It started on the Portuguese Camino.
Walking along the river in Porto, I watched the water lead out toward the Atlantic. At some point, the river gave way to the ocean, and I found myself thinking about something that seemed almost embarrassingly simple: what actually is water?
I knew what water was made of: H₂O. Two hydrogen atoms and one oxygen atom.
But standing there, looking at all that water, the formula suddenly felt incomplete. How could something so familiar have so much going on beneath the surface? And how does a river, which we think of as fresh water, eventually become part of a vast, salty ocean?
That was what made me start reading.
From H₂O to Liquid Water
The first thing I realised was that there is a difference between knowing what a water molecule is and understanding liquid water.
A single molecule is relatively simple. But in liquid water, countless molecules are constantly interacting. Because water is polar, neighbouring molecules can form hydrogen bonds, which are continually breaking and reforming. These interactions help give water many of its unusual properties.
I had never really thought about that distinction before. I had always treated “water” as one simple thing. But a molecule of H₂O and a river full of H₂O are not quite the same scientific problem.
Why Does a Liquid Flow?
That led me to another basic question: what makes a liquid flow?
The particles in a liquid are not locked into a rigid structure like those in a solid. They can move relative to one another, allowing liquids to flow and take the shape of their container. What differs between liquids is how their particles interact, affecting properties such as viscosity.
In water, hydrogen bonding makes this molecular environment particularly complex. I also came across research suggesting that molecules in liquid water can form different local arrangements. This is sometimes described using “two-state” or “two-structure” models. It does not mean there are two separate liquids, but rather that different local structures may exist and shift between one another.
I found this surprisingly difficult to reconcile with the ordinary experience of water. Looking at a glass of it, you would never guess that scientists are still investigating how its molecules organise themselves.
Why Does Ice Float?
Then I came across something much more familiar: ice floats.
I had obviously known that. What I had never really considered was why.
For many substances, the solid state is denser than the liquid state. Water is unusual. As it freezes, its molecules form a more open structure, making ice less dense than liquid water.
That simple fact has enormous consequences. When a lake freezes, the ice remains at the surface and can form an insulating layer, allowing the water underneath to remain liquid. Aquatic organisms can therefore survive cold periods.
The more I read, the more often I found myself encountering things I had known all my life without ever asking why they happened.
Where Does the Salt in the Ocean Come From?
And then I returned, in my head, to the water I had been looking at in Porto.
Why was the ocean salty in the first place?
Around 70% of Earth's surface is covered by water, but most of that water is seawater. Average seawater contains roughly 3.5% dissolved salts, which makes it unsuitable for drinking.
The salts in the ocean come from several sources. Rainwater interacts with rocks and soils, rivers carry dissolved ions into the sea, and geological processes on the seafloor contribute dissolved substances. At the same time, materials are continuously removed through processes such as sedimentation, biological activity and chemical reactions.
So the saltiness of the ocean is not the result of one simple process. It is part of a long-running planetary system in which substances are constantly being added, transformed and removed.
That was another thing I hadn't expected when I started with such a basic question. The water in front of me in Porto was part of processes shaped by chemistry and geology over enormous periods of time.
What I Took Away from a Simple Question
By this point, my original question had taken me much further than I expected.
The answer is still H₂O. But that turns out to be only the beginning.
Looking more closely took me from molecular interactions to the unusual structure of ice, from the salinity of the ocean to geology and the conditions that allow life to exist. What surprised me was not that water turned out to be mysterious in some dramatic sense. It was how much I had been taking for granted.
We know what a water molecule is made of. But knowing the formula is not the same as understanding what happens when countless molecules come together.
And perhaps that is what stayed with me after that walk in Porto. Something as ordinary as the water beside the path can look completely different once you stop taking its simplest description for granted.