1) Unsaturated fat molecules are long chains with double of triple bonds between a pair (or more) of carbons
2) When an organic compound has double or triple bonds between some carbon atoms, it contains less number of hydrogens than the chains with only single bonds can have.
3) When all the carbon atoms only have single bonds, then it is saturated (has the maximum number of hydrogens possible for a moleucles of that size). The opposite is an unsaturated chain.
