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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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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14 Answers

Anonymous 0 Comments

Thankfully Humboldt County is one of my favorite movies and in it Brad Douriff explains the difference.

“Solids and liquids are differentiated from one another by the way they respond to stress. If the material deforms, stays deformed or springs back when the stress is removed then it’s a solid. If instead the material flows when stressed, rearranges itself in such a way as to remove the stress, then it’s a liquid. “

Anonymous 0 Comments

A fluid flows. No matter how viscous it would be, it’s molecules move in either a laminar or turbulent flow when a force is applied. A solid molecules don’t move. If they do, the material would break.

Anonymous 0 Comments

The difference largely comes down to the amount of toilet paper you need to use when finished…a solid often requires just a quick proof wipe to guarantee it’s clean, while a thick liquid often requires a large number of wipes.

Anonymous 0 Comments

Isn’t it simple, liquids have no definitive shape but a solid does?

Anonymous 0 Comments

I propose we classify an in between state of matter to settle such disputes called “Thiquid”

Anonymous 0 Comments

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.

Anonymous 0 Comments

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.

Anonymous 0 Comments

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.

Anonymous 0 Comments

interesting addition to this: a friend once told me that glass in fact also is a liquid. does anybody know facts?

Anonymous 0 Comments

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)