Recrystallisation is a very important technique in synthetic chemistry. A critical step is in selecting a solvent system where the impurities are very soluble but the product is only partly soluble. The mixture for purification is heated to dissolve all of the product in a super saturated solution. On cooling the product starts to crystallise out of solution as it's solubility is poor at room temperature. If you have chosen your solvent correctly your impurity will stay in solution. Watch this short video and answer the following questions about recrystallisation. Note: this is an example recrystallisation and not actually a benzocaine recrystallisation. Video Player is loading.Play VideoPlaySkip BackwardSkip ForwardMuteCurrent Time 0:00/Duration 0:00Loaded: 0%Stream Type LIVESeek to live, currently behind liveLIVERemaining Time -0:00 1xPlayback Rate2x 1.75x 1.5x 1.25x 1x, selected 0.75x 0.5x ChaptersChapters Descriptionsdescriptions off, selected Captionscaptions and subtitles off, selected Audio TrackPicture-in-PictureFullscreenThis is a modal window.Beginning of dialog window. Escape will cancel and close the window.TextColor[select: White, Black, Red, Green, Blue, Yellow, Magenta, Cyan]Opacity[select: Opaque, Semi-Transparent]Text BackgroundColor[select: Black, White, Red, Green, Blue, Yellow, Magenta, Cyan]Opacity[select: Opaque, Semi-Transparent, Transparent]Caption Area BackgroundColor[select: Black, White, Red, Green, Blue, Yellow, Magenta, Cyan]Opacity[select: Transparent, Semi-Transparent, Opaque]Font Size[select: 50%, 75%, 100%, 125%, 150%, 175%, 200%, 300%, 400%]Text Edge Style[select: None, Raised, Depressed, Uniform, Drop shadow]Font Family[select: Proportional Sans-Serif, Monospace Sans-Serif, Proportional Serif, Monospace Serif, Casual, Script, Small Caps]ResetDoneClose Modal DialogEnd of dialog window. Why do we allow our recrystallisation to cool slowly? Please select the most accurate answer.

Question

BlackTom AI · Accepted Answer

The question asks why the recrystallisation process should be allowed to cool slowly. 
First, consider the idea behind slow cooling: as the solution cools gradually, crystals have time to grow and exclude impurities more effectively. This tends to produce purer product because impurities remain dissolved in the solvent rather than being incorporated into the crystal lattice. 
Since only one answer option is provided, we can focus on its claim directly: 
Option b. 'To allow crystals to form slowly so there is less chance impurities get trapped.' This aligns with the fundamental principle of recrystallisation, where slower crystallisation reduces entrapment of impurities and helps form larger, purer crystals. It highlights a balance between solubility and crystallisation rate: if cooling is too rapid, many small crystals can trap impurities or co-crystallise with solvent, leading to a impure solid. 
Because the question presents a single choice, there isn’t another option to evaluate against in this item. In general, other potential statements (not shown here) might claim rapid cooling improves yield or purges impurities, but those notions are incorrect in the context of recrystallisation: rapid cooling often increases impurities trapped in the crystal lattice or yields many small crystals with more defects. 
In summary, the principle behind slow cooling is to promote orderly crystal growth and minimize impurity incorporation, which is exactly what option b states.

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