Question

Some fats, like oil, are liquid at room temperature, while others, such as butter, are solid. A) Explain how the structure of these molecules determines whether they are liquid or solid. B) Somethings, such as sugar, easily dissolve in water while others, like oils, don’t dissolve in water. What aspect of the molecule’s structure determine whether it dissolves in water or not?

Answer 1

a)               To form a solid, molecules need to pack together nicely, while in a liquid there is less order and the molecules flow around each other.

                  Butter has animal fat which is saturated ; like more densely packed due to the structure of carbon bonds ( maximally bonded carbon atoms in the 12-22 carbon fatty chains). This allows the chains to sort of dovetail and next to each other, enhancing intermolecular attractions that encourage solidification of butter.

                  But , when we notice oils ,which is also a fat, comes in the shape of a liquid at the room temperature. The reason is also based on it's molecular structure. In oils, they have more unsaturated bonds( double bonds) creating kinks; bends in the chain. Anyway the unsaturated kinks discourage the geometric complementarity that gives rise to the solidifying attractions. Since oils are rich with unsaturated sites they therefore tend to be in liquid form.

                     

b)                       The sugar we use, is a molecular solid, in which the individual molecules are held together by relatively weak intermolecular forces.When sugar dissolves in water, the weak bonds between the individual sucrose molecules are broken, and these C12H22O11 molecules are released into solution. It takes energy to break the bonds between the C12H22O11 molecules in sucrose. It also takes energy to break the hydrogen bonds in water that must be disrupted to insert one of these sucrose molecules into the solution.

                         Sugar dissolves in water because energy is given off when slightly polar sucrose molecules form intermolecular bonds with polar water molecules. The weak bonds that form between the solute and the solvent compensate for the energy needed to disrupt the structure of both the pure solute and the solvent. Thus sugar gets dissolved in water.

                        But in the case of oils ( fats ) , they do not have any polar part in their molecular structure and so for them to dissolve in water they would have to break some of water hydrogen bonds. So non polar molecules are unable to dissolve in polar solvents. Hence oils are not dissolved in water.              

• Sugar in water;

                    

•                Oil in water ;