Which of the following is not an example of chemical weathering?
a. the oxidation of iron
b. the formation of clay
c. the reaction of limestone with acid rain
d. the breaking of a rock into smaller pieces

D. is not chemical weathering.
We know this because when iron oxides, it is reacting with oxygen in the air, with is therefore a chemical change.
When clay forms, it's from the chemical deposition of rocks like granite, which makes it a type of chemical weathering.
Finally, limestone reacting with acid rain is a chemical change, which makes it chemical weathering.
A rock being broken down does not change the chemical structure of the rock, so it is a physical change, and physical weathering, not chemical.

For cyclohexane compounds drawn in a chair conformation:   -axial bonds are vertical  -equatorial bonds are parallel to two bonds within the ring.  thus, as i assume the chair conformation is drawn:   -the x group is in an axial position, as are the five other vertical bonds.  -three axial bonds are on the top face of the ring  -the other three axial bonds are on the bottom face  the position of the second substituent on each ring carbon is equatorial.  gauche groups must be on an adjacent carbon such as carbon 6 and carbon 2, and also gauche groups are rotated by 60 degrees relative to each other.  drawing the newman projections for c1–c2 and c1–c6 aids in visualizing the groups that are gauche to the y group on c1.  click this link and watch some academic videos that may you more understand the topic