Which is proof that in any synthesis reaction, a chemical change has occurred?

in a synthesis reaction, two reactants combine to form a new compound.

many synthesis reactions involve a phase change.

in a synthesis reaction, oxygen is always converted to carbon dioxide.

all synthesis reactions are exothermic.

Option (a) is the correct answer.

Explanation:

A chemical reaction is a reaction in which there is formation of new compounds due to change in chemical composition of the reactants during the reaction.

For example,

Therefore, we can conclude that a proof that in any synthesis reaction, a chemical change has occurred because in a synthesis reaction, two reactants combine to form a new compound.

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.