# What causes liquids to have different densities?

We know that different liquids have different densities and it is mass over volume. When the mass is more, then its gravitational attraction will be higher (Compared to other lighter atomic mass liquids)but these gravitational attraction has to be too small to be noticeable. My question is what causes these differences in density of liquids? Is it due to atomic attraction of atoms or surface energy of each atoms to near by atoms? I tried searching for an answer and couldn’t find it. pls help.

• – David H Nov 27 '13 at 9:00

It's because the densities of the atoms changes.

The density of an atom is a somewhat vague concept because an atom doesn't have a sharply defined outer surface. nevertheless you can define a size based on the average lengths of bonds formed by atoms, and if you do this you find the size of atoms decreases along a row in the periodic table even though the atoms are getting heavier. This article has a nice diagram showing the size changes.

For example, the density of tetrachloroethylene, at 1.62 g/cm$^3$, is higher than water mainly because the chlorine atoms are denser than oxygen or hydrogen atoms.

It is due to the molecular structure of the respective liquid. The density depends on chemical composition (are there light atoms or heavy atoms?) and the distances between the atoms (are they closer to each other or more separated?). Liquid hydrogen is very light, because hydrogen has only 1 proton in the nucleus. Liquid iron is heavy, because iron atom has 56 nucleons.

• Lighter atom means atomic mass is less. – Vaishakh Rajan K Nov 27 '13 at 15:03

The mass of electrons in atom is $$\sim 1850$$ times smaller than the atomic nucleus so we can neglect electrons, and the radius of the atomic nucleus is much smaller than the distance between atomic nucleus of nearby atoms so a good model here is point like mass dispersed in space. With this model, the density depends on for a given volume how many atomic nucleus are there and what is the mass of each atomic nucleus which depend on the atom. So, the atomic nucleus are just there then the density is just like that.

So the question is why the atomic nucleus are just there instead of some other places? In an atom the EM force stick the electrons as electron cloud around the atomic nucleus, when different atoms come close to each other (all atoms are moving and colliding constantly), there will be EM attractions between them because of the non-uniform distribution of charge in the atom, when they come even closer to each other, the electron cloud overlap and propel each other so the two atoms will slow down the speed to come closer. If they come close enough because of the direction and speed of collision and a covalent bond gives a lower energy, they will form a covalent bond to form a molecule. If not, then if their speed of collision is low enough (macroscopically the temperature of the atom system is low enough) then the EM attraction is strong enough to stick them together and prevent them to just pass each other then they condense and the liquid forms.Different atoms have different electron cloud sizes which give different distances between nearby atomic nucleus. In liquid.

When different atoms forms molecule, since the atoms in a molecule can move relatively to each other only in a limited way (this is all a molecule is about or they are just a bunch of atoms), the structure of the molecule may form spaces where there is no atoms at all (e.g. $$\rm C_{60}$$ molecule) which gives even lower density without considering other factors.

All the above are just intuitive explanation, no body is sure whether the true reason of why the atoms are in their places is because of the electron cloud or some other reason. Behind the scene it is the basic physics laws i.e. standard model that in theory can predict why the atomic nucleus are in their current place instead of other places.