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Molecular weight of petrol is so much higher than water, but when it comes to physical property, weight, one litre of water weighs more than one litre of petrol. How is it possible?

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    $\begingroup$ What do you mean "weighs more"? A kilo of water obviously weighs the same as a kilo of petrol. A molecule weighs less. I suspect you mean a liter of water weighs more. But "weight per volume" or density is a different physical property than weight. Weight by itself is only well-defined for concrete objects. I'm being hyper-precise here, but that is because understanding this starts by understanding the definitions. $\endgroup$ – MSalters Jun 3 at 15:34
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    $\begingroup$ @MSalters BTW, the terminology for what you're getting at is that weight is an extensive property, while density is an intensive property. en.wikipedia.org/wiki/Intensive_and_extensive_properties $\endgroup$ – Acccumulation Jun 3 at 16:19
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    $\begingroup$ Note - water is not denser than all petroleum products. Some of the higher molar mass petroleum species are actually more dense than water. $\endgroup$ – David White Jun 4 at 0:40
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    $\begingroup$ Get a liter of macaroni and a liter of orzo and a liter of flour. Each individual macaroni weighs more than each individual orzo, which weighs massively more than each particle of flour, but the liter of macaroni weighs less than the orzo, and the orzo weighs less than the flour. How is that possible? If you can explain to me how that is possible, then you can probably answer your own question by similar logic. $\endgroup$ – Eric Lippert Jun 4 at 22:33
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    $\begingroup$ One liter of water is also heavier than one liter of ice. How is it possible? :) $\endgroup$ – JonathanReez Jun 5 at 1:01
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Because water molecules are small and pack tightly together, causing water to have a greater density than petrol.

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    $\begingroup$ Specifically, they're packed together tightly due to hydrogen bonds. $\endgroup$ – Sanchises Jun 3 at 14:16
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    $\begingroup$ @Sanchises, when hydrogen bonds form in water, the molecules take on a hexagonal structure, ice forms, and the density goes down. $\endgroup$ – David White Jun 4 at 1:41
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    $\begingroup$ @David That's the case when the number of bonds increases even further, yes. But they are also present in liquid water, and are responsible for the fact that liquid water can exist at STP, as opposed to most molecules this small. $\endgroup$ – Sanchises Jun 4 at 5:47
  • $\begingroup$ In order to evaporate water, you need break the hydrogen bonding in liquid water. $\endgroup$ – Cinaed Simson Jun 5 at 6:11
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Density relates to the mass per unit volume. If your molecules are heavier but take up more space, the net result could be more or less mass per unit volume.

When you look at a typical hydrocarbon, it has a lot of carbon and hydrogen. Now atom for atom, oxygen is heavier than carbon (ignoring isotopic abundance, roughly a 16:12 ratio). So if the molecules were otherwise the same shape, if we replaced the carbons with oxygens the hydrocarbon would become heavier (you can’t do that of course - the chemistry is different).

But the larger and more complex shape of the hydrocarbon molecules has another effect. Imagine two elevators. In one elevator we cram a squad of ballet dancers - tall, elegant, and able to be packed very closely. In the other elevator there are a number of people who just went shopping - they carry big bags and generally take a lot of space. It’s quite possible you could get 15 ballet dancers into the first elevator and only five shoppers in the second. So though the dancers might weigh 100 pounds each and the shoppers 200 pounds, the first elevator car will be heavier.

The same analogy can help explain why density of most materials goes down when temperature goes up. Imagine the dancers hear music and start to dance. Suddenly 15 of them aren’t going to fit in that elevator!

Maybe that’s why elevator music is usually so awful?

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    $\begingroup$ You were doing so well with the elevator analogy... And then you had to stick on a Dad joke... $\endgroup$ – Oscar Bravo Jun 3 at 9:05
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    $\begingroup$ @OscarBravo sorry. Four time dad - it comes naturally... $\endgroup$ – Floris Jun 3 at 9:13
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    $\begingroup$ @UKMonkey pics or it didn’t happen. Make that video. $\endgroup$ – Floris Jun 3 at 16:57
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    $\begingroup$ @Floris hold my beer ... nothing bad can happen from this - no promises they all have their feet on the floor :) $\endgroup$ – UKMonkey Jun 3 at 17:03
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    $\begingroup$ @Floris This is the closest I could find, which falls dismally short of an elevator jam packed with ballerinas: i.pinimg.com/originals/8c/e5/9f/… $\endgroup$ – nasch Jun 4 at 22:18
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One of my Chemistry teachers had an interesting way to describe this type of phenomenon.
Because the water molecule is shaped like a triangle, it is possible for the water molecules to "snuggle up" with each other, and pack themselves close together. Like this:
>>>>>>>>>
He claimed that Water molecules behave more like H(100) O(50) that H(2) O.
Now, I'm sure we could think of a dozen reasons why this isn't correct, but: this does show a unique property of water. It relates closely to the answers provided by @Floris and @G. Smith.

