The Basic Setup
You need two clear containers, tap water, salt, an object to test, and something to measure density with. The standard approach is to fill one container with fresh water and the other with salt water at a known concentration, then drop your test object into each and observe what happens. Most people grab a raw egg for this because it hovers right at the edge of neutral buoyancy in plain water. The egg sinks in freshwater but floats in saltwater. That single visual result answers the whole question pretty cleanly. Here's the part most people gloss over when they build this. Density isn't just about the liquid - it's about the relationship between your test object's density and the fluid's density. Saltwater has a higher density than freshwater because dissolved sodium chloride ions take up space between water molecules without significantly increasing volume. Room temperature freshwater sits around 1.00 g/cm³. Saturated saltwater at the same temperature pushes to about 1.20 g/cm³. That twenty percent jump matters a lot if your object has a density near 1.05 or 1.10. I ran into a problem last year when a student brought in a plastic figurine that floated in both containers. They concluded the hypothesis was wrong, which it wasn't. The figurine had a density of roughly 0.92 g/cm³, which means it was already positively buoyant in freshwater. Adding salt changed nothing visible. The workaround was swapping to a different test object - I used a small aluminum foil boat shaped into a cube, loaded with washers. You can tune the mass precisely. Add one washer at a time until the object barely stays afloat in freshwater, then test it in saltwater. The number of washers it can carry before sinking gives you a quantitative measure instead of just a yes or no observation.
Temperature control is another detail that gets ignored until it ruins your data. Water density shifts roughly 0.0002 g/cm³ per degree Celsius. If your freshwater is at 20°C and your saltwater sits at 30°C because one container was near a window, your comparison is flawed. Keep both containers in the same room, away from direct heat or cold drafts. Measure the temperature of each liquid before testing and note it. If the difference exceeds two degrees, you should account for it in your calculations or re-test after equalizing. The salt concentration itself needs attention. Not everyone realizes that table salt (sodium chloride) doesn't dissolve infinitely. At 20°C, you can only dissolve about 360 grams per liter before the solution becomes saturated and excess salt sits on the bottom. Stirring helps, but once you hit that point, adding more salt does nothing. For a clean experiment, aim for a target concentration and measure it. A simple method: dissolve 50 grams of salt in 500 milliliters of water, stir until fully dissolved, and you get roughly 1.03 g/cm³. If you want higher density, go up to 150 grams in 500 milliliters for about 1.10 g/cm³. Don't guess. Weigh the salt on a scale and measure the water volume precisely. One counter-intuitive thing to keep in mind: some objects that sink in freshwater will actually sink faster in saltwater under certain conditions. This happens with objects that are negatively buoyant but have a shape that creates significant drag in freshwater. In saltwater, the higher density reduces the drag force relative to the buoyant force, which can make the object accelerate downward more quickly. It sounds backwards, but it's a real effect. If you're dropping a smooth stone through the column of water, time its descent with a stopwatch. You might find it hits the bottom sooner in saltwater even though the water is "thicker." This doesn't change the overall principle - buoyancy is still greater in saltwater - but it catches people off guard.
Another pitfall involves air bubbles clinging to your test object. If your object has a rough surface or irregular shape, tiny air pockets can form when you lower it into the liquid. These bubbles add buoyancy artificially, making an object float higher than it should. Tap the object gently against the side of the container as you lower it, or use a thin rod to dislodge bubbles before you start measuring. I learned this the hard way when a pitted metal sphere kept floating unexpectedly in my saltwater trial. A quick rinse and a few taps freed the bubbles, and the sphere sank normally. For the actual procedure, here's a straightforward sequence that works reliably. Fill container A with 500 milliliters of tap water at room temperature. Record the temperature. Fill container B with 500 milliliters of tap water and dissolve your measured salt, stirring until clear. Record the temperature again. Select your test object, dry it thoroughly, and weigh it on a digital scale. Lower it gently into container A using a string or tweezers. Note whether it floats, sinks, or remains suspended. Remove it, dry it, and repeat in container B. If the object floats in both, switch to a denser test item. If it sinks in both, try a less dense one. The goal is to find an object that shows different behavior in each liquid. Data collection matters more than the visual result alone. Instead of just recording "floats" or "sinks," measure how much of the object is submerged. Mark a reference line on the container and note the water level before and after adding the object. The displacement volume divided by the object's mass gives you a direct density calculation. You can compare your calculated density against the known densities of freshwater and saltwater at your measured temperatures. This turns a simple observation into actual quantitative data.
Get the Full Details
If you want to push this further, test multiple salt concentrations. Prepare three solutions: 50 grams per liter, 100 grams per liter, and 150 grams per liter. Plot the fraction of the object that remains above the waterline against the solution density. You'll get a linear relationship, and the slope tells you something about the object's density relative to the fluid. This is Archimedes' principle in practice, and it works consistently across a wide range of materials. The main limitation of this experiment is that it only tells you about static buoyancy. It doesn't account for waves, currents, or dynamic forces. An object that floats well in still saltwater might behave completely differently in a moving body of water. Real ocean conditions introduce variables like salinity gradients, temperature layers, and biological growth on surfaces that your classroom experiment can't replicate. That's fine - the project has a clear scope, and staying within it keeps the results valid. Just don't claim your findings apply to open ocean navigation without acknowledging the difference. Another practical constraint: not all salts produce the same density increase. Table salt works for this project, but sea salt contains magnesium and calcium compounds that precipitate differently and can cloud the water, making observations harder. Epsom salt (magnesium sulfate) creates a different density profile than sodium chloride at the same mass. Stick to one type of salt and note it in your write-up. Mixing salts introduces an unnecessary variable that complicates the analysis.
For the write-up, include your temperatures, your salt masses, your object's mass and dimensions, and your displacement measurements. Calculate the expected densities using the temperature correction factor if you want to be precise. The calculation adjusts freshwater density by subtracting 0.0002 g/cm³ for every degree above 4°C, where water reaches its maximum density. It's a small correction, but including it shows you understand the underlying physics rather than just reporting observations. Most teachers accept a simple float-or-sink result for younger grade levels. For older students or science fairs, the quantitative approach with multiple concentrations and temperature records will stand out. It doesn't require fancy equipment - just a kitchen scale, a graduated cylinder, a thermometer, and a few hours of careful measurement. The experiment is reliable if you control the variables, and it fails gracefully when you don't, which is exactly what you want from a science project.
