A material that gives up hydrogen ions in a chemical reaction. Compare with base . A material that accepts hydrogen ions in a chemical reaction. A substance that increases the rate of a chemical reaction, without being consumed or produced by the reaction. Catalysts speed both the forward and reverse reactions, without changing the position of equilibrium .
Enzymes are catalysts for many biochemical reactions.
Compare with anion . A cation is a positively charged ion. Metals typically form cations. Compare with physical change . A chemical change is a dissociation, recombination, or rearrangement of atoms.