Not sure how this can get, but how is that two atoms of hydrogen and one atom of oxygen combine into something (water) that’s heavier than air even though they’re lighter than air separately?
In: Chemistry
The key here is being careful with the words “lighter” and “heavier”. Obviously, 1 pound of water weighs the same as 1 pound of air: they both weigh 1 pound.
The difference is that, in normal circumstances on earth, 1 pound of air takes up a lot more space. It takes about 12 cubic feet of air to weigh one pound, according to Google, while it only takes about 0.016 cubic feet of water.
And this is what we usually mean when we say air is lighter than water: the *same volume* of water weighs more than the same volume of air, under normal conditions.
A follow up question could be “why is it that the same volume of water weighs more?” The answer is that water molecules are funny shaped with a positively charged side and a negatively charged side, so that one side will attract the other side of another molecule. This means they tend to pull together and you get more molecules in the same volume. H_2 and O_2 molecules do not have that property, and tend to repell each other more, causing the molecules to spread out.
Of course, you could then ask why water is funny shaped, and that’s because of a balance of electric forces that really would be better described with pictures, and should probably involve words like “valence”.
If you stacked bricks neatly you would fit more in a box making it heavier then if they were loosely thrown in. The atoms become more densely packed making it heavier.
Water quite close in density to air if they are in the same state(liquid, solid or gas). So if you have both the air and water as gas the density of water is less
Att 100C the density of air is 0.947 kg/m3 compared to 0.6 kg/m3 for water
An H2O molecule has the mass of 18U and the air is primary N2 at 28U and O2 at 32U
As a liquid O2 have a density of 1.141 g/cm3 but need to be at at -182. Liquid N2 has a density of 0.808 g/cm3 and needs to be at -195.79C. Water has a density of 1g/cm3 so oxygen is a bit denser but nitrogen is less dense.
If you include liquid hydrogen the density is only 0.07085g/cm3 and it needs to be −252.87 °C
The difference is not density but boiling temperature. It is because of the strong hydrogen bonds between the H2O molecules where the 3D shape of it has a huge effect.
> water is heavier than air
Oh but that’s the typical riddle: which one is **heavier**, 1 Kg of water or 1 Kg of air? Answer: naturally, they weight exactly the same, 1Kg.
What to want to look at is not the *weight* but the weight *per unity of volume*. While it’s true that one molecule of water (H2O) is lighter than one molecule of oxigen (O2), 1 meter cube of liquid waters packs way more water molecules than there are air molecules (oxigen and others) in 1 mt cube of air. That’s because in air there’s much more empty space between molecules, as it’s typically the case with any gas. This makes the cubic meter of air much much lighter than the cubic meter of water.
How can a truckload of feathers be heavier than a brick, if they’re all lighter than the brick separately? Because their masses and weights are combined when they become one substance.
Hydrogen bonding is the answer. The way the three atoms of H2O are aligned three-dimensionally causes a weak electric force on opposite sides, sort of like poles of a magnet or static electricity. This force (called hydrogen bonding) causes the water molecule to stick to other molecules of water stronger than normally. When the molecules stick to each other, they stay in the more dense liquid water form for higher temperatures before becoming a gas. As you heat water, eventually the energy of the water becomes high enough to break the static electricity force, which is when it evaporates into gas (steam).
If it weren’t for the extra static force, water would be a gas at room temperature, just like similar other molecules are, like methane. But methane is arranged symmetrically, so it doesn’t have opposite charges on different ends, and therefore no extra static electricity holding it together.
*EDIT*: Note that water isn’t “heavier” than air. You mean liquid water is denser than gaseous air.
Strong inter-molecular forces make water pack together much more tightly than air, at the same temperature.
More tightly packed, per unit volume, allows the mass rise compared to free gaseous oxygen and hydrogen.
Diatomic oxygen molecules don’t particularly care about their neighbors. Neither does diatomic hydrogen. Water has an unequal distribution of charge and feels the presence of it’s neighbors.
If I understand correctly, the answer is basically that water is more compact (by space) as a result of the atoms joinging. How do I mark this as solved? Thanks for answering, everyone.
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