Chromium(III) oxide, often called chromic oxide, has been used as a green paint pigment, as a catalyst in organic synthesis, as a polishing powder, and to make metallic chromium. One way to make chromium(III) oxide is by reacting sodium dichromate, Na2Cr2O7, with ammonium chloride at 800 to 1000°C to form chromium(III) oxide, sodium chloride, nitrogen, and water.

c. heat could easily travel through the molecules in the air and escape/enter the flask. removing the air slows down the heat transfer.  

b.

the accuracy of the data.

each chemical family is represented as a group or column on the periodic table. based on chemical properties, there are 6 types of chemical families. group 1 represents alkali metals, group 2 represents alkaline earth metals, groups 3 to 12 represent transition metals, groups 13 to 16 represent non-metals and other metals, group 17 represents halogens and group 18 represents noble gases.