Salt water egg float lab
01 // Question
How does the amount of salt dissolved in water change whether an egg floats or sinks?
A testable question names one thing you change (salt amount) and one thing you measure (float or sink).
02 // Variables
Independent
Amount of salt dissolved in the water- 0 tablespoons (plain water)
- 3 tablespoons
- 6 tablespoons
Dependent
Whether the egg floats or sinks- Floats or sinks
- Height of the egg above the surface (mm), if it floats
03 // Hypothesis
IF more salt is dissolved into the water an egg is placed in, THEN the egg is more likely to float, and float higher, BECAUSE dissolved salt adds mass to the water without adding much volume, making the salt water denser than the egg.
04 // Procedure
1. Measure the water
Measure the same volume of water into three clear cups.
2. Add the salt
Stir the measured amount of salt into two of the cups until fully dissolved, leaving the third as plain water.
3. Lower the egg
Gently lower a raw egg into the plain water cup and observe whether it floats or sinks.
4. Test each cup
Rinse the egg and repeat in each salt water cup, observing and recording the result.
5. Measure the float height
For any cup where the egg floats, measure how far above the surface it sits.
05 // Data table
| Salt amount | Floats or sinks? | Height above surface (mm) |
|---|---|---|
| 0 tbsp (plain water) | ____ | ____ |
| 3 tbsp | ____ | ____ |
| 6 tbsp | ____ | ____ |
06 // What to expect
The egg sinks in plain water but floats once enough salt is dissolved in, because a raw egg is slightly denser than plain water but less dense than salt water once the salt concentration is high enough.
The egg floats higher in the 6-tablespoon cup than the 3-tablespoon cup, since more dissolved salt makes the water denser, and denser water pushes up harder against the same egg.
07 // Lab notes
- Stir until the salt is completely dissolved — undissolved salt at the bottom won't add to the water's density where the egg actually sits.
- Lower the egg in gently with a spoon rather than dropping it, so a sinking egg doesn't crack on the bottom of the cup.
- Safety. Raw egg can carry Salmonella — wash hands and wipe the area after handling.
08 // Remember
Salt doesn't make water heavier by itself. It makes the same volume of water hold more mass — that's what raises its density.
09 // Questions people ask
Why does an egg float in salt water but sink in plain water?
An object floats when the liquid around it is denser than the object itself. A raw egg is slightly denser than plain water, so it sinks. Dissolving salt into the water adds mass without adding much volume, raising the water's density until it becomes denser than the egg, which then floats.
How much salt does it take to make an egg float?
It varies by egg and by how the salt is measured, but many classroom versions find an egg starts floating somewhere around 3 to 6 tablespoons of salt dissolved into roughly 300 mL (about 1.25 cups) of water — testing your own water and egg is the point of the lab.
Can this experiment tell you if an egg is fresh?
A related trick uses plain water instead of salt water: a very fresh egg sinks and lies flat, while an older egg — which has lost moisture and gained an air pocket through its shell over time — sinks but tilts upright, or floats if it's old enough. That's a different variable (egg age) from the one this lab tests (salt concentration).