THE ELEMENTS OF PLANT FOOD 13 



Exercise. Read the following symbols, telling the 

 number of atoms indicated in each symbol : S, Fe, 2 N, 

 O, H, CO 2 , 7HC1, HNO 3 , P 2 O 5 , 5H 3 PO 4 , 2 NaCl, 

 7MgO, CaCO 3 , 4 NaCl, A1 2 O 3 . 



Note. In ordinary composition scientific books do not use the 

 symbol in place of the term which it represents. In order to familiar- 

 ize the learner with the symbols, we have hi this book used the symbol 

 alone in many cases. 



Chemical Equations. Chemical action, or reaction 

 between substances, is represented by equations. If we 

 mix iron filings (Fe) and fine sulphur (S) and then heat 

 the mixture in a spoon over a hot flame, chemical action 

 takes place. The iron unites with the sulphur, making a 

 compound called iron sulphide. This union may be rep- 

 resented thus : Fe + S = FeS. It may be interpreted 

 one atom of iron combined with one atom of sulphur is 

 converted into one molecule of iron sulphide. The 

 above is a very simple equation. Suppose we mix the 

 FeS formed in the above experiment with hydrochloric 

 acid, we shall then have the following reaction : 



Iron sulphide Hydrochloric acid Iron chloride Hydrogen sulphide 



FeS + 2HC1 = FeCl 2 + H 2 S 



In this case one molecule of FeS is acted on by two 

 molecules of HC1 and is converted into one molecule 

 of FeCl 2 and one molecule of H 2 S. Note that the sum 

 of the atoms before the sign of equality is the same as 

 the sum of the atoms after the sign ; that is, we have 

 exactly the same number of atoms, but arranged in 

 different groups. Changes both physical and chemical 

 may take place, but there is just the same amount 

 of matter in existence after as there was before the 



