Chemistry and crystallography are interlinked. I suggests challenging problems of chemistry solved by crystallography:
1. Ionic compounds and electrolysis - Bragss structure of NaCl and KCl
2. Tetrahedral carbon - Vant Hoff and structure of diamond - Kathleen Lonsdale
3. Structure of benzene - Kekule and works of Lonsdale on hexamethylbenzene and hexachlorobenzene.
and finally something more advanced eg penicyllin and vitamin B12 - Doroty Hodgkin.