Compare Fresh vs. Salt Water Floating

This experiment demonstrates one of the most counterintuitive and fascinating principles in physics: an egg that sinks in fresh water floats in salt water. The difference is density—salt water is denser than fresh water because the dissolved salt adds mass to the same volume of water. A denser fluid provides more buoyant force, allowing objects to float that would sink in less dense water.

This principle explains why people float more easily in the Dead Sea than in a swimming pool, why ocean fish are adapted for salt water, and why ships can carry heavier loads in salt water than fresh water. Understanding it at a kitchen-table level builds the physical intuition that makes these larger phenomena immediately comprehensible.

What You'll Need

• Two identical clear glasses or containers — Transparency allows children to see floating behavior clearly.
• Water — To fill each glass approximately three-quarters full.
• Salt — About 5–6 tablespoons for the salt water glass.
• A stirring spoon — For dissolving the salt completely.
• Two fresh eggs — Or alternatively, two similar small objects (small potatoes work nicely).
• Food coloring (optional) — Add different colors to each glass for clearer visual distinction.
• A permanent marker — To label each glass: "Fresh Water" and "Salt Water."

How to Do It

1. Prepare the two solutions.

Fill both glasses three-quarters with water. To the second glass, add 5–6 tablespoons of salt and stir vigorously until completely dissolved—no crystals visible on the bottom. Label each glass.

2. Predict before testing.

Ask: "What do you think will happen when we put an egg in this water? Will it float or sink? Will both eggs do the same thing?" Record the predictions.

3. Test the fresh water egg.

Gently lower an egg into the fresh water glass. It sinks to the bottom. Observe and describe.

4. Test the salt water egg.

Gently lower an identical egg into the salt water glass. It floats. The contrast with the fresh water glass, viewed side by side, is dramatic.

5. Investigate the boundary.

For a spectacular demonstration: in a tall clear glass, carefully pour fresh water over a layer of salt water (pour slowly over the back of a spoon to keep layers separate). Drop an egg: it will sink through the fresh water layer and float exactly at the boundary between fresh and salt water—suspended in the middle of the glass. This demonstration is visually extraordinary.

6. Add more salt progressively.

Add more salt to the fresh water glass, one tablespoon at a time, stirring after each addition. At some point, the egg that was sinking begins to hover, then float. This progressive demonstration shows that there's a specific threshold density at which the egg transitions from sinking to floating.

🎓 Skills Your Child Will Develop

• Density Concepts — Understanding that equal volumes of different liquids can have different masses, and that this difference in density determines floating behavior, is foundational physics that underlies fluid mechanics, oceanography, and engineering.

• Buoyancy — Understanding that a denser fluid provides more upward force on a submerged object—more buoyancy—explains why objects float differently in different fluids.

• Controlled Comparison — Testing the same object in two different conditions while keeping everything else the same is a controlled experiment. Seeing the different result allows the difference to be attributed to the single changed variable.

• Prediction and Revision — Making a prediction before testing, then discovering whether it was correct, and revising understanding based on evidence is the scientific method in its most basic form.

Tips & Variations

• Dead Sea connection: Tell the story of the Dead Sea—a lake so salty that people float on the surface without swimming, because the water is dense enough to support their body weight. Show a photo. The real-world application makes the kitchen experiment globally significant.

• Floating grape experiment: A grape sinks in fresh water. Add enough salt to make it float. This works with slightly different salt concentrations than the egg and adds another data point.

My Two Cents

The side-by-side comparison—one egg floating, one egg sunk, in two identical glasses of visually similar water—is one of the most memorable science moments available in a kitchen. The eggs reveal something invisible (the difference in water density) through their visible behavior. Making invisible properties visible is what good science experiments do.