Which of the following is true of the different types of crystals? A. The different types of crystals differ in angles between the faces and differ in the number of edges of equal length on each face. B. The different types of crystals differ in angles between the faces, but do not differ in the number of edges of equal length on each face. C. The different types of crystals have the same angles between the faces and the same number of edges of equal length on each face. D. The different types of crystals have the same angles between the faces but differ in the number of edges of equal length on each face.

your answer is a. the enthalpy of formation for a pure element in its standard state is always positive.

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.