of its abundance and how big of a role it plays in our universe, we can take water for granted.

But it's really strange stuff. And understanding why can help us advance lots of scientific fields. So the next time you take a swig of water or see a rain cloud, know that you're looking at something truly extraordinary.

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