Why does salt disappear in water?

Imagine you have a glass of water. You add a spoonful of salt.

At first, you can clearly see the salt crystals. Then you stir. After a few seconds…

The salt seems to disappear.

Materials. If you're doing this mission in person, get:

• 📋 A clear cup
• 📋 Water
• 📋 A small amount of table salt
• 📋 A spoon

Before you mix. Place a small amount of salt on a plate or spoon. Look closely at it.

Add a small amount of salt to your water. Stir for about 20–30 seconds. Now look carefully.

Scientists don't rely only on what they can see. They look for other evidence.

Imagine you had two cups:

One particularly useful test: what happens if we remove the water?

If the water evaporates, salt crystals are left behind. So the salt did not vanish — it was there the entire time.

Imagine we could keep magnifying a grain of salt. A grain that looks tiny to us is actually made of an enormous number of particles.

Everything you can touch is made of matter.

• 📝 Your desk is matter.
• 💧 Water is matter.
• 🧂 Salt is matter.
• 👤 Your body is matter.

Matter is built from incredibly small particles called atoms.

An atom is one of the basic building blocks of matter. Atoms are so small that you cannot see an individual atom with your eyes or an ordinary classroom microscope. But scientists have developed ways to study how atoms behave.

Atoms themselves contain even smaller particles. There are three especially important ones:

• ➕ Protons — positive charge (+)
• ⚫ Neutrons — no electrical charge
• ➖ Electrons — negative charge (−)

Protons and neutrons are found together in the center of the atom — this center is called the nucleus. Electrons occupy the space around the nucleus.

✅ Quick check

Not every atom is the same. The type of atom is determined by how many protons it has.

Each type of atom belongs to an element. An element is a substance made from one type of atom.

For example:

There are many different elements, and they can interact in different ways to create the substances around us.

The water in your cup is made of water molecules. A molecule forms when atoms are chemically joined together. One water molecule contains two hydrogen atoms and one oxygen atom.

So even though a drop of water looks like one continuous substance, zoom in far enough and you would find enormous numbers of water molecules.

Table salt is called sodium chloride. Its chemical formula is:

But there is something interesting about salt.

Remember: protons have positive charge, electrons have negative charge.

Normally, an atom has equal numbers of protons and electrons — the positive and negative charges balance. But atoms can lose or gain electrons. When that happens, the particle becomes electrically charged. A charged atom or group of atoms is called an ion.

Opposite charges attract — that's why Na⁺ and Cl⁻ are drawn to each other.

A salt crystal contains huge numbers of Na⁺ ions and Cl⁻ ions arranged in a repeating structure. The positive and negative charges attract each other — this helps hold the salt crystal together.

Now let's return to our cup. It contains both salt and water, and water plays a big role in what happens next.

Water molecules have regions with slightly different electrical charges. Unlike ions, which have full electrical charges, water molecules have areas that are slightly more positive and areas that are slightly more negative. Because of this, water molecules can interact with charged particles, such as ions.

When salt is placed in water, the water molecules begin interacting with the sodium and chloride ions at the surface of the salt crystal. The water molecules surround these ions and help pull them away from the crystal. This process is what we commonly call dissolving.

Imagine the salt crystal like this — ions packed tightly together. When water surrounds the crystal:

The salt has now dissolved.

No.

The salt particles became separated and spread throughout the water. They are simply much too small for us to see individually.

Look at your cup again. You started with visible salt crystals. Now you may see only clear water.

But based on what we learned, finish this sentence:

The salt is still in the cup because...

Try to include at least one of these ideas: ions · particles · water molecules · dissolved · too small to see

Let's put all of our evidence together. Where does salt go when it dissolves?

So… the salt never disappeared. It dissolved.

Scientists collect observations during experiments. Computers can help us store and organize that information.

One way we store information in Python is with a variable. A variable is a name that stores a value. For example:

salt_grams = 5

The variable is called salt_grams, and the value stored inside it is 5. But variables can store different types of information.

salt_grams = 5

substance = "salt"

crystals_visible = False

Now let's use variables to record our actual salt experiment. Think back to what you observed. Our experiment might look like this:

substance = "salt"
salt_grams = 5
water_mL = 100
crystals_visible = False
water_clear = True

Each variable stores a different piece of information.

Storing information is useful, but sometimes we want the computer to show us what it stored. We can do this using the print() function.

print() tells Python: display something on the screen. For example:

print("Hello!")

displays: Hello!

We can also print a variable:

print(salt_grams)

which displays: 5

Printing text and variables together. Sometimes we want to display both a label and a variable. For example:

print("Salt added:", salt_grams)

The comma tells Python to display both pieces of information together. Python displays: Salt added: 5

We can also write:

print("Salt added:", salt_grams, "grams")

which displays: Salt added: 5 grams

What did we just use code to do? We used code to record and display observations from an experiment.

But computers can do more than remember what we already know. They can also calculate new information for us.

Imagine you have two cups of salt water. Both look clear.

At first glance, the two cups may look exactly the same.

But scientists can describe a solution using characteristics that we cannot always see just by looking.

For example, we can ask:

• How much salt is dissolved?
• How much water is present?
• How concentrated is the solution?

These measurements help us describe and compare the two cups more precisely.

One useful characteristic is concentration.

Concentration tells us how much of a substance is present compared with the amount of liquid.

Even if two cups both look clear, they can have very different concentrations.

This is where computation becomes useful.

We can give the computer our measurements and ask it to calculate the concentration for us.

Imagine these two cups:

Scientists use the idea of concentration to describe how much of a substance is present in a certain amount of liquid. For this mission, we'll use a simple measurement: grams of salt per milliliter of water.

For our experiment:

salt_grams = 5
water_mL = 100

We could calculate the answer ourselves. But Python can do it for us.

salt_per_mL = salt_grams / water_mL

The / symbol means divide. So Python calculates 5 ÷ 100 and stores the answer in a new variable: salt_per_mL.

Read the code:

salt_grams = 5
water_mL = 100

salt_per_mL = salt_grams / water_mL

print("Salt per mL:", salt_per_mL)

Run the code below. You should get Salt per mL: 0.05.

Output will appear here.

This tells us that our simple solution contains 0.05 grams of salt for every 1 mL of water.

Now let's let the computer compare two experiments.

salt_A = 5
water_A = 100

concentration_A = salt_A / water_A

print("Cup A:", concentration_A, "grams per mL")

salt_B = 10
water_B = 100

concentration_B = salt_B / water_B

print("Cup B:", concentration_B, "grams per mL")

Output will appear here.

Both cups might look like ordinary clear water. But the computer helped us calculate something we couldn't determine just by looking.

We started with measurements: grams of salt, and milliliters of water. Then the computer used those measurements to calculate how much salt there is compared with the amount of water.

Record data → perform calculations → discover information that isn't obvious from observation alone.

Now you get to change the code. You do not need to write a program from scratch — use the template below and change only the numbers.

Output will appear here.

Today, we learned that the world around us is made of atoms — and that electrons help determine the electrical charge of particles.

But electrons can do much more than create ions. During chemical reactions, the way electrons are arranged can change. And sometimes, those reactions produce something we can actually see.