Potato osmosis lab
01 // Question
How does the salt concentration of a solution change the mass of a potato strip soaked in it overnight?
A testable question names one thing you change (salt concentration) and one thing you measure (mass or length change).
02 // Variables
Independent
Salt concentration of the soaking solution- Distilled water (0% salt)
- 5% salt solution
- 10% salt solution
Dependent
Change in potato strip mass- Grams, to 0.1 g
- Percent change
- Length in cm
03 // Hypothesis
IF a potato strip soaks in a higher-concentration salt solution, THEN it will lose more mass, BECAUSE water inside the potato's cells moves out toward the solution with more dissolved salt, through the same osmosis process seen across any semi-permeable cell membrane.
04 // Procedure
1. Cut identical strips
Cut potato strips of the same size using a cutter or ruler, from the same potato where possible.
2. Weigh each strip
Blot dry and record the starting mass and length of each strip before soaking.
3. Mix the solutions
Prepare distilled water and two salt solutions at different concentrations, using the same total volume for each.
4. Soak for 24 hours
Submerge one strip per container and let them sit undisturbed for 24 hours at room temperature.
5. Reweigh and calculate
Blot each strip dry, reweigh and remeasure, then calculate the percent change in mass for each.
05 // Data table
| Solution | Start mass (g) | After 24 h (g) | Change (g) | % change |
|---|---|---|---|---|
| Distilled water | ____ | ____ | ____ | ____ |
| 5% salt solution | ____ | ____ | ____ | ____ |
| 10% salt solution | ____ | ____ | ____ | ____ |
Formula % change = (mass after − start mass) ÷ start mass × 100
06 // What to expect
The strip in distilled water usually gains mass and stays firm, while strips in salt solution lose mass and turn limp, with the 10% solution losing the most, because a bigger difference in salt concentration across the cell membrane pulls water out faster.
The limpness in the salt-soaked strips is a visible sign of plasmolysis, where plant cells lose enough internal water pressure that they can no longer hold their normal rigid shape.
07 // Lab notes
- Cut all strips from a similar part of the potato — strips near the skin can behave differently than strips from the center.
- Blot each strip the same way before every weighing, since surface water adds mass that has nothing to do with osmosis.
- Use a clean cutting board and wash hands after handling raw potato.
08 // Remember
Cells don't lose water randomly. Water always moves toward the side with more dissolved solute.
09 // Questions people ask
Why does a potato strip lose mass in salt water?
Osmosis moves water across a cell membrane from an area of lower solute concentration to an area of higher solute concentration. Salt water outside the potato's cells has a higher salt concentration than the water inside the cells, so water moves out of the potato and into the solution, lowering the strip's mass.
Why does the potato strip in distilled water gain mass instead of losing it?
Distilled water has no dissolved solutes, so it has a lower solute concentration than the inside of the potato's cells. Water moves into the cells through osmosis, causing the strip to swell and gain mass.
How is the potato osmosis lab different from the osmosis egg lab?
Both demonstrate osmosis, but the egg lab uses a semi-permeable membrane at the scale of a single large cell, while the potato lab uses many plant cells at once and can show plasmolysis — the loss of internal water pressure that makes plant tissue go limp.