eli5: Why are some molecules a liquid, others a gas and some are solids?


For example, at room temperature H2O (water) is a liquid, CH4 (methane) is a gas, NaCl (salt) is a solid, why?

What are the properties of a molecule that determines its state?


In: 0

It’s the wiggle. Cold molecules wiggle *slowly* , like a baby randomly kicking or waving arms. Its easy for the chemical bonding between atoms to hold the two molecules together.

When the molecules heat up the wiggle faster. This is like when the toddler is squirming so much mom can barely hold on and only has a grasp by one hand. This is a liquid as the two molecules are semi-attached but can move almost at free will.

Heat it up more and the wiggle gets extreme. So much that it has the energy to break the chemical bonds holding it together and as a gas molecules go flying off. So now the toddler saw a puppy and bolts away at speeds unknown to mankind and in every direction.

Different molecules have different properties that allow them to hold each other strongly or weakly. Hydrogen is like asking a book old to hold the attention a baby. They might have its attention for a moment but they will fly apart super easily. Some molecules like that makes up rock is the marriage of your great aunt Ruth who been married for 65 years and it isn’t conceivable the heat that would be needed to separate her from great uncle buck.

Basically the interactions between the individual molecules, notably how attracted they are to each other.

The molecules are always moving. The temperature of stuff is related to the the energy of the molecules’ movement.

If the attraction is so strong that the molecules are stuck in place and can only vibrate, that’s a solid.

If the attraction is strong enough that the molecules are stuck together but can move around each other, that’s a liquid: the molecules can move around so the material can take the shape of its container–but because they’re all stuck together, the volume is effectively constant.

If the attraction is too weak to prevent the molecules from wanting to fly everywhere, that’s a gas: it will expand to fit its container.

The inter-molecular interactions are determined largely by the way/shape the negative electric charge (of the electrons) get distributed around the positive electric charges (of the protons in the atoms’ nuclei).

What causes the different states of matter is how the molecules interact with each other, if the forces between the molecules are very strong the substance is a solid, an example of very strong union is ionic or metallic bonds, ionic bonds is where atoms rip electrons of each other and are both left with different electrical charges.

If the forces are weaker the substance is a liquid, here it gets more complicated, as some liquids are composed of molecules that have something called polarity, which means that different parts of the molecule have more positive and some have more negative charge. And some other molecules are completely non polar. The more polar a liquid is, the stronger the attraction between the different polarities in the molecules, the stronger the attraction the harder it is for that liquid to become a gas.

Gases have next to no force between the molecules.

So it comes to the forces between the particles, strong forces means solid, weaker forces means liquid, next to no forces means gas

Aah, this is due to what is called “Hydrogen liaisons” or “Van der Vaals liaisons”. (Let’s stick to hydrogen).

TL:DR at the bottom, but I tried to keep it understandable below.H2O and CH4 are perfect examples.There are a few notions to unpack:


– 1 First: an atom is made of a *core* of neutrons and protons. Orbiting this core are as many electrons as there are protons.


– 2 Second: in both example you have a central atom, Oxygen (O) or Carbon (C), surrounded by Hydrogen (H) atoms. Each hydrogen atom has a *covalent* liaison with the center atom. It’s liaison made by two electrons, one from each atom, that are now common to both. It’s extremely solid, hard to break. That makes a molecule.


– 3 Third: the core of those two center atoms are more massive than the Hydrogen. O has 8 protons and 8 neutrons, C has 6 protons and 6 neutrons. H only has… 1 proton.


– 4 Fourth: because of this, electrons in a O-H liaison and in a C-H liaison are closer to O and C than they are to H. This means that you have in average a more *negative* electric charge around O and C and a more *positive* charge arouns each H. (Electrons are charged negatively).


– 5 Fifth: Due to reasons, O is better at attracting those electrons than C.- 6 Sixth: Structure ! It gets more complicated here:So, electrons always go in pairs, and an atom has “layers” of “boxes” in which you can put 2 electrons. And only the “outer” layer is used to create liaisons.For reasons, the outer layer has 4 boxes. The one from O has 6 electrons while C has 4.

