What is the difference between an extremely thick liquid and a solid? At which point does the difference stop mattering, it at all?
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The pitch in the University of Queensland pitch drop experiment (google it, it’s really interesting) is liquid, but there is about a decade between each drop falling.
Glass on the other hand is solid, despite the myth that really old windows are thicker at the base because they’ve flowed (they were just made that way back then).
The biggest difference is that if you put increasing stress on the pitch, it’d flow faster, but glass wouldn’t flow at all as long as it could withstand the forces.
>What is the difference between an extremely thick liquid and a solid?
None. Almost every solid will flow if you have enough of it and apply enough stress. [Ice](https://www.nature.com/articles/s43247-022-00385-x), [rocks](https://press.princeton.edu/books/hardcover/9780691644929/viscosity-of-the-earths-mantle), [steel](https://www.jstor.org/stable/24100561#metadata_info_tab_contents), even [diamond](https://www.sciencedirect.com/science/article/abs/pii/S0925963500004003).
Size and scale really matter. We think of molasses as a very thick viscous fluid, but if you have enough of it … Dozens of people were killed in the [Great Molasses Flood](https://www.history.com/news/great-molasses-flood-science) in Boston in 1919, when a burst storage tank released a wave of molasses that traveled up to 35 miles an hour, far too fast to outrun.
The only real difference between a liquid and a solid is how much you have, how strong the forces on it are, and how long you’re willing to wait for it to move.
The difference between a liquid and a solid is the organization of the particles at a molecular level. There is in fact a bright-line difference between solids and liquids, a point at which liquids suddenly stop being liquid.
It’s not gradual! It is much more of a “poof” now I’m not a liquid anymore moment. Think of ice and water – water doesn’t gradually thicken until it is ice, it gets cold and then poof, it doesn’t flow at all anymore!
To describe the difference more technically, **solids have a crystal structure** – a fancy way of saying that the molecules are arranged in a repeating geometry – rows of tiny rectangles or hexagons or whatever. Solids have a fixed volume and do not fill the space they inhabit.
**Liquids do not have a crystal structure.** Liquids have molecules that are all in contact with one another, but roll past each other randomly, any old way, with no repeating patterns. Think of this like digits in pi, they never repeat, and there’s no way of predicting which one will come next. Liquids fill spaces they inhabit and have a fixed volume.
(Gasses fill the space they inhabit and fill the volume they inhabit.)
The transition between being a liquid and a solid happens as the molecules lose the energy that keeps them from forming crystals. Temperature stays constant in this transition phase, and so does the state of matter.
If this seems weird and counterintuitive – it is! Liquids are weird and frankly exotic as a state of matter. Under most possible conditions for a given material, being liquid simply isn’t a thing. Your guess in your initial post, that a substance would get gradually thicker and thicker as it cools is what happens to most stuff; it simply skips becoming a liquid or solid and becomes a very dense gas.
The difference between a thick liquid and a solid is that a solid is less compressible and a liquid can flow.
The problem is that these depend on pressure and temperature. Look at this example
When I skate on ice skates, the pressure of my skate blade on the ice causes the solid ice to change to liquid water under my blade. I do not “cut” the ice, but melt a tiny track of water that refreezes after my skate passes by.
If I take a substance like roofing tar or pitch (a thick oily substance that looks like a solid) and put enough pressure on it (or warm it up) it will flow like a liquid.
At one pressure and temperature it is a solid, at another it is liquid.
Look at [this lava flow in Hawaii](https://www.youtube.com/watch?v=_hyE2NO7HnU)
You question has lots of complexities.
The quick answer is that many materials have distinct behaviours at various temperature / pressure combinations.
So water behaves like a gas (water vapour or steam) above 100 C at 1000 kpa
Cool this down to 90 C and you have mostly liquid water
Cool this down to -4 degrees and you get mostly solid ice.
This is all at 1 Atmosphere of pressure.
If you play with temperature and pressure you find that each substance has its own behaviour for solid, liquid and gas. Many substances have multiple solid or liquid states. This is often charted in something called a phase diagram.
Some materials have a very clean [phase diagram](https://en.wikipedia.org/wiki/Phase_diagram) describing the temp and pressure combinations tha
The dictionary definitions below show how imprecise this is.
* Fluid – having particles that easily move and change their relative position without a separation of the mass and that easily yield to pressure : capable of flowing [source](https://www.merriam-webster.com/dictionary/fluid)
* Liquid – a fluid (such as water) that has no independent shape but has a definite volume and does not expand indefinitely and that is only slightly compressible [source](https://www.merriam-webster.com/dictionary/liquid)
* Solid – a substance that does not flow perceptibly under moderate stress, has a definite capacity for resisting forces (such as compression or tension) which tend to deform it, and under ordinary conditions retains a definite size and shape [source](https://www.merriam-webster.com/dictionary/solid)
interesting addition to this: a friend once told me that glass in fact also is a liquid. does anybody know facts?
The most viscous liquid is, from engineering pov, still a liquid. You can’t trust it to keep its shape over time.
On the other hand, if you design something with plastic, rubber, or similar materials, despite them being solids, their plasticity is to be kept in mind and they may flow, change their shape into their container’s shape, or being squeezed away from their place. Which are all properties of a liquid. This is the most close example I can imagine to the liquid vs solid border.
Digression:
All in all, remember, the definitions we use are for ease of defining things, matter does what it wants despite our description.
I think the first argument I won vs my manager was about replacing a spring that I declared being plastically deformed (elongated). He rightfully said that the material and the spring itself are by definition not plastic. To which I demonstrated that the spring did what it wanted to do, without caring of our definitions. “Collective control balance spring replaced, aircraft ok for service” was my next written line. This said, my manager and the materials we used did their job the other 99.99999% of the time. So yeah… definitions… good the 99.99999% of times.
The top answers are not wrong, but maybe aren’t basic enough to count as ELI5. They way I look at it is that in a liquid, the molecules can move around each other. They might move really slowly, but they can still move relative to each other. In a solid, the molecules can’t. They are either too big, or too tangled up, or stuck together too well. If you apply enough energy to force them to move past each other, they will break apart completely.
There are materials that are both liquid-like and solid-like depending on how fast you apply stress. “Apply stress” just means that you try to change their shape by pushing and pulling on them. Silly putty is the classic example. If you throw it hard against a table, it acts like an elastic solid and will bounce back. The molecules can’t get out of each other’s way in the amount of time it takes to hit the table, so they kind of coil up or stretch out and act like billions of little springs instead. But if you leave it sitting out on the table, over time it will melt into a puddle. That’s because the molecules will eventually wiggle and slide around each other if you give them enough time. This behavior is called “viscoelasticity.”
Temperature also matters, because molecules move faster when things are warmer. So at higher temperatures, viscoelastic materials will flow more easily, and at lower temperatures they will act more like solids. Most substances will become completely solid at a low enough temperature, which you already know happens with things like water turning to ice.
Even a thick liquid will eventually flow to settle in the bottom of its container. This is essentially the definition of a liquid; it is a fluid but not a gas (air is also a fluid). The thickest liquids may take years to move noticeably.
A solid does not flow.
I propose we classify an in between state of matter to settle such disputes called “Thiquid”
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