ZIMSEC O Level Combined Science Notes: Chemical Reactions

Introduction to reactions

  • A compound is formed when a chemical reaction takes place
  • Chemical change takes place to form the compound
  • Word equations can be used as a short hand way to show what has happened
  • An example of a chemical equation is:
  • \text{iron + sulpher}\rightarrow \text{iron sulphide}
  • As already stated a mixture is made by a physical (as opposed to chemical) change
  • Iron and sulpher are elements
  • Iron is made up entirely of iron atoms
  • Sulpher is made up entirely of sulpher atoms
  • These two elements can be combined in a chemical reaction to form a new substance called a compound
  • These two have to be heated first before they can combine to form iron sulphide
  • The heat is required to start the reaction for once it starts it gives out heat
  • The mixture glows red as it reacts even if the burner is removed
  • Iron and sulpher combine at a molecular level
  • This is done when iron atoms form bonds with sulpher atoms
  • These bonds can be broken by chemical means to obtain sulpher and iron again
  • Since iron and sulpher are elements they cannot be further broken down
  • Iron and sulpher are thus the simplest substances (elements) that can be obtained if iron sulphide is broken down
  • Chemical reactions always result in the formation of new substances
  • Some reactions require heat in order to start/take place

Chemical reactions and Oxides

  • Oxygen is an that is found in the atmosphere
  • When elements burn up in air they combine with oxygen to form new compounds
  • These compounds are known asĀ oxides
  • Oxides-are compounds formed when elements combine with oxygen
  • For example carbon in firewood combines with oxygen when it is burnt to form carbon dioxide
  • And sometimes carbon monoxide

To access more topics go to the Combined Science Notes page.