Now, the way electrons fill those boxes is they find an empty one. If there are none, then they go pair up with an electron already in a box.

So C has 4 boxes with 1 electron each. That’s why it can have 4 H around: C has 4 lonely electrons waiting to pair up.

O has two full boxes and two boxes with one electron. That’s why it can only have 2 H around.

BUT ! Electrons are charged, and similar charges push against each other until equilibrium (like magnet).

CH4 has 4 *identical* liaisons. They push each other the same way to CH4 is perfectly simmetrical in 3D (as seen here: [https://csg-prd.tinkercad.com/things/aTu6bqeu0ac/t725.png?rev=120&s=&v=0](https://csg-prd.tinkercad.com/things/aTu6bqeu0ac/t725.png?rev=120&s=&v=0))

The O in H2O has two full boxes and two liaisons. Take the above CH4 molecule. Replace two H atoms with full boxes and you now have this [https://1.bp.blogspot.com/-OlnUXIHKtDg/Teh5ekJUTjI/AAAAAAAAAD8/UIgoC2JK2PI/s1600/770px-Water-2D-flat.png](https://1.bp.blogspot.com/-OlnUXIHKtDg/Teh5ekJUTjI/AAAAAAAAAD8/UIgoC2JK2PI/s1600/770px-Water-2D-flat.png).

This is 2D, but the placement is very much 3D. However, since it’s only 3 atoms, you can consider H2O as a “2D” particule.

Notice that now you have 4 electrons on one side of H2O and 2 Hydrogen atoms on the other. And remember point 4: *The two H in H2O have a slightly positive charge*


So now H2O is negative on one side and positive on the other side. What happens when you put two H2O next to each-other ? The positive side of one attracts the negative side of the other. And now those molecule will be harder to separate. It’s why it’s a liquid. We call this a *hydrogen liaison* (because it’s made with hydrogen atoms).


CH4 now: It’s all symmetrical. Negative charges *all around*. CH4 molecules cannot go together. They are free to roam around and that’s why it’s a gas.

Now for NaCl it’s because it’s a crystal. Strong liaisons all around. Very hard to break.


**TL;DR: H2O molecules are electrically positive on one side and negative on the other side. Those two sides attract the opposite side of another H2O molecule and they stick more to each other. CH4 doesn’t have this property and thus its molecules can move more freely.**


(If you feel I wasn’t clear on some points, feel free to ask).

The size of the molecule/atom and the forces between them.

For example, it requires very high temperature to vapourise most metals, because most metal atoms bond very well with eachother. They’re not molecules, but I’m just illustrating the point.

A lot of molecules which are gaseous, like O2, N2, CO2 etc. don’t liquify or solidify until very low temperatures because they’re both light and they don’t attract eachother very much. It’s very easy for them to be knocked about by other molecules.

Then you have two cases:

Light but strongly interacting molecules. For example H2O is lighter than the above three gasses, but it interacts very strongly with itself and other molecules. It’s a dipole, that means it has a noticeable positively and negatively charged ends. And electrostatic forces are strong. So water should be flying freely, but can’t, because other water molecules hold onto it.

Other end are molecules which interact weaker, or maybe not at all, but are heavy. A lot of organic molecules fall into this category. Many interact weaker than water but are heavier than it, so they remain liquid in similar circumstances. Then some are so heavy that they can’t evaporate at all, and will burn if you apply enough heat hoping to force them.

The same goes for things that aren’t made from molecules, so interatomic bonding. Some metal bonds are relatively weak, like mercury or sodium. Some metallic bonds are extremely strong. There’s no such thing as a molecule of NaCl either, it’s a crystal of electrostatically attracted ions, and they’re bound fairly strongly, so their melting point is high. But these bonds allow for phase transitions because they’re in a way “flexible”. They’re non-directional and allow for “stretching”. You can have a liquid of atoms kept together with ionic and metallic bonds.

The one exception, really, are covalent bonds. They’re the strongest bonds and they are very stiff and ordered. Those are the bonds that make molecules. Something is either covalently bonded or it is not, there’s no equivalent of “liquid” here. You can have a liquid OF molecules, but if you try to break the molecule itself that’s no longer a phase transition, that’s a chemical reaction.