An inexperienced chemist adds too little boiling solvent to an impure sample of product and then leaves this mixture to cool. the crystals form with product coated onto impure crystals. that form are pure. do not form. form with impurities coated onto the pure crystals

Recrystallization of a solid is an experimental technique used to separate out impurities from a pure product using differential solubilities of the solids in a solvent at different temperatures. We chose a solvent in which the pure solid has lower solubility at low temperatures but is completely soluble at higher temperatures. The amount of solvent taken must be sufficient to completely dissolve the impure solid formed in a reaction. The impurities must be soluble in cold solvent, so that they can remain in solution after crystallization of pure compound.  When the amount of solvent is too little then we cannot achieve the separation of pure crystals as the pure solid crystallizes on the impurities.

So the correct answer will be the crystals form with product coated onto impure crystals

substituting a hydrogen atom with a halogen in a hydrocarbon:

- the single bond remains single. a is wrong.

- the bond with hydrogen is as strong as with a halogen. c is wrong

- a hydrocarbon is saturated if there is no double bond. we do not know that so d is wrong.

- carbon will always have four valence electrons. e is wrong.

by elimination, the correct answer is b. the boiling point of the new compound increases.