A catalyst is a chemical which changes the speed of a chemical reaction without being changed itself. Within living organisms, chemical reactions take place all the time. These are called metabolic reactions. An enzyme is a biological catalyst which controls the speed of metabolic reactions in living organisms. Most chemical reactions happen faster when the temperature is higher. The pH of a solution is a measure of how acid or alkaline it is.The scale runs from 1-14. A pH of 7 is neutral. A pH below 7 is acid, and a pH above 7 is alkalineThe shape, and therefore the activity, of enzyme molecules is affected by pH.