The Science of Making Your Own Crystals

When you dissolve salt in boiling water, you are creating a supersaturated solution. This is a special state where the water contains more dissolved material (solute) than it would normally be able to hold at room temperature. Because hot water molecules move faster and are spaced further apart, they can “tuck” more salt molecules between them.

As the water cools, the solution becomes unstable. The water can no longer hold all that salt, and the excess starts to “fall out” of the liquid. In this experiment, the water travels up the string through a process called capillary action. This happens because the water molecules are attracted to the fibres of the string (adhesion) and to each other (cohesion), allowing the liquid to climb upward against gravity.

As the water reaches the string and begins to evaporate into the air, it leaves the salt molecules behind. These molecules begin to stack together in a very organized, repeating geometric pattern, which is what creates a crystal!

You can dive deeper into the mechanics of capillary action at USGS Water Science.

Variables to test

Find out more on variables here.

• Solute Variety
  Try using Epsom salts or sugar instead of table salt. Do different molecular structures result in different crystal shapes or growth speeds?
• Thermal Energy
  Compare using cold water versus boiling water. Does the initial temperature change the total mass of the crystals you are able to grow?
• Substrate Texture
  Does it matter which material you use for the crystals to form on? Compare a rough cotton string, a smooth nylon fishing line, or a wooden stick. This tests how surface area and “nucleation points” affect growth.
• Evaporation Rate
  Place one experiment in a sunny window and another in a dark cupboard. Does the rate of evaporation change whether you get many tiny crystals or a few large ones?

To keep your experiment valid, remember to only change one variable at a time while keeping everything else exactly the same!