Physical changes do not change the chemical make up of a substance. Chemical changes do.
Slicing a tomato only changes its proportion and size (which are physical properties), instead of anything inside the tomato. while burning toast changes something inside the toast, making it a chemical change, instead of a physical change.
<span>In chemistry, a catalyst can speed up the reaction (or make it initiate easier) by altering the activation energy, lowering it enough to allow the reactants to react more easily. Some negative catalysts or inhibitors can do the same by increasing the activation energy. </span>
