In essence, the question is asking which of the options is an acid or a neutral compound; thus not a base. A base is a compound that accepts free hydrogen ions while in aqueous solution and as such they usually lack hydrogen ions that can be released in solution. If you ionize all the compounds/molecules above, the only on that has free hydrogen ions is OPTION D (which is an organic acid and not a base).
A buffer system usually will need a weak acid, like H2CO3, which is carbonic acid, and a strong base, and the only compound that could figure a fairly strong base among the options given is Na2CO3. NH3 and NaHCO3 are weak bases and HF is a strong acid. Therefore the best answer will be letter A
Recrystallization is the process in which a compound is dissolved in a hot solvent and then allowed to cool which forms the crystals.
It is a technique used for purification of solids.
Recrystallization mainly depends on the solubility of the solvent that means the solid should be soluble in the solvent when it is hot and should be insoluble in the same solvent when it is cool.
The observations while doing recrystallization is to observe the purified crystals obtained and to filter it out.
By heating the solution a saturated solution can be changed into an unsaturated solution. Without adding any solvent it can be changed into an unsaturated solution. Example: on heating a saturated solution of sugar in high temperature, it starts dissolving.
