Your friend is having a hard time with the terminology in this unit. He is confused between the terms pure substances, mixtures, elements, compounds, homogeneous, and heterogeneous, as well as the difference between physical and chemical changes. Without just reciting the definitions, explain each term to your friend and provide at least two examples of each that will help him understand the concepts better.

Answer and Explanation:

Pure substances are basically chemical substances with distinct composition and properties that are constant. They may consist of one atom, molecule or compound that cannot be easily separated by physical separation technique. Examples of pure substances are iron, water, gold etc. Mixtures are made up of two or more substances that are not chemically combined together, in which each substance that make up the mixture still retains their chemical identity, and can separated by physical method. Examples of mixtures are brass made up of different metals, salt water, milk etc. Elements are pure chemical substances that are made up of a single type of atom which cannot be easily broken down. For example, hydrogen, oxygen, iron. Compounds are made up of two more different elements that are chemically bonded together by chemical bonds. Examples include NaCl or salt, which is made up of Na and Cl; Water which is made up of Hydrogen and Oxygen Homogenous mixture contains substances of uniform composition, e.g, air which consists of mixture of gases; a mixture of salt and water; milk etc. Heterogenous mixture, on the other hand contains substances whose distribution are not uniform, i.e. non-uniform. Examples are a mixture of sand and water; water and oil etc. Physical change is a type of change that involves the change of form of a matter without the chemical properties being altered. This is a reversible change. On the other hand, a chemical change is an irreversible change in which the chemical properties of a matter are changed. Examples of physical change are freezing of water, melting an ice cube, melting of wax etc.  Examples of chemical change are burning of wood, rusting of iron etc.

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