Step 1 - Understanding the relation between structure and melting point
The melting point of a substance of a measure of the intensity of its intermolecular forces. If a molecule interacts rather strongly with the other molecules, it will be harder to pull them apart.
Therefore, since the structure of a molecule may determine the strenght of its molecular forces, the melting point can be explained based on the chemical structure. Double bonds, for example, are an important structural feature that can significantly change the melting point.
Step 2 - Understanding how double bonds change the melting point of triglycerides
The molecules in the picture are triglycerides. Note that some of them have double bonds, while others do not, or differ in the number of double bonds. Molecules without double bonds tend to be more linear than molecules with double bonds, thus interacting more strongly with neighbour molecules:
Note that while the first molecule is linear, the second one, which has a double bond, has rather a rough shape. The consequence is that its intermolecular interactions are less strong.
We can state thus that the greater the number of double bonds, the lowest the melting point, due to structural properties.
Step 3 - Ranking the melting points of the molecules in the picture
Let's call the molecules A, B, C and D, respectively. Note that:
molecule A has no double bonds
molecule B has two double bonds
molecule C has six double bonds
molecule D has three double bonds
According to the rule of thumb we established in step 2, the melting points, from lowest to highest, would be:
[tex]C