If water really were H(100) O(50), would that explain the property of your original question? I believe it would.
$0.02

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    $\begingroup$ This is closely related to the fact that water expands when it freezes - because the molecules then find a preferred way of packing that is less dense (tarragon always structure). Not sure about the H100O50 part of the argument but tight packing is relevant. $\endgroup$ – Floris Jun 4 at 6:11
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    $\begingroup$ When you add just a few very long thin molecules into the water, dramatic things happen. The water adjusts its hydrogen bonding to minimise energy and coordinate with the impurity. Witness jelly (Jello) which turns (mostly) water into something more like a solid than a liquid at room temperature. The 50-molecule average coordination count seems very plausible to me. $\endgroup$ – nigel222 Jun 4 at 8:18
  • $\begingroup$ @Floris I don't think you mean tarragon. $\endgroup$ – David Conrad Jun 4 at 20:20
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    $\begingroup$ @DavidConrad indeed - I meant tetragonal. Never type on a phone with autocorrect when you are not wearing glasses... $\endgroup$ – Floris Jun 4 at 21:45
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    $\begingroup$ @DavidRicherby > is (kinda) the structure of a simple H2O molecule. $\endgroup$ – Martin Bonner Jun 5 at 13:31
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Another approach to answer this for youself might be to run with the idea and look at molecules with even higher molecular weight.

Like, why aren't plastics far heavier than lead? Why isn't DNA? These are truly massive molecules, yet their densities are pretty low (comparable to water).

With polymers, you should pretty quickly get the idea, that the arrangement of the atoms does matter. While the molecules themselves are very heavy, they also take up more space and hence are less dense. With petrol, the usual molecules are not that much bigger than water, so arrangement effects are not that obvious.

On top of that, intermolecular force also play a role - here water's attraction to other water molecules comes into play. Plastics on the other hand do leave holes and often are not particularly strongly attracted to each other, leading to lower densities.

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Picture this. You have a basket of foam balls and a basket of wooden balls. Say each basket has the same volume and each ball the same volume as well. Stuff as many foam balls as you can into the 1st basket and do the same for the other basket but with wooden balls. Each of the foam balls weigh 10 grams, and each of the wooden balls weigh 20 grams. You find that you can stuff 3 times more foam balls into the basket than wooden balls. Doing the math, you find that:

10 grams * 3 times more balls > 20 grams.

30 > 20.

This means that even though the foam balls weigh less than the wooden balls, they can be packed together more densely, resulting in a larger total mass than the wooden balls. Hope this clarified anything!

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    $\begingroup$ In your analogy, you can pack more foam balls in because they can be compressed, but that's not what's happening wth water vs petrol. Both of those liquids don't compress easily. $\endgroup$ – PM 2Ring Jun 3 at 23:36
  • $\begingroup$ It'd be more accurate to liken the balls to two different forms - the water are wooden cubes with slightly smaller volume than the balls representing petrol. But if I stack them in the basket, the water actively reduces the space between them (they stack nice) so I can fit more cubes in there (even if the two are actually the same volume/unit). Which is much more like what water does, since the molecules themselves can't compress. $\endgroup$ – Delioth Jun 5 at 19:35
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Water is denser than petrol, so there essentially more molecules of water in a given volume. For example in a given volume, there may be 15,000 molecules of petrol but 20,000 molecules of water.

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  • $\begingroup$ You're supposed to be explaining why water is denser than petrol but, instead, you pre-suppose that water is denser than petrol and conclude that water molecules must be more densely packed. This is the wrong direction. Also, typical molecules in petrol are much larger and heavier than water molecules, so an extra 30% of water molecules wouldn't be nearly enough to compensate. $\endgroup$ – David Richerby Jun 5 at 15:24

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