Appearance and structure of organic compounds

About the appearance of organic substances we may say that this is connected with:
  1. the number of C-atoms per molecule
  2. the number of unsaturated bondings
  3. the character of substituents / special or functional groups

in other words:
Unsaturated compounds will tend to be oily, where saturated substances are fattyer, more solid.
The molecules become less rigid when the number of double or triple bonds increases, and with that the relative number of H-atoms decreases.
Fat is more saturated than oil.
But at the same time you may not forget the possible polar character of a substance, which also has an important influence: the more polar, the less volatile.

for example: Ethane is a gas, but as soon as you substitute one H-atom with an OH group, (than you have ethanol = alcohol), then the substance becomes liquid.

att.: the character of a molecule determines also how such a molecule participates in any reaction.