Why is water heavier than air?

Factual Questions
21

I’m not an expert on the subject freshman chemistry was decades ago, but I have a 10-year-old daughter that I explain science to because her mother was a history major.

It is tricky explaining something this complicated in terms that they can understand, without resorting to “come back in 10 years.”

There are two questions here. The title of the OP asks a different question than the text, although they are related.

The title is asking why water is heaver than air while the OP itself is asking why water is a liquid rather while the two component elements are gases.

As others have pointed out, water is heavier than air because of density. For a child, an explanation of density would be a good place to start.

Density is the amount of weight in a given volume.

Liquid hydrogen and liquid oxygen are also going to be “heavier” than air because they have greater densities than air.

This leads us to the second question, which is why water is liquid while oxygen and hydrogen are gasses.

We think of water as a liquid because we live in environments which are usually within the range between 0 deg C to 100 deg C (32 deg F and 212 deg F). At temperatures below -297 deg F, for example, oxygen is in its liquid state and is slightly denser (“heavier”) than liquid water.

The second question needs to reframed. The question needs be “Why is water a liquid at a much higher temperature than the component gasses?”

That is because of the hydrogen bonds which Dir Trihs discussed back in Post 2. The linked article is a good place to start and I wouldn’t attempt to explain this concept without that diagram there.

22

No, not weight vs mass; the problem with your question is heavy vs dense.

There some relationship between density and molecular weight for substances in the same phase of matter. Water vapor is less dense than nitrogen and oxygen at the same temperature and pressure.

23

My wife is a science teacher and kids learn about covalent bonds in middle school. It’s a simple concept – in a covalent bond, atoms share electrons, which binds them together. This is how molecules form.

24

I will note though – the whole concept of the way bonds work isn’t really true. Electrons don’t really orbit atoms in a shell based on energy level; their energy level simply tells us where we would be most likely to find them, but at this scale, there’s no absolutes – it’s a matter of probability.

The simple model we teach kids, like Newtonian Physics, is good enough for everyday applications. But it isn’t really TRUE.

25

Here’s how my 11 year old and I tried to understand :

  1. Compressibility : we bought a helium balloon kit from Walmart for around $20 and made balloons out of them. We got around 29 balloons. Comparing the volume of the balloons to the volume of the small gas cylinder gave her (and me) the idea of compressibility. So instead of compressing 30 balloons into one balloon, if we could compress about 600 of them into one, it will behave like a liquid.

  2. Put a 1x1 inch piece of paper on a sheet of paper. Next we spread half a teaspoon of iron filings on the sheet. Gave her a pair of tweezers and asked her to lift the 1x1 piece and it was easy. Next used a magnet (under the sheet) to get all the filings on the 1x1 piece. Asked her to pick up the piece with the tweezers again - “it felt like solid now”.

My favorite aphorism borrowed from Box - “All models are wrong; some are useful “

26

Liquid oxygen is denser than liquid water (1.14 g/cm^3) but liquid hydrogen is much less dense (only 0.07 g/cm^3).

27

This reminds me of one of my favorite pieces of science trivia: there’s more hydrogen in a gallon of gasoline than in a gallon of liquid hydrogen.

28

So the actual answer is : liquid air is denser (heavier in a given volume) than water. It just requires low temperatures to reach because water bonds to itself more strongly than air bonds to itself. And then you can explain why gasses are less dense than liquids.

29

And note that there is nothing unique about the observation anyway. Ammonia (NH3) is a gas at room temperature, but is a liquid at a temperature far higher than the boiling points of nitrogen and hydrogen.

30

Same thing with “mass” and “weight”. The “mass” of a water molecule is less than the mass of it’s constituent parts. Potential energy is always negative and both air and water as an ensemble would increase if you increased distances between the molecules.

This is a side effect of us teaching civil war era science as absolute truth.

The mass of an object is related to it’s energy content, and mass and weight have been different ways to measure the same fundamental property for over a century now.

