Teachers Notes:

Problem 1:
a. PbS, lead(II) sulfide
b. Fe₂O₃, iron(III) oxide
c. CaCO₃, calcium carbonate
d. BaSO₄, barium sulfate

Problem 2:
Fluorite is the mineral with the greatest hardness and the highest melting point due to the least amount of space between the ions and the greater charge on the calcium ion. The melting point of sylvite and its hardness are lower than halite because the radius of the potassium ion is greater than the radius of the sodium ion.

Extension:
Have students grow their own crystals.

A common process in growing crystals:

1. Start with a water solution containing ions, for example a glass of salt water. Salt water contains sodium and chloride ions.
2. Place the glass of salt water in the sun. As the water evaporates, the concentration of dissolved sodium and chloride ions increases. Eventually, the concentration of ions in solution reaches saturation, the point at which the water contains the maximum amount of ions it can hold. As still more water evaporates, the ions coalesce into a regular repeating arrangement in a process called crystallization. Once a formula unit of salt is created and the water continues to evaporate, additional formula units of salt attach themselves to the already existing ones to form a cubic structure. As the process continues, the cubes of salt become larger. Eventually, all the water evaporates and the salt is completely crystallized.
3. Have students use a magnifying glass to observe the salt crystals.