2 | Pure Substances

Pure Substances

  • Pure substances are those which have fixed physical and chemical properties.
  • For example, they have a consistent composition throughout, fixed melting and boiling points, and chemically react in predictable ways.
  • Physical combinations of pure substances are known as mixtures.
  • Mixtures have variable physical and chemical properties.

 

Elements

  • The simplest type of pure substance is an element.
  • Elements are substances that cannot be chemically broken down into separate substances.
  • For example, iron and sulfur are both elements. Once they are purified, they cannot be chemically separated into simpler substances.
  • There are 118 known elements – 91 naturally occurring elements and 27 synthetic (man-made) elements.
  • All of these are represented on the periodic table of the elements.

 

 iron element pure substance  sulfur element pure substance

Iron (left) and sulfur (right) are both elements. They cannot be chemically broken down into simpler substances.

(Images: Chemicalinterest, Wikimedia Commons; Ben Mills, Wikimedia Commons)

 

Compounds

  • Different elements can chemically combine to form substances known as compounds.
  • Compounds are pure substances, but since they are formed from more than one element, they can be chemically broken down back to these elements.
  • For example, the elements iron and sulfur can chemically combine to form the compound iron sulfide; iron sulfide can therefore be chemically broken down back to iron and sulfur.

 
iron sulfide compound pure substance

Iron sulfide is a compound. It can be chemically broken down into the elements iron and sulfur.

(Image: Benjah-bmm27, Wikimedia Commons)