- Water is confusing.

It's a lot more complicated than the basics you get into in chemistry class.

It turns out scientists don't all agree on what's happening at the molecular level.

There's even an argument that water isn't one liquid, but two.

The case of water chemistry is not closed.

There's controversy in the water world.

Yeah, controversy, heated debates, over the chemistry of plain old water.

(upbeat music) The simplest explanation of how water behaves comes down to hydrogen bonding, but the scientists who study water don't think that's the whole story.

Hydrogen bonds alone don't really account for all of water's weirdness.

Yes, hydrogen bonding is a part of the reason why water becomes less dense when it transitions from a liquid to a solid, which is already weird.

But the reality of what's going on at the molecular level is actually much weirder.

Most fluids become more dense as they get colder, and even more dense as they freeze from a liquid to a solid.

Water is not like that.

Water is most dense at four degrees Celsius.

Warmer than 4C, and the molecules move away from each other as the temperature increases, becoming less dense.

As the temperature drops back down and approaches four degrees Celsius, the molecules get closer and closer together.

But when you pass four degrees Celsius and get even colder, that's where the weirdness starts.

Instead of continuing to get more dense, you start to get molecular clusters with big gaps of space between them forming a rigid crystal structure that becomes ice at zero degrees Celsius.

The gaps make the ordered hexagonal structure less dense than liquid water.

And the expansion is bigger than you might think.

Water expands by 9%, making it way less dense and taking up a lot more space as ice.

This is why a frozen lake has a sheet of ice at the top rather than the bottom.

But oddly, the lake gets warmer the farther down you go.

The densest 4C water sinks to the bottom.

This is super different from other materials where a dense solid would sink to the bottom and warmer, less dense layers would float on top.

That's because water molecules, either in ice or in the liquid form are held together by hydrogen bonds.

Hydrogen bonds form between water molecules because the oxygen and the hydrogen don't equally share the electrons that keep them covalently bonded together.

The oxygen pulls the electrons just a little bit harder, giving the oxygen a slight negative charge and the hydrogens a slight positive charge.

This means that a slightly positive hydrogen on one molecule can create a hydrogen bond with a slightly negative oxygen on another.

All of this hydrogen bonding on a large scale form hydrogen bonded network that creates a really high surface tension.

Hydrogen bonds between water molecules are constantly breaking and forming in the liquid as it moves.

But each molecule is always reaching out to the next.

It's as if the water molecules are holding hands and able to form long chains and sheets of water that can support a tiny water bug or pull a stream of water up a thin channel by capillary action, or bead up on a hydrophobic surface.

And those hydrogen bonds have to be just the right strength to do all of these things and support life as we know it.

It is a super incredible balance.

They have to be strong enough to provide some structure and surface tension, but weak enough to provide flexibility.

Too weak and the molecules in your cells might fall apart, but too strong in your water filled blood might move through your veins like honey.

In fact, if the hydrogen bonds were just 7% stronger, water would be a solid at most temperatures found on Earth.

Without just the right hydrogen bond conditions, life may not have evolved at all.

So that's hydrogen bonding, but hydrogen bonding is not the whole story.

It turns out that when you look very closely at molecular interactions in water, they're doing something strange and totally unexpected.

Studying the bonds between water molecules is hard because they're just so small and light, and because the hydrogen bonds are really easily broken.

People have tried using X-rays, Inspectra, and neutron methods of observing water, but no method has ever been sensitive enough to watch what hydrogen bonds in water are doing in real time.

So one group of researchers has built the tool to do it, the amazingly named Mega-Electron Volt Ultra Fast Electron Diffraction Instrument.

I love it.

It sounds like a super villain tool.

They used it to make a stop motion movie of how the water molecules moved when excited by infrared light.

First, the scientists excited the molecules in a really thin stream of water with an infrared laser.

Then they fired super high energy mega electrons at the water to see how they would bounce and reflect off of the individual water molecules.

By capturing a series of these defractions, they were then able to then put them together to create a little water molecule movie.

Imagine that the water molecules were like marbles in a dish, and you started carefully shaking just one marble really fast.

What you'd expect would be that the excited marble or water molecule would move first, and then the surrounding molecules would be pushed away a bit.

But what they saw was that the hydrogen bonds between the molecules were actually the first thing that changed.

Initially, the hydrogen bonds between the water molecules strengthened the hydrogens of an excited water molecule pulled neighboring oxygens toward them, then the bonds pushed the oxygens away.

This means that the excited molecule didn't move first, its neighbors did.

This is the first time that scientists have directly observed the nuclear quantum effect of hydrogen bonds.

Because hydrogen is so small, its nucleus isn't bound by the rules of classical mechanics and can display weird quantum properties, like tunneling, the ability to pass straight through a barrier rather than traveling over it.

This effect where the excited water molecule's hydrogen bonds pull oxygens in and then push them away is called a quantum tug.

