why does bleeding stop when alum is applied on a fresh wound?

In earlier post, you have been answered this question based upon the blood physiology. Now, let us understand the chemistry involved in it.
The chemical formula of alum is K2SO4. Al2 (SO4)3. 24 H2O
Blood contains negatively charged particles. On the other hand, alum contains positively charged particles Al3+. On applying alum on fresh wound, coagulation of negatively charged particles with Al3+ ions takes place and bleeding stops.
Hope, the concept is clear to you.

Good luck! 

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