12 GENERAL SCIENCE 



When a substance is composed of atoms which are all 

 alike, the substance is said to be a simple substance or 

 element, such as iron, gold, lead, silver, oxygen, hydrogen, 

 sulphur. When a substance is composed of molecules 

 made up of two or more different kinds of atoms, it 

 is said to be a compound, such as water, salt, sugar, 

 chalk. 



Molecules are very small, but atoms are smaller than 

 most molecules, as many molecules are made up of two 

 or more atoms. A molecule of common salt has two 

 atoms, while a molecule of cane sugar has 45 atoms. 



In ordinary chemistry the atom is the smallest division 

 that can be made of any substance. So the atom is the 

 smallest chemical unit. The kind and number of atoms 

 that are used to make a molecule determine the nature 

 or properties of the molecule and hence the properties 

 of the substance. Most substances used by man are 

 composed of complex molecules and so they are called 

 compounds. These compounds are classified according to 

 their own properties and according to the way in which 

 they affect other substances or compounds. Most of 

 the compounds discussed in this book can be classed as 

 acids, bases, and salts. 



11. Acids. Acids occur in plants, in animals, and 

 in the earth. Citric acid is found in lemons and oranges 

 and gives them their sour taste. It can be prepared from 

 lemon juice, and consists of beautiful crystals which 

 will easily dissolve in water. Malic acid is present in 

 sour apples. Grapes contain tartaric acid, which is 

 used in making cream of tartar and baking powder. 

 The jack-in-the-pulpit and rhubarb contain oxalic acid. 

 Lactic acid is formed when milk sours. Lactic acid is 

 also found in the muscles of man and other animals, 



