Make an Egg Float: A Salt Water Buoyancy Experiment

Every object immersed in a liquid feels an upward push called buoyancy. Whether the object floats or sinks comes down to one comparison: is its own density greater or smaller than the density of the liquid? With nothing more than a glass of water, an egg and some table salt, you can demonstrate this rule with your own hands.

First watch the egg rise as if coming back to life, then use the drag-and-play simulation to see the two force arrows — buoyancy and gravity — clearly.

Safety first (read this section before you start)

• Everything here is just water and table salt, so it is very safe; the glass is the only concern — handle and set it down carefully so it doesn't break.
• Never eat the egg or drink the salt water after the experiment. Pour it all out when you're done and wash your hands.

Materials and equipment

• One clear drinking glass or large cup;
• One raw egg (a fresh one shows the effect best);
• Plenty of table salt (keep most of a package on hand) and a spoon;
• Fresh water.

Step-by-step procedure

1. Fill the glass about three-quarters full with fresh water and gently lower in the egg. It will sink to the bottom.
2. Add salt one spoonful at a time, stirring after each spoonful until it fully dissolves — take care not to crack the egg while stirring.
3. As the salt builds up, watch the egg first stand upright and hover in mid-water, then rise to the surface with a small patch poking out.
4. Now slowly pour in fresh water (trickle it down the inside wall of the glass to limit mixing). The egg sinks again, because the density of the upper layer has been diluted.

What you should observe

The single most important thing to record is how much salt it takes to make the egg just barely float. A typical data set looks like this (one ordinary egg, 300 mL of fresh water):

• Around spoonful 4–5 (roughly 30 g of salt), the egg starts to "stand up" off the bottom and wobble, wide end pointing upward;
• Around spoonful 6–7 (roughly 45 g of salt), the egg leaves the bottom entirely and hangs in the middle of the liquid — this is neutral buoyancy (suspension);
• Add another spoonful or two and the egg rises to the surface, with a coin-sized patch above the water. This is floating.

Converting those numbers makes them even more interesting: 45 g of salt dissolved in 300 mL of water gives a solute mass fraction of about 13%, raising the solution's density to roughly 1.09 g/cm³ — and an egg's density typically falls in the 1.03–1.09 g/cm³ range. In other words, the "just suspended" critical point your students hunt for is actually a measurement of that particular egg's density.

Two easily missed details are worth writing down as well:

• The egg floats wide end up. The blunt end of an egg contains an air cell, so that end is less dense. The air cell grows as an egg ages, which is why an old egg floats more readily than a fresh one — and why the kitchen rule says "sinkers are fresh, floaters shouldn't be eaten."
• After you trickle fresh water down the wall, the egg parks itself right at the boundary between the two layers. Denser salt water sits below it, lighter fresh water above it, so the egg is pinned in between, seemingly hovering in mid-air.

The science: where buoyancy comes from, and what decides sink or float

For an object immersed in a liquid, the water pressure on its lower surface is greater than on its upper surface, and that pressure difference produces the upward buoyant force. Archimedes discovered that this force equals the weight of the liquid the object displaces:

F_buoyant = ρ_liquid g V_displaced

Whether the object ends up floating, sinking or suspended depends on which is larger, buoyancy or gravity. For a solid object you can simply compare the object's density with the liquid's density:

• ρ_object < ρ_liquid → it rises and ends up floating at the surface (part of it sticks out until buoyancy again equals gravity);
• ρ_object = ρ_liquid → it is neutrally buoyant, resting at any depth in the liquid;
• ρ_object > ρ_liquid → it sinks to the bottom.

An egg is slightly denser than fresh water, so it sinks in fresh water; adding salt keeps raising the salt water's density, and the moment it exceeds the egg's density, the egg floats up. The egg itself never changed — the liquid did.

Buoyancy in everyday life

• The Dead Sea is so salty and dense that a person can lie on it without sinking;
• A ship made of steel floats because its hollow hull displaces an enormous volume of water;
• A submarine dives and surfaces by flooding or emptying its ballast tanks, changing its own weight;
• A hydrometer measures a liquid's density using exactly the floating condition "buoyancy equals gravity."
• Gases exert buoyancy too. Hot-air balloons and helium balloons rise because the gas inside is less dense than the surrounding air; the same equation F_buoyant = ρ g V_displaced holds with air's density plugged in. Right now the air is pushing you up with about 0.5 N of buoyant force — it's just far too small compared with your weight to notice.
• When swimming, a deep breath makes you rise and exhaling makes you sink. The air in your lungs changes your body's overall density — the physics behind the swim-class advice that "relax and you'll float."

Four things students most often get wrong

• "Buoyancy equals the object's weight." They are equal only when the object is floating or suspended. A sinking object still feels buoyancy — it's just smaller than gravity. Hang a stone from a spring scale and lower it into water: the reading drops, and the amount it drops by is exactly the buoyant force.
• "The deeper you go, the bigger the buoyant force." Not true. There is no depth term in F_buoyant = ρ_liquid g V_displaced. Once an object is fully submerged, sinking another half meter changes neither the displaced volume nor the buoyant force. Only while it is still partly above the surface does sinking deeper displace more water and increase the buoyancy.
• "Heavy objects must sink." A ten-thousand-ton ship is vastly heavier than a coin, yet it floats. What decides sink or float is density, not mass — or put another way, whether the weight of the displaced water can hold up the object's own weight.
• "V_displaced is just the object's volume." They are equal only when the object is fully submerged. A floating object has part of itself above the surface, and V_displaced counts only the part below the waterline. Once the egg floats with its coin-sized patch exposed, that patch no longer counts toward V_displaced.

Notes for teachers and parents

• Time and grouping: 25 minutes, easily done at home; work in pairs, one student adding salt while the other stirs.
• Say this before hands touch anything: the glass is fragile, and the salt water must not be drunk.

The step most often skipped. Add the salt one spoonful at a time and have students record what they see after every spoonful. Landing on the "suspended" state is the hardest and most valuable moment — remind them to switch to tiny additions there, instead of dumping in a spoonful that sends the egg straight to the surface.

How to tell they truly understand. A student who can predict sink-or-float by comparing the object's density with the liquid's density, and can explain that the egg never changed — only the liquid did — has met the goal.

Take it further

• In the simulation, tune the object's density to exactly match the liquid's and watch the "suspended" state — then think about why that state is so hard to hold steady with real objects.
• Try the experiment with an orange: it floats with its peel on and sinks once peeled. Why? (The peel is full of tiny air pockets, which lower the fruit's overall density.)
• Build a homemade hydrometer. Take a drinking straw, plug one end with a small ball of modeling clay as ballast so it floats upright in water, and mark the waterline with a marker. Move it into salt water and it rides higher — the higher the mark sits above the surface, the denser the liquid. That is the entire principle of a hydrometer.
• Verify Archimedes' principle. Set a cup filled to the brim inside a dish, slowly lower a stone into it, then collect and weigh the overflow. You'll find the weight of the spilled water almost exactly matches the drop in the spring-scale reading as the stone went under.
• Think it over: as a ship sails from a river into the sea, does its waterline sink deeper or rise? (Hint: seawater is denser than river water, so displacing the same weight of liquid takes less volume.)

When a student can explain why the egg floats after adding salt in terms of "whose density is bigger — the object's or the liquid's," this experiment has done its job.