In addition, how long a reaction is observed for depends on the rate of reaction. Reactions can vary from being almost instantaneous to taking years to complete. In the lab, reactions are usually followed over a few seconds or minutes.
The change in mass of a reactant or product can be followed during a reaction. This method is useful when carbon dioxide is a product which leaves the reaction container. It is not suitable for hydrogen and other gases with a small relative formula mass, Mr. The units for rate are usually g/s or g/min.
The change in volume of a reactant or product can be followed during a reaction. This method is useful when a gas leaves the reaction container. The volume of a gas is measured using a gas syringe, or an upside down burette. An upturned measuring cylinder can also be used but this is less accurate. The units for rate are usually cm3/s or cm3/min.
A few reactions produce a colour change, for example due to the production of iodine. A colorimeter measures colour change.
The formation of a precipitate causes the reacting mixture to change from transparent to opaque. The rate of reaction can be measured by timing how long it takes for a cross (drawn on a white tile) to disappear.
The rate of reaction can be analysed by plotting a graph of mass or volume of product formed against time. The graph shows this for two reactions.
The gradient or steepness of the line is equal to the rate of reaction: