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Is the following statement always true?

The more $\ce{-OH}$ functional groups in the molecule, the more is its viscosity?

I think it is true, because it is known that weak intermolecular forces lead to lower viscosities and strong intermolecular forces lead to higher viscosities and because the more $\ce{OH}$ functional groups in the molecule will give rise to a molecule having stronger molecular forces. As the case in glycerol ($\ce{C3H8O3}$) and water ($\ce{H2O}$)

Therefore the statement must be true. What do you think?

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It is a reasonable rule of thumb, but certainly not always true.

Compare for example the viscosities of Dodecane ($\ce{C12H26}$, $\mu=1.374 \ce{\; mPa\cdot s}$, no $\ce{OH}$ groups) and Ethanol ($\ce{C2H5OH}$, $\mu=1.095 \ce{\; mPa\cdot s}$, 1 $\ce{OH}$ group) (source). There you can see that ethanol has a higher number of $\ce{OH}$-groups, but a lower viscosity.

That said, I do think you are on to something regarding the strenght of intermolecular interactions, it is just not purely the number of $\ce{OH}$-groups that matters for that. The length of the carbon chain for example (like in the example I mention above) also plays a role.

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    $\begingroup$ I think the OP was asking about viscosity vs. the number of hydroxyl groups in a molecule. $\endgroup$
    – ron
    Commented Jul 27, 2014 at 15:16
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    $\begingroup$ @ron Yes, that is what I answered right?! OP wants to know whether more hydroxyl groups means higher viscosity, I show that that is not necessarily the case. $\endgroup$
    – Michiel
    Commented Jul 27, 2014 at 15:20
  • $\begingroup$ basically i wanted to compare the viscosity of molecules which have different number of oh groups .. but the piece of information that you have presented is really good to know ! Thanks $\endgroup$
    – Maher
    Commented Jul 27, 2014 at 15:27
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    $\begingroup$ @maher ok, I didn't get that you wanted to compare molecules that are identical except for the OH groups, since you mentioned glycerol and water in your example I thought you were just thinking about all molecules $\endgroup$
    – Michiel
    Commented Jul 27, 2014 at 15:38
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    $\begingroup$ I'm sure someone could come up with a molecule where the addition of an -OH group decreases viscosity (maybe by forming an internal hydrogen bond that stabilizes a less "sticky" conformation?). But yes, I'd say that, all else being equal, it's almost always true, because, as ron notes, -OH groups like to stick to each other. $\endgroup$
    – Ilmari Karonen
    Commented Jul 27, 2014 at 18:42
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Is the following statement always true? The more −OH functional groups in the molecule, the more is its viscosity?

It's hard to find anything that is "always" true in chemistry, but I'd bet that within a series of molecules where the only variable is the number of $\ce{OH}$ groups, your statement is generally true, so I basically agree with your position. Your analysis and examples are also good. As you pointed out, it is about intermolecular forces. Specifically, molecules with hydroxyl groups can form intermolecular hydrogen bonds (see first picture below). These hydrogen bonds cause the molecules to "stick" together and act as if they had a higher molecular weight. Sugars have many hydroxyl groups that give rise to many intermolecular hydrogen bonds and cause sugars to flow in a slow, syrupy manner.

enter image description here

Here is a comparative series of alcohols all involving a 3 carbon chain. When the number of hydroxyl groups remains the same and we just vary their position on the chain the viscosity hardly changes. On the other hand, note how the viscosity increases markedly each time we add another hydroxyl group to the chain. It appears that the total number of hydroxyl groups is what really matters, so your statement does appear to be generally true as long as the number of hydroxyl groups is the only variable.

enter image description here

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