When a mixture of ice and water originally at 0 is heated, the temperature remains constant until all of the ice melts?

When heat is applied during a phase change, there is zero change in temperature. This is because all the heat supplied is used to break the bonds of the solid molecules to form liquid molecules. So in essence, there the temperature remains constant until all solid molecules bond are broken down (until all ice melts into liquid).

So the answer is Yes or True.

alchemy hadn't scientific methods of study, appeal to mysticism and fantasy, the knowledge of chemistry and physics were modest at that time.

alchemy was - but as new rules were discovered it either changed into chemistry or it became "unscientific".