Hello, Iris,
The term "closed system" in calorimetry is used to describe measurements that are made in an experiment in which no energy enters or leaves the area of the experiment. It is most easily understood in a exothermic reaction in which heat is released. A researcher may want to know how much heat a large reaction in a production facility will be released in order to design proper cooling and safety systems. That prediction can be made with small amounts in a calorimeter. If 100 g of the reactants release 1000 Joules of energy, then a design can be made to safely accommodate that. But if the test reaction is done outside a calorimeter, much of the heat will simply escape into the surroundings (air, table, test stand, and even researcher in some cases).
This lose may cause the heat measurement to be lower than actual, say 500 Joules. That would create a 50% error in future designs and lead to an unstable reaction.
Bob
