Is toothpaste solid or liquid?

Question

My teacher didn't answer this properly:

Is toothpaste solid or liquid?

You can't say toothpaste is a solid because solid material have a fixed shape but toothpaste doesn't. However, you can't say it's a liquid because liquids flow easily but toothpaste needs a certain force to push it out of the tube. So is it a solid or liquid? And are there any other example just like toothpaste?

Though the definitions might seem similar, "solid" and "liquid" (especially "liquid") have crude meanings for anyone. I gave up using them a while ago. :) — M.A.R., Jan 18 '15 at 15:29
just out of curiosity, what "improper" answer did your teacher give you? — KutuluMike, Jan 19 '15 at 19:21
@Simon-Nail-It That certainly is an improper answer. It may not be obvious what toothpaste is, but it's definitely not a plasma. — Reinstate Monica -- notmaynard, Jan 20 '15 at 15:42
@iamnotmaynard His teacher probably did not mean the state-of-matter or phase Plasma, but rather the archaic/colloquial term (more often "Plasm") for a highly viscous/collodial liquid. — RBarryYoung, Jan 20 '15 at 21:16

score 106 · Accepted Answer · edited May 31 '20 at 03:24

106

Toothpaste is what is called a non-newtonian fluid, more specifically toothpaste is a Bingham plastic. This means that the viscosity of the fluid is linearly dependent on the shear stress, but with an offset called the yield stress (see figure below). This yield stress is what makes it hard to say whether it is liquid or solid. The fact that toothpaste is viscous alone is not sufficient to explain this, because water is also viscous, but doesn't behave like a solid (unless frozen, but that's another phenomenon).

enter image description here

What the yield stress does is the following. Below a certain shear threshold the fluid responds as if it were a solid, as you can see happening when you have put toothpaste on your toothbrush, it just sits there without flowing away. A highly viscous but newtonian fluid would flow away (although slowly as pointed out by @ron in his comment to the answer of @freddy).

Now if you put sufficient shear stress on the toothpaste, when you squeeze the tube of paste, it will start flowing and respond as a liquid.

Other examples, as mentioned in the Wikipedia link in my first sentence, are e.g. mayonnaise and mustard. Another example is silly putty.

edited May 31 '20 at 03:24

andselisk

37,604
14
131
217

answered Jan 18 '15 at 16:02

Michiel

6,700
3
31
61

57

I tried explaining this to the TSA, they didn't buy it. – Anubian Noob Jan 19 '15 at 23:14
10

@AnubianNoob What is a TSA? Some sort of teaching assistant? – Michiel Jan 20 '15 at 08:20
2

And what exactly didn't they buy about it? Just curious :) – Michiel Jan 20 '15 at 08:26
16

@Michiel, TSA is the American border agency. There are rules dictating the volume of liquid you're allowed to take on a plane with you. – Holloway Jan 20 '15 at 09:28
2

@Trengot it is not a border agency, but a transportation authority. Any mass transit system (planes, trains, boats) has TSA agents at security check points, not just those entering or exiting the borders of the nation. – TylerH Jan 21 '15 at 16:56
2

@TylerH, Thanks, I stand corrected (although it was close enough to explain the joke). – Holloway Jan 21 '15 at 17:13
1

Mayonnaise is a liquid? I thought that emulsions are outside of the pure solid/liquid/gas scale. – rumtscho Jan 21 '15 at 17:59
1

@Trengot Sure, I just wanted to make the distinction in case the definition you gave is used elsewhere :-) – TylerH Jan 21 '15 at 18:19
3

I just have to post this non-newtonian fluid video here, sorry: https://www.youtube.com/watch?v=3zoTKXXNQIU – Tomáš Zato Mar 13 '15 at 11:51

Fred Senese · Answer 2 · 2015-01-18T18:59:15.173

Here's a genchem-level answer for a genchem-level question about the classification of matter:

Toothpaste is a sol: a stable suspension of tiny solid particles in a liquid. When the toothpaste dries out you can see what the solid part alone looks like.

