### 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 as 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.

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