Explain why the catalyst is used as a very fine powder and larger pieces of iron are not used.

Catalyst is used as a very fine powder and larger pieces of iron are not used. This is because the surface area of catalyst needs to be large so that more of the surface is exposed to the substrate and more of the substrate is catalyzed.

=> Larger Pieces of Iron has a smaller surface area than the fine particles.

=> Larger the surface area of catalysts/enzymes , more will be the reaction rate and vice versa.

Hope this helped!

there is none because earth is stationary and flat.

chemical changes occur when a substance combines with another to form a new substance, called chemical synthesis or, alternatively, chemical decomposition into two or more different substances. these processes are called chemical reactions and, in general, are not reversible except by further chemical reactions. some reactions produce heat and are called exothermic reactions and others may require heat to enable the reaction to occur, which are called endothermic reactions. understanding chemical changes is a major part of the science of chemistry.