180 OUTLINE OF CHEMICAL STUDY 



206. Heat from Chemical Action. Not only does heat 

 aid chemical action, but it is also given off during such 

 activity. Some of the chemical energy set free during a 

 chemical change is transformed into heat, and this is one 

 of our important sources of heat ( 74). 



Experiment 113. Put a small piece of zinc into a test tube 

 with hydrochloric acid. Do you see any sign of chemical action? 

 Grasp that part of the tube where the acid is. What further evi- 

 dence of chemical action do you discover? 



Experiment 114. Cut a bit of metallic potassium the size of 

 a small pea. Throw it upon water and stand away. The potas- 

 sium decomposes the water, setting- free its hydrogen and oxygen. 

 Is this a chemical action? Do you note any sign that heat is 

 given off during this action ? 



In burning a substance we have first to apply heat 

 to it. The chemical action that is caused by this gives 

 off enough energy itself to heat more of the mass ; and 

 so the combustion keeps on by its own heat, until 

 stopped by some means. 



207. Compounds and Mixtures. We have seen that 

 in a compound the molecules are all alike, and every 

 one contains the same number of atoms of each element 

 in the substance. That is, the molecules of each ele- 

 ment have been broken up, their atoms combining with 

 other atoms to form a new sort of molecule. In a mix- 

 ture no chemical combination takes place. The molecules 

 of each element or compound lie side by side in the 

 mixed mass ; each is unchanged and no new molecule 

 is formed. The proportion of substances in a mixture is 

 not definitely fixed, as in a compound, but the same ones 

 may be mixed in any proportion whatever. 