Mixtures with more than one phase often have interesting properties and behaviors that the components alone don't have; the other answers to your question touch on some of these.

Other examples of sols are paints and solid-pigment inks. Again, you can see the solid part when they dry.

Tanner Swett · Answer 3 · 2019-07-23T18:36:10.920

Here's the boring answer: Toothpaste is a mixture of some solids and some liquids.

The question "is it solid or liquid?" makes sense when you're talking about a substance or a mixture with a single phase—that is, a substance or mixture that's pretty much completely uniform throughout space. Examples of single-phase materials include pure water (a liquid), a chunk of copper (a solid), and sugar water (which is a liquid—including the sugar).

But some materials consist of multiple phases, and in this case, we have to ask the question "is it solid or liquid?" separately for each phase. Examples of multiple-phase materials include ice water (a mixture containing both a solid (ice) and a liquid (water)), vinaigrette (a mixture of oil (a liquid) and vinegar (another liquid)), and toothpaste (a mixture of several kinds of solids and liquids).

Freddy · Answer 4 · 2015-01-18T16:01:45.923

14

It is Viscous.

Viscous mean "having a thick, sticky consistency between solid and liquid"(dictionary meaning)

There are many more examples like tomato ketchup, honey, wax, toothpaste, etc.

To know more check out Wikipedia

edited Jan 18 '15 at 16:01

answered Jan 18 '15 at 15:00

Freddy

5,087
9
41
68

2

It is all about viscosity and the timescale of the measurement. On a timescale of a microsecond, toothpaste seems pretty solid; on a time scale of centuries, glass seems somewhat liquid. – ron Jan 18 '15 at 15:35
16

Where did you find this definition of viscous? Because I believe it is wrong. The physics definition of viscosity is the degree of resistance to gradual deformation by a shear stress, thus for a liquid to be viscous means it has a resistance to deformation, but it doesn't mean anything in the context of a solid. – Michiel Jan 18 '15 at 16:07
@Michiel I got it from Google. Just type "viscous meaning" It's normal dictionary meaning. Click here – Freddy Jan 18 '15 at 16:25
3

@Freddy well, apparently Google picked the definition from a dodgy dictionary, because any physicist will tell you that this definition is wrong. – Michiel Jan 18 '15 at 16:35
2

"glass seems somewhat liquid" - not at room temperature: http://www.cmog.org/article/does-glass-flow As @Michiel explains, viscosity is not sufficient to explain what is going on. – Francis Davey Jan 18 '15 at 16:36
@Michiel I agree with "non-newtonian fluid." On thinking more I realize we cant use viscous as a state. Sorry for that :( – Freddy Jan 18 '15 at 16:40
So, it is a viscous liquid? – L.B. Jan 21 '15 at 15:25

score 14 · Answer 5 · answered Jan 19 '15 at 16:10

Toothpaste is a colloid. just like dust particles suspended in water form suspension, colloids are much finer particles suspended in a medium. for example jelly, it is solid particles suspended finely in a liquid....and foam; that is gas suspended in a liquid. As such is deodorant spray that is liquid suspended in air(gas)

score 13 · Answer 6 · answered Jan 18 '15 at 22:26

Actually, toothpaste is both. I'm no chemist, but I am pretty sure that it can be correctly classified as a semisolid, which means exactly what you'd think. Semisolids have properties of both solids and liquids. Slime would be another example of a semisolid.

score 6 · Answer 7 · answered Apr 20 '17 at 03:59

Toothpaste is a colloid, more specifically a type of liquid dispersed in a solid dispersing medium. The solid component is much more in proportion than the liquid one, so the paste is quite thick. You can't really call it a solid or a liquid, its a combination of both. As Michiel says, it may be considered as a non-Newtonian fluid. If you have seen a toothpase, you could observe these:

Has no definite shape (liquid property) Cannot flow (solid property) Slight tendency to decrease surface area (liquid property)

So you can see that it has properties similar to both solids and liquids. Its a colloid of a liquid in solid.

Is toothpaste solid or liquid?

7 Answers7

Linked

Related