Evaporation to dryness recovers a dissolved solid from its solution by boiling away every drop of the solvent. The solvent (usually water) leaves as vapour, and the solute is left as a solid in the dish.
An evaporating dish is wide and shallow, so the liquid has a large surface area. The bigger the surface, the faster the water escapes — evaporation happens at the surface, so a shallow puddle dries far quicker than a deep, narrow beaker of the same liquid.
Turn the collar to close the air-hole first, then strike the sparkler at the top of the barrel. The gas lights with a yellow, wavy luminous flame — easy to see but cool. Opening the air-hole turns the flame blue — the hot non-luminous flame — which actually boils the solution. Never light a Bunsen with the air-hole already open.
When you need pure, well-shaped crystals instead, use crystallisation — stop heating at the saturation point and let the solution cool slowly.
Sea water left in shallow pans in the sun leaves behind sea salt. A drying puddle on a hot day is the same idea: the water evaporates and any dissolved dirt is left as a mark.