Amixture that results when substances dissolve to form a homogeneous mixture

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

since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.