What makes liquids miscible

All gases are miscible with each other at normal pressures. For example, helium and nitrogen gases are miscible. Air and argon are miscible. Ethanol vapor and water vapor are miscible.

Miscible solids work a bit differently because they form from liquid melts and then solidify. Elements that form alloys are miscible. So, iron and carbon are miscible to make steel. Copper and zinc are miscible to make brass. Miscibility also produces minerals. Oil and water are a classic example of immiscible liquids. You can mix oil and water, but they will separate. Other immiscible liquids are water and benzene, water and toluene, and methanol and cyclohexane.

While all gases are miscible at normal pressures, gas-gas immiscibility can occur at high temperatures and pressures. Under these conditions, the compressed particles behave more like liquids, but the temperature exceeds the critical temperature. For example, benzene vapor and water vapor become immiscible at high pressure. They may mix as liquids, but separate upon solidification. Two completely miscible liquids will form a homogeneous uniform solution in any amount. When first mixed, miscible liquids often show oily bands—called striations—in the bulk of the solution; these disappear when mixing is complete.

Like any other solubility phenomenon, miscibility depends on the forces of attraction between the molecules of the different liquids. Some of the water molecules would not be able to get close enough to each other to hydrogen bond anymore. The loss of that very stabilizing interaction would be too costly. This problem is similar to the previous one, but in this case the attraction between the strong dipoles of the nitrile groups would be too much to overcome.

Sometimes, the attractions between molecules are a little more complicated. Two molecules may have different interactions on their own, but when placed together still manage to interact with each other. For example, dichloromethane and hexane mix together pretty well.

How can that be? Dichloromethane has dipole interactions. Hexane has nothing more than London interactions. Ethanenitrile and hexane didn't mix for that very reason.

But the problem was not whether those two molecules could interact with each other; they could. The problem was that ethanenitrile would not give up its dipole-dipole interactions for the small amount it could gain by mixing with hexane.

In this case, the dipoles between dichloromethane are much smaller; they aren't held back from the hexane molecules as strongly. On the other hand, the interaction between the the hexane and dichloromethane is actually amplified a little bit. Whereas hexane molecules rely solely on weak, transient London interactions to cling to each other, dichloromethane has a permanent dipole.