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In Harry Potter and the Half-Blood Prince, during their first lesson with Slughorn, the students were asked to identify various potions, one of which is Veritaserum:

“It’s Veritaserum, a colourless, odourless potion that forces the drinker to tell the truth,’ said Hermione.”

Harry Potter and the Half-Blood Prince, Chapter 9

Now the question is, if the potion is colorless and odorless, how is it possible to identify the potion just by looking into a cauldron full of it? For all we know, it could be simply water (colorless and odorless).

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Lacking color and odor is distinctive, and she had information to deduce it.

First of all, being colorless and odorless does help identify the potion - it eliminates all potions that have either a color or scent. In addition, she knew from what Slughorn said that it would be a N.E.W.T. level potion.

“Now then,’ said Slughorn, returning to the front of the class and inflating his already bulging chest, so that the buttons on his waistcoat threatened to burst off, ‘I’ve prepared a few potions for you to have a look at, just out of interest, you know. These are the kind of thing you ought to be able to make after completing your N.E.W.T.s. You ought to have heard of ’em, even if you haven’t made ’em yet. Anyone tell me what this one is?” - Harry Potter and the Half-Blood Prince, Chapter 9 (The Half-Blood Prince)

She'd also presume it wouldn't be water, which would be a reasonable assumption. Water is indeed colorless and odorless, but in the context of the Potions class, it's highly unlikely that Slughorn would have filled a cauldron with water to show his students. His goal was to show them the types of potions they should be able to brew once they become more skilled.

It's entirely possible that Veritaserum is the only N.E.W.T. level potion that's odorless and colorless, and if it is, it would have been simple for Hermione to figure it out.

It seems unlikely that she saw another characteristic that no one mentioned.

In all the mentions of Veritaserum, the only distinctive characteristics it's said to have is it having no scent or color, and looking like water. Although Harry isn't the most observant and isn't particularly good at Potions, he had mentally described the other potions as well, and seen things like the way Polyjuice Potion bubbled slowly and was mud like, and the way Felix Felicis bubbled merrily about in its cauldron. However, when looking at the Veritaserum, all he thinks about it is that it looks like a cauldron of boiling water.

“He indicated the cauldron nearest the Slytherin table. Harry raised himself slightly in his seat and saw what looked like plain water boiling away inside it.

Hermione’s well-practised hand hit the air before anybody else’s; Slughorn pointed at her. ‘It’s Veritaserum, a colourless, odourless potion that forces the drinker to tell the truth,’ said Hermione.” - Harry Potter and the Half-Blood Prince, Chapter 9 (The Half-Blood Prince)

Now, Hermione is certainly more attentive than Harry, and more likely to notice a subtle characteristic distinct to Veritaserum other than its similarities to water. Frankly, though, it seems highly unlikely that Hermione would have noticed something subtle that most people wouldn't have noticed and not mentioned it to Slughorn. She tells Slughorn how she recognized the distinctive spirals of smoke she noticed on the cauldron of Amortentia, as well as what she knew about how it's supposed to smell. It seems odd that she'd pick up on something similar with the Veritaserum and not take the opportunity to show her knowledge to the new professor, who was also more easily impressed than Snape, instead of saying how she noticed a distinctive trait not many people would.

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    So this is basically a pure guess work (unless of course your last paragraph is true). I remember a similar situation in my Chemistry class when I had to identify a gas which has certain physical property and behave in a certain way with certain chemicals. I answered CO_2 (and I answered pretty fast :)). After that teacher asked me 'how did you answer so fast?' My answer was "The only gas with these physical property I know is CO_2" (it's ridiculous, I know, but it was the truth). And Hermione is way to smart for this kind of answer. May be there is no canonical answer to this. – Krish Oct 1 '17 at 5:12
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    @Krish Yes, I'd say unless Veritaserum is the only colorless and odorless NEWT-level potion (which does seem likely enough), she made an educated guess based on what she knew of NEWT-level potions. Basically exactly what you did in your chemistry class. Which, by the way - that's pretty cool!! :) – Bellatrix Oct 1 '17 at 5:18
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    It would have been pretty funny if Slughorn had filled a cauldron full of water to make them think it was Veritaserum and then was like, "Nah, water, bruvs. Gotcha, lol". – The Dark Lord Oct 1 '17 at 8:00
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    Nice answer, but I'd say that there could easily be more properties than color or odor. For example, viscosity (how quickly is it bubbling), smoke patterns, stuff like that. – Adamant Oct 1 '17 at 10:35
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    @TheDarkLord Yes, that would have been really funny! :) – Bellatrix Oct 1 '17 at 23:53
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It might have other properties that make it identifiable.

Being colourless and odourless means it's virtually undetectable if added to someone's food or drink. However, the potion itself (as presented in a cauldron or vial) might be recognisable by other properties, such as:

  • reflection (the way it breaks the light)
  • viscosity (how "liquid" it looks/behaves compared to water)
  • freezing/boiling point (the way it reacts to the temperature)

Now these are probably rather subtle but Veritaserum is not exactly an obscure potion and Hermione would know to look for these signs, especially as (as Bellatrix has pointed out) Slughorn is unlikely to show them a cauldron full of water.

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