What is the difference between an extremely thick liquid and a solid? At which point does the difference stop mattering, it at all?


What is the difference between an extremely thick liquid and a solid? At which point does the difference stop mattering, it at all?

In: 58

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)