Measuring instantaneous rates is the most direct way of determining the rate law of a reaction, but it is not always convenient, and it may not be possible to do so with precision.
piece (pÄs) n. 1. A thing considered as a unit or an element of a larger thing, quantity, or class; a portion: a piece of string. 2. A portion or part that has been separated from a whole: a piece of pie. 3. An object that is one member of a group or class: a piece of furniture.
piece - an item that is an instance of some type; he designed a new piece of equipment; she bought a lovely piece of china; item - a whole individual unit; especially when included in a list or collection; they reduced the price on many items 3.