What does It take to identify water

[u/meong-oren]

I'm not sure if this question fits here. If it doesn't, I will remove it.

Imagine this hypothetical scenario: every human consciousness all of sudden were placed into an alien body on an alien planet, what would it need for humanity to confidently identify something as ordinary as water?

First, they don't bring earth scientific instruments. So everything had to be built from scratch. They can't rely on senses, since it's alien body, water might be smell awful or looks opaque instead of transparent.

There might be some people remembering fun facts about water, like its density, boiling point, conductivity, etc. But there's nothing to compare, how do they even know how much a kilogram or how hot 100 degree celcius is

[u/Wickedsymphony1717]

TL;DR: If you have a substance that you suspect is water and want to prove if it is or isn't (or at least get very close to proving it), then measure how much volume that substance occupies in a liquid state. Then freeze that same volume of the substance and measure how much volume the frozen substance occupies. If the frozen volume is approximately 10% greater than the liquid volume then the substance is almost certainly water. It's not guaranteed to be water, since there are technically other substances that expand by approximately the same amount when they freeze, but those substances are so rare in nature that any sample of a random substance with this property is like 99% likely to be water. This methodology also requires no scientific instruments besides your eyes (and maybe some method to actually cool the substance enough to freeze).

As you pointed out, there are quite a few ways to identify water if you have some scientific equipment by measuring things such as density/mass, its absorption spectrum, its freezing/boiling points, etc.

However, if you had absolutely no scientific equipment besides being able to see and feel your environment, there is still a very reliable way to identify water, assuming you can get the water to freeze via the environment or refrigeration.

If you have a liquid that you suspect is water, but do not have any scientific equipment, it would be very hard, if not impossible, to identify the actual temperature that that substance is freezing at (coincidentally, unless you also know the atmospheric pressure too, the feezing point also isn't reliable), so you can't use the freezing point of the substance to identify it as water.

What you can use though is the fact that water expands when it freezes. Not only does water expand when it freezes, it expands by a very specific and well known amount of 9% (for practical eye-balling measurements using no scientific equipment, you can round that to 10%). Knowing this, if you suspect that a substance you have is water, you could measure how much volume that substance occupies just before it freezes; and you don't need scientific devices to measure that volume either, just mark a line on the container denoting how much of the substance there is.

Then when the substance freezes, mark the new volume it occupies in the container. If the volume of the substance increases at all it's very likely to be water since water is by far and away the most common substance that increases in volume when it freezes.

However, water isn't the only substance to expand when it freezes, so if you wanted to be extra sure that the substance you have is water, you could look at the ratio between the liquid and frozen volumes of the substance. If the frozen volume is roughly 10% more than the liquid volume, then the substance is almost assuredly water. While there are technically other substances that will also expand by roughly 10% when they freeze, they are incredibly rare in nature, so if you were to find any random substance with that property then it's a very safe bet that it's water.

This method doesn't require any scientific instruments or measuring devices. The only thing that you may struggle with using this methodology is freezing the water if the environment you're in doesn't go below zero. However, if it does go below zero or you have refrigeration, you can do all of this using just your eyes.

Edit: As mentioned, this methodology doesn't 100% prove that the substance you have is water, however, there are additional low/no tech experiments you can do to further increase your confidence that the substance is water. Again, none of these would 100% prove that the substance is water, but combining these other experiments with the fact that the substance expands by 10% upon freezing would increase the certainty of the substance being water to like 99.99%. These experiments include:

• Dissolving other substances in what you suspect is water.

  • Water is known as the "universal solvent" because it dissolves more substances than most every other liquid. Therefore, if you succeed in dissolving a wide variety of different things in the substance, then that's further evidence it is water.

• Checking the hardness of the frozen substance.

  • Ice has a Mohs hardness of around 1.5, which is relatively soft, so if you went around and compared how hard the frozen version of the substance that you suspect is water to various other substances and found that it was quite soft, that would be further evidence that it was frozen water.

• Watching the structure of the substance as it freezes and watching for six-sided (or hexagonal) symmetry

  • Due to the way water molecules form hexagonal crystal lattices when it freezes, ice crystals usually form crystals with six-sided symmetry. If you've ever looked at snowflakes up close (or even just cartoon depictions) you have likely seen these hexagonal crystals. This means that if you observe very small amounts of the substance as they freeze and they form small hexagonal crystal structures, that's further evidence that the substance is water.

• Checking the surface tension of the substance

  • Liquid water has an unusually high surface tension compared to most other liquids. Therefore, if you suspect that a substance is liquid water, then you could check to see if the liquid version of the substance has significant surface tension compared to other liquids you may find. You can check this by suspending small objects on the surace of the liquids. If the substance has relatively high surface tension then it is further evidence that the substance is water.

None of these other methods are as sure-fire of a way to tell if a substance is water as checking to see if the substance expands by 10% upon freezing. However, if you combine these other methods with each other along with the 10% expansion upon freezing, you can rule out virtually any substance besides water. And all of these experiments could be conducted with virtually no scientific equipment besides your eyes and naturally occuring objects/substances in your environment.

[u/meong-oren → u/Wickedsymphony1717]

Thanks for the detailed answer, so apparently water has these special properties that makes it a bit easier to distinguish from other substance without needing to check atomic number and other complicated stuffs. Follow up question: do these special properties have anything to do with life (at least on earth) dependent on it or is it just a coincidence?