Mass differs based on the arrangement of the components but obviously in this case the number of those components within a set volume is the major reason for the difference.

For the text you quoted, the equality of inertial and gravitational mass a an postulate for the theory of General Relativity.

31

Just because simple models are outdated or untrue doesn’t make them useless though. Newtonian physics gives you an answer that’s close enough to correct that you won’t notice the difference unless your speed is a significant fraction of the speed of light. And if we want to get down to it – isn’t mass just a measure of the energy contained in the massless subatomic particles that form your atoms?

It’s like how some people claim, centrifugal force isn’t real! Well… neither is gravity, according to general relativity. Gravity is indistinguishable from acceleration.

32

I’m not a chemist but none of these answers have been real great for a ten year old.

I’m depending on my understanding, based on the Periodic Table of Videos series on YouTube, but here’s the way I would describe it.

Atoms can be thought of like little Lego pieces made of rubber, bouncing around all over the place. So long as you keep putting energy into them, they will keep shaking and bouncng and vibrating all over the place. So, when you’re doing that, the pieces are really far apart from one another just because they can’t be contained and they’re pushing each other away.

If you can get them to connect, like the actual little Legos that they are then, well obviously that little clump is all packed together tightly. You know have 3X the atoms all together where you used to have 1, so obviously it’s bigger and massier.

Here on Earth, we have to Sun throwing out energy 24/7. But it’s always, more or less, the same amount of energy. So it’s pushing on little hydrogen Legos and oxygen Legos with the same amount of juice as it is trying to push on our clump of three Legos that have been tied to each other. The upshot is that they move slower, and so they’re not kicking each other away so much. So not only are your clumps larger but they’re also closer to one another.

Our little Lego clump is also less round. It used to be a little perfect cube that would just ricochet off all the other Legos. Now it’s a little gangly mess of three Legos arranged like a coat hanger. Now there’s a lot more opportunity for things to interfere with each other and “catch”.

If you had a room with like 20 rubber balls - and there’s no telling to the room, just super tall walls that go up for millions of miles, and the walls/floor vibrate back and forth real quick, keeping the balls bouncing around - you wouldn’t have much issue driving a car through. It would be mostly empty space and the balls would just hit your car and bounce away.

If you take that same room and fill it with a thousand long rubberized batons, that’s going to be a tougher prospect. The energy of the walls might not be strong enough to keep the batons from being able to jump up more than a couple of feet. And the batons are going to catch onto each other as they bounce, so if you move one, it will hit another and change its direction of travel to hit another one, which will change its direction of travel to hit another one, etc.

You can still drive through it, but it’s a tougher proposition. You’re going to have to push your way through a bit because even though each baton doesn’t weigh that much, since they’re catching on each other, you’re moving a bunch of them every time you move.

But, I mean, if you get the batons moving fast enough and give them room to bounce just as high and far apart as you could ever want then regardless that they’re big gangly beasts, they’re never really going to touch one another and they’re mostly going to be up high over your head not wacking the radiator of your car. It’s a similar experience to driving through the lower energy version with the small rubber balls.

In outer space, water will immediately turn into a gas because it can just freely go everywhere. There’s no gravity or pressure by having things resting on it from above to keep it sort of contained. Being a gas is a matter of having your Legos all spread out all over the place. If you can get everything vibrating fast enough to be able to overcome everything trying to contain it, then no matter what it is, it will become a gas. Put into a place with no container, many things will just pull themselves apart and fly away into a cloud of steam with just the energy of the sun to push them.

Here on Earth, water molecules are too heavy and too gangly to overcome pressure. They get locked up together, jiggling and catching on each other, and are difficult to move through but still moving around enough and loose enough that you can go through without too much issue.

33

There is interstellar water ice.

And please stop using the word “heavy” here. Maybe “sticky”.

34

Would it remain ice if it wasn’t “interstellar”? I.e., if it was just hanging out orbiting the Earth for a while.

One slipped through, at the end. But still, this is for 10 year olds.

