What Happens When Saltwater Evaporates? The Science of Salt Crystallization

Disclosure: this post contains affiliate links, which means I may receive a commission if you click a link and purchase something at no extra cost to you. Please check out our policies page for more details.

Why is the ocean salty, even though the rain that falls into the ocean is fresh?

Great question, my friend!

And today’s experiment will help us understand why oceans are salty, all while we make some super cool salt crystals and learn about evaporation.

Ready to grow your own crystals? Here we go!

How to make the Salt Crystallization STEM experiment

Supplies you will need

For this experiment, you will need the following:

Supplies needed for the Salt Crystallization STEM experiment

Before you start

This experiment takes several days of sitting in a window and waiting for the water to evaporate.

Instructions

Here is how to do this experiment with your child:

Step 1: Make the saltwater solution

Start out with about 1-2 cups of hot water, depending on how large your bowl is for the experiment.

Pour salt gradually into the cup of hot water and stir, making sure the salt dissolves in the cup. Continue adding salt and stirring to mix it into the solution.

The solution is ready to go when you add salt, and it will not mix into the solution (you’ll see some salt at the bottom of the solution).

Pouring salt into the hot water cup and mixing

Step 2: Add modeling clay to the bowl

Make little balls out of non-drying modeling clay (see the Supplies section for what we used) and place them at the bottom of the empty bowl for the experiment.

We tried using Play-Doh first and it disintegrated, so definitely use the right type of clay!

Modeling clay balls at the bottom of the bowl

Step 3: Add toothpicks to the balls of clay

Place a toothpick into each ball of clay, sticking straight up.

Placing toothpicks in each modeling clay ball

Step 4: Add saltwater solution to the bowl

Slowly add the saltwater solution to the bowl with the clay and toothpicks. Key word: SLOWLY! If you pour too fast, you risk knocking the balls of clay over.

We filled our bowl so a little less than half of the toothpicks sticking out from the saltwater solution.

Once you have your saltwater solution in the bowl, place the bowl in a sunny window and give it a few days. You’ll start to see the saltwater solution dissolving and the salt will adhere to the toothpicks (and the sides of the bowl).

Slowly pouring the saltwater solution into the bowl with the modeling clay and toothpicks

The STEM behind the Salt Crystallization Experiment

This experiment teaches:

Physical and chemical changes
Solubility and saturation
The water cycle

How it works

This experiment shows the process of salt crystallizing through evaporation. It’s a great experiment to show physical changes to solutions, solubility (the ability to be dissolved), and even showcase the water cycle!

First, we create a supersaturated solution by dissolving salt (the solute) into hot water (the solvent) until no more salt can be dissolved into the solution.

We place the clay and toothpicks in the bowl to create a nucleation point for the salt to crystallize.

As the water evaporates over several days, the salt begins to form crystals on the toothpicks. Once all of the water is evaporated, you’ll see lots of salt crystallized on the toothpicks!

Physical changes

This STEM experiment is a great way to observe the process of evaporation of a liquid.

As the water in the bowl turns into a gas (water vapor) and escapes into the air, it leaves the salt behind. With the water slowly evaporating over the course of several days, it leaves the salt behind, and that salt begins to form salt crystals, called crystallization. That crystallization is a physical change, as the salt changes from a dissolved solid to a crystalline solid.

What we’re witnessing is a physical change!

But why does the salt form crystals on the toothpicks? Why not in the water?

The salt begins to stick to the toothpicks because the toothpicks provide a surface for the crystallization to begin. The wood on the toothpicks becomes a nucleation point, which is essentially where the salt can start to grab onto something and form a crystal.

Solubility and saturation

This experiment is great for demonstrating solubility and saturation!

In our experiment, we are taking a solute (the salt) and mixing it in a solvent (the water) to make a solution (the saltwater mixture).

When we continue adding salt into the saltwater mixture, or solution, we reach a point where no more salt can dissolve in the mixture. That makes our solution a saturated solution, or a solution where the maximum amount of solute has been dissolved in the solvent at a given temperature.

The goal when adding salt to the water in our experiment is to make the solution saturated. When the solvent (the water) starts to evaporate, there is less solvent to hold the solute (salt).

That leftover salt starts to adhere to the toothpicks (and the sides of the bowl).

It’s a good demonstration on how saturated solutions can no longer hold the solute when the solvent starts to evaporate.

The water cycle

This experiment is also a great supplement to learning about the water cycle!

As the sun beats down on oceans, lakes, and rivers, that heat starts to evaporate the water. And as the sun evaporates the water in these salty bodies of water, what happens? Salt is left behind!

That’s why oceans are salty even when the rain that falls from the sky is fresh and without salt.

We can see this happening in our experiment by the salt left behind over several days as the water starts to evaporate.

Real-world Applications of this Experiment

Here are a few interesting applications of this experiment in the real world:

Salt production

One of the most direct real-world applications of this experiment is the production of salt itself.

In warmer and dry climates, evaporation ponds are filled up with saltwater from the sea or lakes. As the water evaporates from the sun beating down on it, the salt concentration begins to increase until the pond reaches super saturation and the salt crystallizes.

Once it’s crystallized, they can harvest and sell it!

Removing impurities

In our experiment, we are crystallizing the salt. Crystallization is a very effective way to remove impurities in a solution!

If impurities are present, the process of crystallization can remove those impurities.

How?

When we heat up the water to allow the salt to dissolve, the salt molecules can move about freely in the solution. Once the solution cools down, the water can no longer hold all of the salt molecules and those salt molecules start to form crystals (on the toothpicks in our experiment).

That crystal keeps growing and molecules are added to it over time, as the water evaporates. If the geometry of the molecule fits the crystal structure, it’s more likely to actually stay on the crystal structure. If it’s an impurity and doesn’t match the crystal structure, then a molecule that actually does match the structure can come along and boot the impurity.

So the salt we end up with is actually cleaner than what we started with!