Freezing point depression is: A. When a substance freezes and then quickly drops in temperature.
B. When a substance drops in temperature until it freezes, then quickly rises in temperature but stays frozen.
C. When a substance does not need to be as cold as normal in order to freeze.
D. When a substance must be colder than normal in order to freeze.

Freezing point depression is when a substance must be colder than normal in order to freeze.

Freezing point depression often occurs when a solute is added to a solvent. This reduces its freezing point. Since temperature is reduced, in other words, this means that the substance must become colder than normal for it to start freezing.

Hence, freezing point depression is when a substance must be colder than normal in order to freeze.

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the dissolution of polymer can be thermodynamically described in 2 steps.

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.