Why does a raisin swell in water?
Drop a raisin into a glass of water and leave it overnight. By morning it is fat and smooth, almost a grape again. Nothing pumped the water in. This notebook takes that apart: five small experiments, from one drop of dye to a potato that tells you what is inside it.
Grade 8 science · Cells Diffusion and osmosis
Watch it happen first
Drag the clock to leave the raisin in the water longer.
Size: 1.0 × what it started at
Your guess: why does it swell?
One drop of dye
Here is a tank of water, zoomed in until you can see the particles. The small blue dots are water; every one of them is jiggling. Tap anywhere to drop in some dye.
Look at the two counters. Dye crosses the middle both ways. But while most of the dye is on one side, more particles are there to wander across, so more go one way than come back. That difference is the spreading. Scientists call it diffusion: particles spread from where they are crowded to where they are not, just by jiggling.
Which spreads faster: dye in ice water or in hot water?
Hotter particles jiggle faster, so the dye spreads faster. That jiggle is the only "engine" here. Nothing pushes the dye; the heat that is already in the water keeps every particle moving.
A wall that picks
Now put a membrane down the middle of the tank, like the membrane around a cell. Its holes are so tiny that only water particles fit through. Sugar particles are too big: they bounce off.
Before you add any sugar: if you stir sugar into the right side, which way will the water go?
Switch the membrane to a plastic bag. The same thing happens, and nothing in a plastic bag is alive. Osmosis doesn't need a cell to do any work. The water is just jiggling, like the dye on page 1. Sugar can't leave the right side, so more water ends up there, and the bag swells.
Put real cells in it
A cell is a bag of water with stuff dissolved in it, wrapped in a membrane that water can cross. Change how salty the water around these two cells is.
The plant cell has a wall (you took it apart in the 3D cell). In fresh water the wall stops the cell from bursting, and the water pressing on it is what makes carrots and celery crunchy. The red blood cell has no wall, so pure water makes it burst.
The potato-strip lab
A real lab you can also do at home. Cut six potato strips, weigh each one, and leave each in a cup with a different amount of salt for a day. Then weigh them again. First, predict: tap each cup to say what happens to its strip.
| Salt (g per 100 mL) | Before (g) | After (g) | Change |
|---|
Where does a strip neither gain nor lose?
Slide the dashed line to where the graph crosses zero.
Osmosis in real life
For each one, which way does water move for the cells named?
So why does a raisin swell?
A raisin is a little bag of sugar, about 70% sugar inside, wrapped in skin and cell membranes that water can cross. Tap water has almost nothing dissolved in it. So water particles wander into the raisin more often than they wander out, all night long, and the raisin swells. Nothing pumps; the jiggling does it.
Now try the test that proves it: put the raisin in thick syrup instead. The cup switch is unlocked at the top of the notebook. In syrup more concentrated than the raisin, it doesn't swell at all.