An Osmosis Race Between Syrup, Salt Water and Tap Water
Three shell-less eggs in three liquids gain or lose water at measurably different rates. Over four hours the syrup egg shrivels to 80 percent of its mass while the water egg swells and tightens. This is the clearest osmosis demonstration you can run with a kitchen scale.
Materials
| Item | Amount | If you do not have it |
|---|---|---|
| Shell-less eggs from the vinegar card | 3 | Eggs soaked 24 hours in 5% vinegar, rinsed |
| Golden syrup | 250 ml | Honey, which is denser and draws water harder |
| Saturated salt water | 250 ml, about 90 g of salt | Strong brine from pickling |
| Tap water | 250 ml | Distilled water, which gives a larger effect |
| Three identical glasses | 3 | Three jars of the same shape, so the volumes match |
| Kitchen scale | 1, accurate to 0.1 g | 0.5 g accuracy loses the early part of the curve |
Method
Blot all three eggs dry on kitchen paper and weigh each one. Record the starting masses.
Watch for: Three eggs from the same box typically start within 1.5 g of each other.
Label the glasses syrup, salt and water, pour 250 ml of each, and lower one egg into each glass.
Watch for: The syrup egg floats because syrup is denser than the egg; the water egg sinks immediately.
Weigh each egg every 30 minutes: lift it out, blot, weigh, and put it straight back.
Watch for: The water egg gains 1.5 to 2 g per half hour; the syrup egg loses 1 g per half hour but slows after the first hour.
At the four-hour mark, weigh all three and compute the percentage change from the starting mass.
Watch for: Typical results are plus 12 percent for water, minus 6 percent for salt water and minus 19 percent for syrup.
Transfer the shrivelled syrup egg into a glass of tap water and leave it for another two hours.
Watch for: It re-inflates and becomes firm again, which shows the membrane itself was never damaged.
Why it works
The shell membrane is semi-permeable: water crosses it freely but dissolved sugar and salt ions largely do not. Water therefore moves from the side with the lower concentration of dissolved particles to the side with the higher one, until the concentrations match or the membrane's elastic resistance balances the osmotic pressure. Syrup is a very concentrated sugar solution, so water leaves the egg, the yolk and white shrink, and the membrane wrinkles. Tap water has almost no dissolved solids compared with egg contents, so water floods in and the membrane stretches. The salt water sits in between because a saturated salt solution and egg contents are closer in total dissolved particle concentration than most people expect.
Think of it like: Picture two rooms separated by a door that only children can pass through. Put ten adults in one room and one in the other, and every child will end up crowded into the room with the adult. Water is the child; sugar and salt are the adults.
Clean-up and disposal
All three liquids are food-grade, so pour them away and wash the glasses with hot soapy water. The raw eggs must not be eaten: they have spent a day in vinegar and then hours in warm syrup. Dispose of them in the bin, wash your hands, and wipe down any surface the raw egg touched.
Bench notes
Preparing the shell-less eggs
This card depends on the vinegar card being done properly. A shell that is only partly gone will leak a chalky suspension into every liquid and ruin the mass readings.
Observed / 实测An egg straight out of the vinegar soak weighs about 8 percent more than its original shelled mass, so record the post-soak mass as your starting point rather than the mass of the unpeeled egg.
Reading the curve
Plot the three mass curves on the same axes and the shape is the lesson: the water curve is steep and then flattens, the syrup curve is steep then nearly flat, and the salt curve is almost linear and shallow.
Observed / 实测Between hour one and hour four the syrup egg loses only 0.7 g, against 2.6 g in the first hour.
Handling raw eggs and concentrated syrup
Variations worth running
- Run the same four-hour test with sugar solutions at 10, 30 and 60 percent and find the concentration at which the egg mass does not change at all.
- Weigh the syrup before and after the four hours and confirm that the water the egg lost is the water the syrup gained.
- Repeat the whole experiment at 5 C in the fridge and compare the rates: the equilibrium is the same but it takes about twice as long.
Questions that come up on the bench
- Why does the syrup egg stop shrinking after the first hour?
- Two reasons. The membrane becomes stiffer as it loses water and resists further shrinkage, and the layer of syrup immediately against the membrane becomes diluted, which flattens the concentration gradient. Stirring the syrup gently restores some of the gradient.
- Why is the salt water result smaller than the syrup result?
- A saturated salt solution is about 26 percent salt by mass, while golden syrup is around 80 percent dissolved sugar. Per unit volume, syrup holds far more dissolved particles, so the osmotic pull is much stronger.
- Does the egg survive all this?
- The membrane does, provided you never let it dry out. A shrivelled egg put back into water recovers to about 95 percent of its original mass within six hours, though the yolk stays slightly firmer than before.