Apolypeptide in its native conformation has weak interactions between its r groups. however, when that same polypeptide is denatured, these r groups similarly interact with water. why, then, is the native structure more stable than the denatured one

A protein is more stable in its native form, because apart of weak interactions between R groups, it also presents other stronger interactions, as those including covalent bonds  

Explanation:

For example, covalent bonds between sulfur atoms when disulfide bridges are built.  These links are very difficult to break and maintains the protein shape.  Disulfide bonds are a few but they use to incide in the structure of native proteins

c is the answer because when something is left after something evaporates that shows the chemical change.

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.