A Quick Kitchen Experiment on the Difference Between Ocean and River Water Density

Round up your little scientists! You can explore how salt affects the density of water with this fun, quick, and easy experiment with items already in your own kitchen.

A little bit of background:

Ocean water is salty, while river water is fresh. When you first share this with kids, their immediate reaction is likely to be, “Why?”

Scientists believe that during our planet’s early days, the seas and oceans were probably only slightly salty. But over millions of years, the water changed from the water found in rivers because of a process called the “hydrological cycle.” This cycle has been happening for a pretty long time (the word-forming element hydr comes from the Greek language and means “water”).

In a nutshell, it follows the voyage of water from rain through rivers to oceans. Over time, as rain falls to the earth’s surface and runs over land and upper layers of soil, it slowly breaks up tiny parts of rocks and stones, transporting the minerals they contain to the oceans. The oceans collect all the water from the rivers that flow into them, which brings a lot of salt and minerals!

When that water reaches the seas and the oceans, it evaporates again and forms new clouds containing fresh water, while the salt is left behind. That means the oceans have been slowly gathering salt washed off the land by the flow of fresh water. And that’s how the ocean water has become a lot saltier!

On the other hand, rain refills rivers and streams with new fresh water.

Apart from taste, is there another difference between salt and fresh water? There IS!

We’ll explore that other distinction with this practical kitchen science experiment.

What you need:

  • 4 tall glasses (wide enough to fit an egg)

  • 4 eggs (they can be raw or boiled) or several grapes

  • Warm water

  • Salt

  • Sugar

  • Baking Soda

  • Feel free to test other substances, perhaps baking powder, sand, or cornstarch.

Let’s experiment!

  • Dissolve 2 tablespoons of salt in one cup, 2 tablespoons of sugar in another cup, and 2 tablespoons of baking soda in a third cup. Stir well!

  • The fourth cup is the controlled variable and it has just plain, fresh water.

  • Label each cup for easier tracking of the changes that are about to happen.

  • Ask your kids about their prediction on what might happen when they drop an egg into each cup. Will it sink or float?

  • Drop the eggs into each cup to find out if the kids’ guesses were correct!

  • Hint: To see the effect more clearly, you may need to add more salt/sugar/baking soda to the water. Add enough that you can see some extra sediment on the bottom of the cup.

  • The salt water and baking soda water should make the eggs float. Read below to find out why!

The science:

Water is denser when it contains salt. When you add salt to water, it breaks into sodium and chlorine ions. These charged particles change the forces between water molecules. This means water gets heavier when the level of salt in it goes up. That’s why many objects that usually sink in freshwater “change” their behavior when in saltwater, and they…float!

The egg floats in baking soda water too because baking soda is a kind of salt. It dissolves in water and makes it denser, just like table salt does. In addition to that, baking soda has another property! When dissolved in water, some part of the baking soda forms carbon dioxide gas. Look closer and you’ll see tiny bubbles rising from the bottom. If you add just the right amount of baking soda to the water, the egg in the cup will hover right in the middle!

Over time, the tiny carbon dioxide bubbles attached to the egg start to bring it to the top of the cup. It is so cool to witness that process, and kids will be AMAZED by it.

If you end up giving this fun experiment a try to help your little ones learn about the oceans and rivers, we’d love to hear about how it goes. Share your stories and photos with us on Instagram and tag us at @KvaroyKids!