Exercise 5.5
Allow the solution to cool slowly without disturbing it (Fig. 5.8c). It gives enough time to the particles in the solution to come together, resulting in the formation of larger, shiny, well-shaped and blue-coloured crystals.
Not cross-checked
NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.
Yes — the faster the evaporation, the smaller the crystals; slow evaporation gives larger, better-shaped crystals.
A crystal is a solid whose particles are arranged in a regular geometric pattern, and building that pattern takes time.
When evaporation is slow, the particles have enough time to come together in an orderly way, so they grow into large, shiny, well-formed crystals.
When evaporation is fast (strong heating, or a thin film left on a glass plate), many tiny crystals start forming at once and none can grow, so the salt is a powder of small, poorly-shaped crystals.
The chapter's own example: panga salt, made by boiling concentrated sea brine, and karkatch salt, made by the slow evaporation of seawater, came out as salt crystals of different sizes.