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.
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.