The floating egg saltwater experiment is a simple way to see density and buoyancy in action. A fresh egg usually sinks in plain water because the egg is denser than the water around it. When salt is mixed into the water, the water becomes denser and can push up on the egg more strongly.
This project is useful because it turns an invisible idea, water density, into something students can observe directly.
To do the experiment, you need a clear glass, water, salt, a spoon, and an egg. Add the egg to plain water first, then slowly stir in salt and watch how the egg rises. The salt dissolves into the water, adding mass without greatly increasing the volume, so the density of the liquid increases.
When the saltwater becomes dense enough, the buoyant force on the egg is large enough to make it float.
Understanding The Floating Egg Saltwater Experiment
The important hidden process is displacement. When the egg enters the glass, it takes up space that water was using. Some water must move out of the way.
The water presses on every part of the shell, but pressure is greater lower down because there is more water above that point. This makes the upward push from the lower part of the egg stronger than the downward push from the upper part. That difference creates buoyancy.
As the egg rises, less of it stays below the surface. It then displaces less water, so the upward force becomes smaller. The egg stops at the level where the upward force matches its weight.
Dissolving salt is more than simply making water taste salty. Solid salt is made of tiny charged particles held in a crystal pattern. In water, many of these particles separate and spread through the liquid.
They fit into spaces between moving water particles. This changes the mass packed into a given amount of liquid. Stirring matters because salt can collect at the bottom of the glass.
If the water is not mixed well, the egg may sit in a denser lower layer while clearer water remains near the top. This can create an interesting result where the egg hangs in the middle rather than resting at the surface.
A fair experiment needs careful control. Use the same egg, glass, water amount, and temperature for each trial. Add measured amounts of salt, such as one spoonful at a time, and stir until no grains remain visible before recording the egg position.
Make a table with the number of spoonfuls and the egg's height in the glass. Repeating the test improves confidence because a small difference in spoon size or water level can affect the result. Avoid cracking the shell.
A crack lets water enter the egg and changes its mass. Eggs vary naturally too. An older egg can float in plain water because water slowly leaves through the shell and an air space inside grows larger.
This idea appears outside the classroom in lakes, oceans, and engineering. Ocean water contains dissolved salts, so it is denser than fresh river water. People float more easily in very salty places such as the Dead Sea.
Ships are designed to displace enough water for an upward force that supports their weight. A cargo ship sits lower when it carries more load because it must displace more water. Submarines change their average density by taking water into tanks or pushing water out with air.
When learning this topic, separate the ideas of mass, volume, weight, density, and force. They are connected, but they are not the same thing. The egg experiment works because the properties of the liquid change, not because salt somehow makes the egg lighter.
Key Facts
- Density = mass ÷ volume, or ρ = m/V.
- An object sinks if its density is greater than the density of the liquid.
- An object floats if its density is less than the density of the liquid.
- Adding salt increases the density of water because dissolved salt adds mass.
- Buoyant force is the upward force a fluid exerts on an object.
- An egg floats in saltwater when the buoyant force balances the egg's weight.
Vocabulary
- Density
- Density is the amount of mass packed into a certain volume of a substance.
- Buoyancy
- Buoyancy is the upward push from a fluid that helps objects float.
- Saltwater
- Saltwater is water with dissolved salt particles mixed throughout it.
- Solution
- A solution is a mixture in which one substance is evenly dissolved in another.
- Displacement
- Displacement is the movement of fluid out of the way when an object is placed in it.
Common Mistakes to Avoid
- Adding too much salt all at once, which makes it hard to observe the gradual change in floating height. Add salt one spoonful at a time and record what happens after each addition.
- Not stirring until the salt dissolves, which can leave salt sitting at the bottom instead of increasing the water's density evenly. Stir gently until the water looks mostly clear before testing the egg again.
- Assuming the egg changes during the experiment, which is usually not the main reason it floats. The important change is the density of the water around the egg.
- Using a cracked or very old egg, which can change the result and make the test unfair. Use the same uncracked egg for each trial so the liquid is the only major variable.
Practice Questions
- 1 A glass contains 250 mL of water. A student dissolves 30 g of salt into it. If the final volume is about 260 mL, what is the approximate density of the saltwater if the original water mass was 250 g?
- 2 An egg has a mass of 60 g and a volume of 55 mL. What is the density of the egg in g/mL, and would it sink or float in pure water with density 1.0 g/mL?
- 3 A student notices that the egg rests halfway down in weak saltwater but floats near the top in stronger saltwater. Explain how the water density and buoyant force changed.