Materials
- Table salt
- Warm water
- Clear cups or jars (3 or more)
- Spoon for mixing
- Small objects (e.g., plastic beads, grapes, paper clips)
- Optional: Food coloring for each saltwater layer
Procedure
- Fill each cup halfway with warm water.
- Add different amounts of salt to each cup (e.g., 0 tsp, 3 tsp, 6 tsp) and stir well until fully dissolved.
- Add a different food coloring to each layer if you’d like to see the separation clearly.
- Carefully layer the liquids by slowly pouring them into one clear jar from most dense (most salt) to least dense (least or no salt).
- Gently drop small objects into the container and observe which layers they float in.
Choose Your Learning Level
Elementary
When you add salt to water, the water gets “heavier.” This means things might float in salty water but sink in plain water! You can even make colorful water layers and see where things float the best.
Discussion Questions
- What did you notice about the objects in salty vs. plain water?
- Which cup had the most salt?
- Was it fun to stack the colors?
Middle School
Salt increases the density of water, which means it becomes heavier per unit volume. When layered properly, the denser saltwater stays at the bottom. Objects float or sink depending on whether their density is more or less than the layer they’re in.
Discussion Questions
- Why did some objects float in one layer but sink in another?
- How does salt make water more dense?
- What other liquids might have different densities?
High School
By dissolving NaCl in water, you increase its mass without significantly increasing its volume, thus raising the solution's density. This experiment visually models stratification and principles of buoyancy (Archimedes’ principle), and is relevant to oceanography and chemical engineering.
Discussion Questions
- How can you quantitatively calculate each layer’s density?
- What physical principles explain why layers don’t immediately mix?
- Where do you see salinity gradients in nature?