organic chem

1. same molecular formula; there are structural isomer of the other.

They have different dispersion of charge and hence different Van der Waals force and finally different intermolecular forces and different boiling points.

n-butane has a higher boiling point than 2-methlypropane does.

2. both (1) and (3) can form only one more bond that it can only be attached at the end of the chain. For (2) and (4), they can bond to two more groups as illustrated in your question, so that either one can be inserted into the middle of the carbon chain. (1) and (3) are well known terminal functional groups while (2) and (4) are not.

3. At the beginning when the temperature is increased to a value high enough but not so strong for cracking that the decane vapour forms and leaves the tube and be collected over water.