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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?

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    $\begingroup$ Though the definitions might seem similar, "solid" and "liquid" (especially "liquid") have crude meanings for anyone. I gave up using them a while ago. :) $\endgroup$
    – M.A.R.
    Commented Jan 18, 2015 at 15:29
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    $\begingroup$ She says it is plasma $\endgroup$
    – Simon-Nail-It
    Commented Jan 20, 2015 at 9:58
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    $\begingroup$ @Simon-Nail-It That certainly is an improper answer. It may not be obvious what toothpaste is, but it's definitely not a plasma. $\endgroup$
    – Reinstate Monica -- notmaynard
    Commented Jan 20, 2015 at 15:42
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    $\begingroup$ @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. $\endgroup$
    – RBarryYoung
    Commented Jan 20, 2015 at 21:16
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    $\begingroup$ As in "Blood Plasma". $\endgroup$
    – RBarryYoung
    Commented Jan 20, 2015 at 21:24

7 Answers 7

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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.

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    $\begingroup$ I tried explaining this to the TSA, they didn't buy it. $\endgroup$
    – Anubian Noob
    Commented Jan 19, 2015 at 23:14
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    $\begingroup$ @AnubianNoob What is a TSA? Some sort of teaching assistant? $\endgroup$
    – Michiel
    Commented Jan 20, 2015 at 8:20
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    $\begingroup$ And what exactly didn't they buy about it? Just curious :) $\endgroup$
    – Michiel
    Commented Jan 20, 2015 at 8:26
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    $\begingroup$ @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. $\endgroup$
    – Holloway
    Commented Jan 20, 2015 at 9:28
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    $\begingroup$ I just have to post this non-newtonian fluid video here, sorry: youtube.com/watch?v=3zoTKXXNQIU $\endgroup$
    – Tomáš Zato
    Commented Mar 13, 2015 at 11:51
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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.

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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).

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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

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    $\begingroup$ 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. $\endgroup$
    – ron
    Commented Jan 18, 2015 at 15:35
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    $\begingroup$ 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. $\endgroup$
    – Michiel
    Commented Jan 18, 2015 at 16:07
  • $\begingroup$ @Michiel I got it from Google. Just type "viscous meaning" It's normal dictionary meaning. Click here $\endgroup$
    – Freddy
    Commented Jan 18, 2015 at 16:25
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    $\begingroup$ @Freddy well, apparently Google picked the definition from a dodgy dictionary, because any physicist will tell you that this definition is wrong. $\endgroup$
    – Michiel
    Commented Jan 18, 2015 at 16:35
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    $\begingroup$ "glass seems somewhat liquid" - not at room temperature: cmog.org/article/does-glass-flow As @Michiel explains, viscosity is not sufficient to explain what is going on. $\endgroup$
    – Francis Davey
    Commented Jan 18, 2015 at 16:36
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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)

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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.

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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.

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