CLASSES OF SUBSTANCES 183 



7. Explain how heat may be given off during chemical changes. 

 Show how the heat set free during combustion serves to keep up 

 the burning. 



8. Explain the difference between a compound and a mixture. 



9. Tell everything about these substances that you can learn 

 from their symbols : H 2 O (water) ; H 2 SO 4 (sulphuric acid) ; NaCl 

 (common salt) ; C 12 H 22 O 11 (sugar) ; C 2 H 5 OH (alcohol). 



SECTION III 

 CLASSES OF SUBSTANCES 



209. Acid-Forming and Base-Forming Elements. Two 

 important classes of substances are adds and bases. 

 Some elements have the power to unite with others to 

 form acids, while different elements in a similar way 

 usually form bases. Elements that commonly form acids 

 are called acid-forming, or negative, elements; those 

 that form bases are called base-forming, or positive, 

 elements. 



The common negative (acid-forming) elements are bro- 

 mine, carbon, chlorine, fluorine, iodine, nitrogen, oxygen, 

 phosphorus, silicon, and sulphur. Of positive (base-form- 

 ing) elements there are aluminium, calcium, copper, gold, 

 iron, lead, mercury, nickel, platinum, potassium, radium, 

 silver, sodium, tin, and zinc. The element hydrogen 

 seems to hold a neutral place, being found in both 

 acids and bases. 



Sometimes a group of elements acts like a single atom 

 in combining with other elements ; for example, NO 8 

 in nitric acid, or SO 4 in sulphuric acid. Such a group 

 of elements is called a radical. Like elements, radicals 

 are either positive or negative. 



