108 ELEMENTS AND COMPOUNDS 



115. Summary. Physical changes do not alter the nature of a 

 substance; chemical changes do. 



Water is composed of hydrogen and oxygen. 



The electrolysis of water gives two volumes of hydrogen for one of 

 oxygen. 



Elements are substances that have never been divided. 



Compounds are made up of elements that are united as the result of 

 a chemical change. 



Mixtures are made up of elements or compounds not united chemi- 

 cally. 



Hydrogen is prepared by the action of certain metals upon certain 

 acids. It is the lightest substance known. 



Hydrogen burns with a very hot flame, forming water. Water is 

 also formed in the burning of compounds of hydrogen, such as wood 

 and coal. 



Diffusion is the mixing of substances because of molecular motion. 



Salt is sodium chloride. It is broken up by electrolysis into sodium 

 and chlorine. 



Sodium is a metal, which burns, and which acts upon water. It 

 reunites with chlorine, giving salt. 



Chlorine is a green gas, which has the power of bleaching and dis- 

 infecting. Ordinary combustibles do not burn in it. 



Hydrogen burns in chlorine, giving hydrogen chloride. In solution 

 this is hydrochloric acid. 



Ammonia is a compound of nitrogen and hydrogen. It unites with 

 hydrogen chloride to give ammonium chloride, or "sal ammoniac." 



Sulphur burns to give sulphur dioxide. Hydrogen sulphide is 

 formed by the decay of organic materials containing sulphur, such 

 as eggs. 



The number of compounds is very large ; but most of them are made 

 out of only 2, 3, or 4 elements. 



116. Exercises. 



1. What are the melting points of the following metals : sodium, tin, 

 lead, copper, silver, gold, cast iron, steel? (See Appendix, Table V.) 



