Question

Why does bleeding stop by rubbing moist alum?

Answer 1

:Hint:Moist alum is a coagulating agent. Whenever it is rubbed on the site of blood bleeding, it causes coagulation of the blood that stops bleeding. Bleeding is stopped by the coagulation of the blood, that is clot formation. This coagulation is due to colloidal solution of protein in blood.

Complete answer:
An alum is a type of chemical compound, usually a hydrated double sulphate salt of aluminium with the general formula \[XAl{\left( {S{O_4}} \right)_2}\cdot12{H_2}O\] where X is a monovalent cation such as potassium or ammonium. By itself, ‘alum’ often refers to potassium alum, with the formula \[KAl\left( {SO} \right)\].
Bleeding is stopped by the coagulation of the blood, that is clot formation. This coagulation is due to colloidal solution of protein in blood. When Potash alum is used on the bleeding part the highly charged \[A{l^{3 + }}\] and \[S{O_4}^{2 - }\] ions act as neutralizing ions for the protein cloud in blood. This causes the blood to clot.
It acts as a haemostatic agent, allowing the blood clot to form. The high ionic strength promotes flocculation of the blood and the astringent chemical causes local vasoconstriction. Thus, blood is negative or colloidal sol and sol particles are coagulated more efficiently by Al3+ ions. When we nib the injured part with moist alum then coagulation of blood takes place. Hence the main reason is coagulation, which stops the bleeding. Therefore, bleeding stops by rubbing moist alum.

Note:
Also, alum has Kashaya (astringent) and Rakta Stambhak (haemostatic) properties. These properties stop bleeding as quickly as it is rubbed on the affected area. Thus, Moist alum coagulates the blood and so generally, formed blood clots stop bleeding.