35

My post and Newtonian Mechanics being useful as an approximation are not in conflict.

Note the claim that “centrifugal force isn’t real” is not valid even under Newtonian Mechanics. A “fictitious force” is an apparent force that acts on all masses whose motion is described using a non-inertial frame of reference. Under F = ma, fictitious forces are always proportional to the mass which is exactly why a feather and an hammer fall at the same speed in gravity.

Just as the apparent force of gravity is still a useful construct in some frames of reference so are the apparent centrifugal force and apparent coriolis force.

The problem is in using a model that provides a good approximation, the problem is the erroneous claim that the reason is due to some fundamental truth of our universe.

While the use of the term “weight” is challenging due to multiple conflicting definitions, the equality of gravitational and inertial mass has been tested and has been confirmed with every test we can make. It is destructive to students forward progress in physics to ignore the equivalence principle and teach the false belief based on a model which is a useful approximation.

One can teach the Newtonian model without making a false claim that gravitational mass does not equal the inertial mass by simply abandoning the operational definition of weight. Better yet abandoning the term weight.

http://www.physics.usyd.edu.au/~bedding/papers-weight/galili2001.pdf

The insistence on superseded models leads to the difficult for the learners with the concept of weight.

In the US, the NIST chooses the “Metric version” for packaging.

The Metric system tried to standardize on the gravitational definition.

Resolution of the 3rd CGPM (1901)

Even under the Newtonian mechanics the distinction between mass and weight is unimportant for most practical purposes because the strength of gravity does not vary much on the surface of the Earth. When it does matter one typically cares about accuracy and will use some post-Newtonian corrections anyway and those corrections are about measuring the local acceleration. For standards like the ISO who consider buoyancy and centrifugal force or metrology they are already past the simple need to maintain pretty Newtonian formulas and have to care about local acceleration.

Either way, as others have mentioned density is what is important here. My issue is that the “weight vs mass” difference claim is not objectively true with a gravitational definition of “weight”. This is true even under Newtonian mechanics unless one insists that gravity = 9.8 m/s[sup]2[/sup] everywhere on Earth. Using the Newtonian concepts of apparent weight vs. true weight help clarify this a bit, if you reserve true weight to be the F in F = ma to equal weight and thus a direct measurement of inertial mass.

In the case of the OP, which is really a question about density, this reduces confusion and actually helps students understand the concepts. The apparent weight due to buoyancy and differences in density are what are important.

This model also helps with explaining weather and other aspects of our world if one clarifies one is talking about apparent weight vs mass. But even Newton considered “true weight” to be defined by gravity. Due to a lack of access to non-Euclidean geometry and the use of Galilean transformations he couldn’t describe gravity in the way Einstein did.

Newton himself had serious reservations with his law of gravity, thus the famous passage.

While the term “weight” will probably always be encumbered by differing definitions and thus problems we can stop trying to teach a concept in a way that is so confusing and leads to so many problems.

Choosing to define “Weight” as being the force observed under acceleration of gravity due to an objects mass helps and doesn’t pose barriers if students continue to learn physics to the point where they are within a century of our current best theories.

36

Ah, I see your point, and I think we are in agreement - the term “weight” is encumbered by too much baggage due to everyday use to be a useful scientific concept. With this, I agree. And you’re right, weight isn’t equal to mass unless you’re measuring at a location where Earth’s gravity is a very specific number, in a vacuum.

37

It depends entirely on the conditions. There’s plenty of ice in our solar system that isn’t on planets and moons.

And thus likely capable of understanding density, states of matter, and attractive forces between molecules.

38

Methane is also a good one. It has carbon in it but it’s lighter than air!

If you string CH[sub]2[/sub]'s together you can get liquids and what not.

39

Well, carbon is lighter than both nitrogen and oxygen, so that shouldn’t be too surprising :slight_smile:

40

Hydrogen bonds.
When water is in a gaseous state, it’s actually lighter than air, hence low pressure regions around areas of high humidity or even rain.