How the Crystals Grow:
Salt dissolves better in hot water than cold water, so as the solution cools the salt wants to come out of solution and crystallize. When you pour the solution over the sponge, this causes the liquid to evaporate. This further concentrates the salt so that it will crystallize. The salt crystals will start to form on undissolved salt or on the sponge. Once the crystals start forming, they grow fairly rapidly.
Things to Try:
Table salt crystals have a cubic shape. Adding the vinegar and growing the crystals on a sponge alters the appearance a bit. You can experiment with different types of salt, such as sea salt, iodized salt, Himalayan salt, and other.
Instead of using a sponge, try growing the crystals on another surface. Good choices include a charcoal briquette, a brick, or a rough rock.
If you use a charcoal briquette, another interesting chemical to add to the mixture is laundry bluing or Prussian blue. It is available online as well as in stores in the laundry section (as bluing) or art section (as Prussian blue). This iron-based solution produces intricate white crystals that readily absorb food coloring. While it is safe to work with, it's best to avoid its use around very young children to prevent any chance they might ingest the iron salt.
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