To calculate conservation of mechanical energy, initial mechanical energy is subtracted from final mechanical energy true or false?

True according to a quick google search.

Yes, if it does not change, then the mechanical energy could be conserved.

However, there is a catch, that catch is that the total mechanical energy could have fluctuated in between. A quantity is conserved if it does not change. The only, definite way to calculate conservation of mechanical energy is to show that its time derivative is 0.

With that being said, the total mechanical energy of a system could have been conserved if the difference between the initial and the final mechanical energy is the 0, but it is not certain.

Think of it like a parabola, if the final and initial mechanical energy are the same, there still can be great variation in between.

explanation:

detials on each model with digrams

hydrocarbons are organic compounds that contain only hydrogens and carbons. they can be saturated (those which contain only carbon-carbon single bonds) or unsaturated (those which contain carbon-carbon double or triple bonds) in nature. hydrocarbons can undergo several reactions like substitution, elimination etc.  

on substituting one or more hydrogen atoms in hydrocarbon it results in the formation of haloalkane. as the halogen atoms are large compared to the carbon and hydrogen atoms, due to which the molecular weight increases and the bond become polar because of presence electronegative halogen atom and results in the increase in boiling point of the haloalkane.

thus, the correct option is (b) that is the boiling point of the new compound increases on substituting a hydrogen atom with a halogen in a hydrocarbon.