Why is the higher the ammonia concentration, the smaller the density?

　　Because the molecular mass of water is less than that of ammonia.

　　1. The mass of ammonia molecule is smaller than that of water molecule.

　　2. Ammonia dissolved in water is equivalent to ammonia molecules entering the gap between water molecules (of course, the volume of the solution should be increased).

　　3, for example, you can see that in a container containing soybeans, mix the same volume of small iron particles and foam particles (very light). Suppose the final mixture has a large volume, then who do you think is the most dense? The more foam particles are mixed, the smaller the density is.

　　When the two substances are mixed, there are two reactions, one is chemical and the other is physical. It will not only change the spatial structure of the original molecules or associated compounds, but also form a new arrangement structure. There may be stacking arrangement or chimeric type. When two substances are mixed, the molecules will be embedded into each other due to the different spatial structures of the two substances.

　　In the solution of alcohol and water, small water molecules can exist in the gap between alcohol macromolecules, which is why 20 ml of water and 80 ml of alcohol can not get 100 ml of mixture. Is it understandable that the more alcohol, the more voids that cannot be filled by water?

　　In fact, these are not because of molar mass, but because of density.

　　The density of aggregated ammonia is 0.75g/cm3 and the density of water is 1.0g/cm3. The more ammonia, the closer it is to the density of ammonia, and the more water, the closer it is to the density of water.

　　Of course, the specific density should be very complex. If the solubility is large or close to infinity, the general trend of density is close to the density of solutes in aggregated state (the molecular spacing of solution cannot be the spacing of gaseous state) with the increase of concentration. However, there may be twists and turns in the middle. For example, it is possible to become larger and then smaller, or to become smaller and then larger. The way of intermolecular interaction is complex. We can't generalize.

　　Specifically, you can think from mathematical calculus. For example, for a solute with infinite solubility, when its concentration reaches 99%, what do you think its density is? If it is 99.99%, if it breaks through the modern purity and reaches more than a dozen 9s? It is certain that the density is close to that of pure matter. When the concentration becomes 1%, 0.01%? How about smaller? Isn't that pure water?