Lesson 4

Lesson 4: Pure Substances and Mixtures

How can matter be classified? 

Matter is made up of basic units called atoms. Atoms are the smallest units of an element. They can be combined in different ways to produce different substances. Most of the substances can be classified into 3 types of matter: elements, compounds and mixtures. 

Atoms are the basic building blocks for all 3 types of matter. But they differ in the way their atoms are combined. 

Pure Substances

Elements and compounds are pure substances. They have definite physical and chemical properties, such as appearance, melting point and reactivity. The atoms that make up an element are all the same. Two different kinds of atoms make up a chemically combined particle called molecule.  

Physical changes do not change the identity of the substance. If you slice an apple, it's still an apple. The chemical bonds that hold atoms together cannot be broken easily. To break or form chemical bonds, a chemical change is required. When a pure substance undergoes a chemical change, it is no longer the same substance. 

How can elements and compounds be classified?  

Differences in physical and chemical properties allow us to classify elements. 

• Elements: They are broadly classified as metals, nonmetals or metalloids. There are over 10o elements known to exist. Each one has a place in the periodic table of elements. They all occupy different regions in it. 
• Compounds: They can be grouped into basic categories by their properties. 

1. By their pH: They can be classified as acidic, basic or neutral by measuring their pH. Acids have a pH below 7, bases have pH values greater than 7 and neutral compounds have a pH of 7. 
2. Organic or Inorganic: The word organic refers to compounds that contain carbon and hydrogen. Organic compounds are found in most foods or synthetic goodse. For example, gasoline has octane and heptane. 
1. By their role in the body: Organic compounds that are made by living things are called biochemicals. They are divided into 4 categories, lipids, proteins, carbohydrates and nucleic acids. Carbohydrates are a source of energy (sugars), Lipids store excess energy (fats), Proteins build and repair body structures and Nucleic acids contain genetic information (DNA).