For a chemical reaction to take place substances must have the necessary affinity with each other, and can be mixed in specified proportions. Only certain substances react together in given circumstances, and the different molecular combinations that result, always have the same proportion of elements as the original substances.
A chemical combination between iron (Fe) and sulphur (S) making ferric sulphide (FeS) will follow their atomic weights:
If we begin with 60 grams of Fe, there will be 4 grams of Fe left over after the reaction has taken place. These leftovers may be important. In the production of polymers, the remaining products from the reaction are called residual monomers. These by-products usually follow the plastics in the process as a sort of parasite, even though they are not chemically bound to them. This physical combination is very unreliable and can lead to problematic emissions in the indoor climate.
It is possible to calculate how much of each of the different elements is needed to produce a particular substance. In the same way we can, for example, calculate how much carbon dioxide (CO2) is released when limestone is heated up:
CaCO2 has the following weight, through adding the relative atomic weights:
This means that 44 g of CO2 are given off when 100 g of limestone is burned.
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