Place ½ cup of water in a shallow microwave-safe bowl. Use a microwave to heat the water to a boil. Use hot pads to remove the bowl of hot water from the microwave. Stir ½ cup of table salt into the hot water. Add 2 teaspoons vinegar to the salt water. Put a piece of sponge in the center of the bowl of salt water. Move the bowl to a spot where it can be observed for one or two weeks. Take photos of the crystal growth each day to record the progress. These photos can be used to give a presentation about the science experiment.
Salt crystals will form as the solution cools. Crystals will gather on the sponge because it is a rough surface that will give the crystal support as they increase in size. Once the crystals start forming, they will increase in size and number. Crystals will continue to form as water evaporates from the salt solution. Crystal formation stops when all the water has evaporated.
Growing crystals can be a creative way to introduce chemistry and geology. A supersaturated solution is a chemistry concept. Starting with a saturated salt solution is critical to succeeding at salt crystal experiments. That crystals are produced naturally and artificially is a geology concept. This salt experiment could initiate a discussion about lab-produced gems and natural gems.
Children are often fascinated by changing something in an experiment. There are many ways for them to express their creativity with salt crystal experiments. They could add several drops of food coloring to the sponge before placing it into the saturated salt solution so that color crystals will form. They can make a saturated solution using alum or baking soda to form different-shaped crystals.