How can you detect the point of crystallisation?

• A. Place a stirring rod in the solution and observe the formation of the first crystals on it
• B. Measure the pH change
• C. Observe a sudden color change
• D. Notice a drop in temperature of the solution

Answer: (A)

Crystallisation begins when dissolved solute starts to come out of solution and form solid crystals. The clearest and most practical sign is the first appearance of crystals. Using a stirring rod gives a surface for nucleation, so crystals tend to form on it and you can observe exactly when crystallisation begins.

Measuring pH doesn’t tell you about crystal formation, since acidity or basicity isn’t tied to when solids start to appear. A sudden color change isn’t a typical indicator for crystallisation, as many solutions keep their color even as crystals form. A temperature drop isn’t a reliable cue either, because crystallisation depends on reaching supersaturation and providing a site for crystal growth rather than just a change in temperature.