couple of questions

[akimhaneul]

1) Why is it that out of all hydroxides, only akali metals and akaline earth metal hydroxides are considered strong bases? Why not Al(OH)3?

2) Why does adding impurities to a compound broaden the melting point of the compound?

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[Happy X]

1) Mostly because all other metal hydroxides are insoluble (look for Ksp), while all GroupI&GroupII metal hydroxides are soluble (or slightly soluble, eg slaked lime). Insoluble hydroxides will release insignificant amount of OH- and thus they are not strong base.
Besides, Al(OH)3 is an interesting case in chemistry world because it can react with both acid (HCl) and base (NaOH). this is called an amphoteric compound.

2) Adding impurities will disrupt the lattice force that holds the solid together, which means you need less energy to separate the molecules/ions

Hope that helps.

 

[aldol16]

1) With non group 1/2 metals, you run into the problem of Lewis acidity with the metal even if the ligands release hydroxide into solution. So Al3+ has empty orbitals that can accept electron pairs, making it a Lewis acid as well.

2) Impurities will make a compound more heterogeneous (not in the solution sense) and each new complex will have a slightly different melting point, giving you the appearance of a broad melting point. So say you have metal M and it melts at 500 K. You add oxygen into it and by so doing, you form MO, MO2, MO3, etc. MO melts at 510 K, MO2 melts at 515 K, and MO3 melts at 517 K. Now from 500 K to 517 K, you're going to observe these various solids melting and thus the melting point will appear broad.