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 and verify the salt
Stir the measured amount into the 3- and 6-tablespoon cups; leave the 0-tablespoon cup plain. Record whether all salt dissolves. Six tablespoons is near the room-temperature dissolving limit for 300 mL; if crystals remain, do not treat the full amount as dissolved or as the intended uniform sample.
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.
Materials // Prepare the bench
- Raw eggs
- clear cups
- water
- table salt
- measuring spoons
- stirring spoon
- balance
- labels
- tray
05 // Data table
| Salt amount | All salt dissolved? | Floats or sinks? | Height above surface (mm) |
|---|---|---|---|
| 0 tbsp (plain water) | N/A | ____ | ____ |
| 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.
If all the salt dissolves, the 6-tablespoon sample may support the same egg higher than the 3-tablespoon sample. Measure the actual result: egg density, spoon measurement and whether the salt fully dissolves all affect the outcome.
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.
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 // Separate the density question from the recipe
An egg's position depends on its average density compared with the surrounding liquid. Adding dissolved salt increases the solution's mass per volume, so a solution can eventually become dense enough to support an egg. Use measured salt and water amounts, stir until dissolved, and label each concentration. The amount of salt per cup is not the same thing as the solution's density unless the volume and preparation are controlled.
Keep eggs similar in size and temperature, use cups deep enough for the egg to be fully submerged, and lower each egg gently with a spoon. Start with fresh water and add salt in measured steps to separate cups, or use separate prepared concentrations. Do not add salt during the same observation if the question compares fixed concentrations; that makes the density change over time.
10 // Record a threshold with uncertainty
Record whether the egg sinks, floats at the surface, or remains suspended. Those are useful categories, but the transition may occur between two tested salt concentrations. If it sinks at 5% and floats at 7%, the threshold lies somewhere in that interval under the class conditions; do not report an exact threshold of 6% unless it was measured. Use repeated eggs or repeat trials to see how much the egg itself varies.
A balance and measured volume can provide a more direct solution-density estimate. Calculate mass per volume for each prepared sample and graph the egg's position against measured density. This separates the property that matters physically from a kitchen measure that can be packed or dissolved differently. Explain that an egg's shell condition, size, and trapped air can also affect its average density.
11 // Handle samples responsibly
Do not eat eggs used in the activity. Raw egg can carry bacteria, so wash hands and surfaces and keep the experiment away from food-preparation areas. Use a tray, lower the egg gently, and clean spills promptly. Discard the liquid according to class directions rather than tasting it to check whether the salt has dissolved.
A useful extension compares another object of known mass and volume or changes water temperature while keeping salt concentration constant. Avoid changing temperature and concentration together. Students can predict the direction of the effect, gather evidence, and explain why floating is relative to the liquid rather than an absolute property of the egg. The conclusion should describe the measured range of solutions and the eggs used.
12 // Build a concentration series
To test how salt concentration affects floating, prepare several measured solutions rather than comparing an unlabeled pinch of salt with plain water. Use the same water volume and egg type, dissolve salt fully, and label each concentration. Lower the egg gently and record whether it sinks, floats, or remains partly submerged; take repeated readings because small bubbles or shell differences can affect buoyancy.
Salt changes the liquid's density, which changes the buoyant force for a submerged object. The egg itself also varies in density, so one egg may not represent all eggs. Use a consistent egg across the concentration series when safe, or compare multiple eggs and explain the added variability. Do not eat eggs used in the lab.
13 // 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.
What if salt crystals remain after stirring?
Record that the sample did not fully dissolve. Six tablespoons is near the room-temperature dissolving limit for 300 mL, so the cup does not contain the intended uniform sample if crystals remain; do not describe all of the measured salt as dissolved.
How can I narrow the salt amount where my egg starts to float?
Use the highest tested amount where the egg sank and the lowest where it floated to make an interval. For example, if it sinks at 3 tbsp and floats at 6 tbsp per 300 mL, test a freshly prepared 4.5 tbsp sample next. If it sinks, test halfway between 4.5 and 6; if it floats, test halfway between 3 and 4.5. Keep the water volume the same, dissolve the salt fully, rinse the same intact egg between cups, and describe the result as specific to that egg and preparation. Science Buddies uses additional dilution series to refine this kind of bracket.
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).
14 // Keep going
Can you explain the investigation?
Salt water egg float lab · Student Data Sheet
Question: How does the amount of salt dissolved in water change whether an egg floats or sinks?
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.
Materials
- Raw eggs
- clear cups
- water
- table salt
- measuring spoons
- stirring spoon
- balance
- labels
- tray
Safety before starting
- Raw eggs can carry Salmonella. Do not eat or taste any sample; wash hands, cups, and the work surface after handling, and keep the egg away from food-preparation areas.
- Use stable cups on a tray and wipe spilled water or salt solution promptly.
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.
Variables
Independent: Amount of salt dissolved in the water · Dependent: Whether the egg floats or sinks
Keep constant: same cup size · same volume of water (300 mL) · same egg · salt fully dissolved before testing · room temperature
Procedure
- Measure the water: Measure the same volume of water into three clear cups.
- Add and verify the salt: Stir the measured amount into the 3- and 6-tablespoon cups; leave the 0-tablespoon cup plain. Record whether all salt dissolves. Six tablespoons is near the room-temperature dissolving limit for 300 mL; if crystals remain, do not treat the full amount as dissolved or as the intended uniform sample.
- Lower the egg: Gently lower a raw egg into the plain water cup and observe whether it floats or sinks.
- Test each cup: Rinse the egg and repeat in each salt water cup, observing and recording the result.
- Measure the float height: For any cup where the egg floats, measure how far above the surface it sits.
Record your observations
| Salt amount | All salt dissolved? | Floats or sinks? | Height above surface (mm) |
|---|---|---|---|
Answer key and expected pattern
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.
If all the salt dissolves, the 6-tablespoon sample may support the same egg higher than the 3-tablespoon sample. Measure the actual result: egg density, spoon measurement and whether the salt fully dissolves all affect the outcome.
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.