What Makes Up Our World?

Elements/compounds/mixtures

            Everything on Earth can be classified as:

• an element, characterized by having atoms with the same number of protons in its nucleus (examples: hydrogen, oxygen, iron);
• a compound, consisting of two or more elements joined together by chemical bonds (examples: water, quartz and most minerals, carbon dioxide);
• a mixture, with characteristics that can vary greatly from sample to sample and which can be physically separated into its parts (examples: rocks, chairs).

Four basic types of chemical reactions

            There are four basic ways in which elements can react to form compounds, or the reverse:

• Synthesis or addition reaction

This can be represented as:  A + B ® AB

Example:  Pb (lead) + S (sulfur) ® PbS (galena)

• Decomposition or analysis reaction

This can be represented as:  AB ® A + B

Example:  2H₂O ®  2 H₂ + O₂

• Single replacement (displacement) reaction

This can be represented as:  AB + C ®  AC + B

Example:  Fe₃O₄ + 2 C ®  3 Fe + 2CO₂

• Double replacement (displacement) reaction

This can be represented as:   AB + CD ®  AD + CB

Activity: Models of Mineral Formation

            A simple but effective model of how some minerals can form through a double replacement reaction involves pouring a little potassium iodide (KI) into lead nitrate (PbNO₃) to produce lead iodide (PbI) and potassium nitrate (KNO₃):

KI + PbNO₃  ®  PbI + KNO

The two clear liquids produce a sudden color change that creates what at first appears to be a bright yellow liquid. But if left to stand, the yellow settles to the bottom as a solid precipitate (PbI).
 [Safety note: This reaction involves lead and should be used with caution, if at all!]

            Use your resources to find one or two other simple demonstrations that model mineral formation, and describe them here: