Is the density of a two-component mixture always between the densities of its individual components? There exist eutectic mixtures, in which the freezing point of the mixture is below those of either component of the mixture, i.e., the freezing point of the mixture is not intermediate between that of each component, but is instead lower than both.
Can the same occur for density? Are there mixtures of substances $\ce{A}$ and $\ce{B}$ such that either $\rho_\mathrm{mix}$ is greater than both $\rho_\ce{A}$ and $\rho_\ce{B},$ or $\rho_\mathrm{mix}$ is less than than both $\rho_\ce{A}$ and $\rho_\ce{B}?$
Or is the density of a two-component mixture always bracketed by (i.e., some intermediate value between) the densities of its individual components?
No, there is no law that requires the density of a mixture to fall between the densities of pure components. It does so most of the time, but the exceptions are not unheard of. Here's one.
Water: density $1.00\ \rm{g/cm^3}$
Hydrazine: $1.02\ \rm{g/cm^3}$
Hydrazine hydrate: $1.03\ \rm{g/cm^3}$
So it goes.