And the researchers who built this mega electron volt ultraflacked, ultra, I thought I could do it.

Who built this mega electron volt ultrafast electron defraction instrument were the first to ever actually observe this happening.

And this is really important to biological molecule.

For example, the two sides of the double helix structure of my favorite molecule, DNA, are held together by hydrogen bonds.

But the quantum nature of the bonds means that different atoms in the molecule can actually share the hydrogen protons, which helps to stabilize the overall structure.

Like the different atoms are sharing hydrogen protons, so just keeping 'em all stable.

It's so cool and I had no idea.

Adding quantum mechanics into our understanding of hydrogen bonds starts to make water seem like a complicated liquid, while some scientists think it's not a complicated liquid, but more like two normal liquids with a complicated relationship, the worst Facebook relationship status.

And yeah, you heard me right.

There could be two separate liquids that make up water.

The idea is that there are actually two densities of water simultaneously happening in a glass, HDL and LDL.

And sure, they sound like your cholesterol tests, but they actually stand for high density liquid and low density liquid.

LDL water looks like what you'd think.

There are nice ordered hydrogen bonds between the molecules, and this drives the structure of the water.

It resembles the tetrahedral structure that we see in ice.

But in HDL, the molecules are closer together.

Still hydrogen bonded to some of their neighbors, but compressed closer to others.

At normal room temperature, water molecules are drifting back and forth between groups of LDL or HDL water, constantly forming and reforming local groups.

But to investigate them better, you can change the ratios of the two by changing the environment.

By preventing water from forming ice crystals, usually using something like an oil and water emulsion, scientists found that at cooler temperatures, like negative 100 degrees Celsius cold, higher pressures can lead to more HDL while lower pressures can often lead to more LDL.

Super cooling very, very pure water can also let you look at liquid water below zero degree Celsius.

with nothing to seed ice crystal formation, the water can stay liquid while dipping well below typical ice temperatures.

Observations of liquid water made in these very cold environments, sometimes dubbed the no man's land of water, allow us to see some of these weird density swapping water properties, and researchers believe, see how interactions between LDL and HDL make water you unique.

But not everybody agrees that these two densities alone are what's driving the weirdness of water.

Other groups have looked at the shapes that water molecules form when they interact, also known as their local structures.

So that classic shape of water molecules you always see, the one you've seen several times already in this video, that is an oversimplification.

And the truth is of course, controversial.

Current evidence suggests that some of the water molecules will form nice tetrahedral structures with each other, known as LFTS or locally favored tetrahedral structures, while others will form a kind of shifted elongated pyramid, a disordered normal liquid structure, or DNLS.

So it could be that the presence of these two structures rather than the HDL versus LDL relationship explains some of the weird ways the water behaves, including its expansion when frozen and weird surface tension.

This mixture model actually dates back to Wilhelm Rontgen.

Okay, Wikipedia tells me it is Wilhelm Rontgen.

Wilhelm Rontgen, who metaphorically suggested that water was a mixture of icebergs and fluid seas, implying that water might be a series of ice like crystals suspended in a more fluid liquid, but it wasn't until recently that anyone had actually shown this experimentally.

Rontgen's ice like crystals in a fluid sea have now been observed, except instead of icebergs and water, it's the relationship between locally favored tetrahedral structures and disordered normal liquid structure.

So yeah, we're still learning about the basic shape of interacting water molecules.

Still now, and one more controversy.

And if I'm being fair, this is actually just more of a personal pet peeve than an actual controversy, but I can't do a video about water and not say it.

This glass of water, it is not dinosaur pee.

This myth is all over the internet and I do get it.

The idea is that since matter can't be destroyed or created, and water has been around forever in terms of the earth, the water that you're drinking likely passed through a dinosaur a long time ago, and conservation of matter is important, I agree, but all of that is happening at the atomic level.

And we are talking about a molecule, one of the most important molecules.

It is constantly undergoing reactions.

Just think of every plant, every tree, every organism on Earth that undergoes photosynthesis.

It is constantly smashing carbon dioxide and water together to create sugars, turning sunlight into the energy that powers literally everything on Earth, and the oxygen that plants exhale actually comes from splitting water apart.

Water is constantly reacting with salts and acids, and bases in the oceans and in your kitchens, and water is created all the time too.

Every time you burn fuel, you are creating carbon dioxide and heat and water, and all around us, all the time, water molecules are being broken and destroyed, and created and consumed.

And they're not the same water molecules that existed with the dinosaurs.

Though fine, the atoms in the water were probably once in a dinosaur, but to say that your delicious fresh glass of water was once peed out of a brontosaurus, it denies how infinitely cool water chemistry actually is.

So I'm sorry.

Or you're welcome.

I'm yelling at you about not drinking dinosaur pee, but it just isn't, it's not how any of this works.