Answer:
Aluminium has a very strong affinity
for oxygen. It reduces the oxides of chromium and manganese to the metallic forms.
This can also be explained on the basis of Ellingham diagram. For details,
consult section 6.11.
\[C{{r}_{2}}{{O}_{3}}\,+2Al\xrightarrow{Heat}\,A{{l}_{2}}{{O}_{3}}+2Cr\]
\[3M{{n}_{3}}{{O}_{4}}+8Al\xrightarrow{Heat}\,4A{{l}_{2}}{{O}_{3}}+9Mn\]
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