How can you detect the point of crystallisation?

• A. Place a stirring rod in the solution and see the formation of the first crystals on it.
• B. Observe a color change in the solution to blue.
• C. Look for the solution turning cloudy with no crystals.
• D. The temperature reaches the boiling point.

Answer: (A)

Crystallisation happens when the solution becomes saturated and the dissolved material starts to come out as solid crystals. The moment you see tiny crystals appear, especially on a stirring rod, is the point at which crystallisation has started—nucleation marks the first formation of solid crystals from the solution. The other signs aren’t reliable indicators: a color change to blue doesn’t relate to crystal formation, cloudiness without visible crystals can just mean undissolved particles or precipitation, and heating to the boiling point is about boiling the solution, not forming crystals. So the first appearance of crystals on the stirring rod best signals the crystallisation point.