! the cooling curve below represents the cooling of a sample of gas. according to the cooling curve, what phase(s) would you expect to observe when the substance is at a temperature of 375 kelvin?

According to the cooling curve given above, at temperature 275K, the phases that will be observed is LIQUID AND SOLID.
The flat points on the graph represent the points where phase changes are taking place.The beginning of the first flat point represent the point where the gas become liquid, the extreme end of this point indicate the point where the liquid has started turning to liquid and that is the point of 275K. Thus at that point both phases are present. 

D-no effect on the rate

assuming that your periodic table has eighteen columns, you should highlight   the four elements in the third column from the left of the table.

explanation

neutral atoms of elements after calcium contain d electrons. the energy of d orbitals of a particular main shell lies between the energy of the s and p orbitals of the next main shell. for example, electrons in the have energy higher than those in the orbital while lower than those of the orbital.

by the aufbau principle, electrons fill orbitals one at a time, starting from those of the lowest energy. the orbital is therefore filled right after orbitals and before orbitals.  

potassium has electron configuration and calcium . the s orbital of a main shell holds a maximum of two electrons. the next element would thus have the electron configuration and satisfy the requirements.

a neutral atom of the element contains three electrons on top of the most recent noble gas element. it should thus be placed in the third column of the periodic table. both scandium and yttrium would satisfy the conditions. an aufbau diagram indicates that the 4f orbital is filled before the 5d. however, it appears that exceptions exists for both lanthanum and actinium. neutral atoms of the two d-block elements also demonstrate the and should be highlighted as well.