Potato osmosis lab
01 // Question
How does salt concentration relate to potato-strip mass change after a 24-hour soak?
A testable question names one thing you change (salt concentration) and one primary measurement (mass change); length can be recorded as a supporting observation.
02 // Variables
Independent
Salt concentration of the soaking solution- Distilled water (0% salt)
- 5% salt solution
- 10% salt solution
Dependent
Potato-strip mass change (primary); length (supporting observation)- Starting and final mass to 0.1 g
- Signed mass change and percent change
- Starting and final length in cm as a supporting observation
03 // Hypothesis
IF a potato strip soaks in a higher-concentration salt solution, THEN it will lose more mass, BECAUSE differences in water potential can cause net water movement across selectively permeable cell membranes; the measured tissue response depends on the potato and test conditions.
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 and divide the solutions
Prepare 100 mL final volume of each condition: 0 g, 5 g or 10 g salt per 100 mL. Divide each solution evenly among three labeled cups so every strip has its own replicate cup.
4. Soak nine strips for 24 hours
Place one matched strip in each of nine cups: three replicate cups per concentration. Cover them and leave all samples at room temperature for 24 hours.
5. Compare three replicates
Blot every strip with the same method, reweigh and remeasure, then calculate each percentage change. Compare the three values, their mean and range for each concentration.
Materials // Prepare the bench
- Potato
- distilled water
- measured salt solutions
- cups
- cork borer or knife
- ruler
- balance
- paper towels
- goggles
05 // Data table
| Solution / replicate | Start mass (g) | After 24 h (g) | Change (g) | % change | Length (cm), start → final |
|---|---|---|---|---|---|
| 0% · 1 | ____ | ____ | ____ | ____ | ____ → ____ |
| 0% · 2 | ____ | ____ | ____ | ____ | ____ → ____ |
| 0% · 3 | ____ | ____ | ____ | ____ | ____ → ____ |
| 5% · 1 | ____ | ____ | ____ | ____ | ____ → ____ |
| 5% · 2 | ____ | ____ | ____ | ____ | ____ → ____ |
| 5% · 3 | ____ | ____ | ____ | ____ | ____ → ____ |
| 10% · 1 | ____ | ____ | ____ | ____ | ____ → ____ |
| 10% · 2 | ____ | ____ | ____ | ____ | ____ → ____ |
| 10% · 3 | ____ | ____ | ____ | ____ | ____ → ____ |
Formula Change = final − start; % change = change ÷ start × 100. Compare the three replicate values, mean and range.
06 // What to expect
A strip in distilled water may gain mass while strips in salt solutions may lose mass, but compare the class's measurements instead of assuming exact values or an ordering. Differences in water potential can cause net water movement across cell membranes; three replicate strips help show variation but do not establish a universal result.
A salt-soaked strip may feel less firm if its cells lose water pressure, but a strip-level observation does not show that every cell changed equally or prove plasmolysis in individual cells.
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
Water crosses cell membranes in response to water potential. Use strip measurements to compare the three solutions; repeat before generalizing.
09 // Standardize the potato samples
Cut nine potato strips to the same length and thickness, removing skin consistently. Use one potato where practical, because potatoes and even regions within one tuber vary. For each condition, prepare 100 mL final volume: 0 g salt for 0%, 5 g for 5%, and 10 g for 10% (weight per volume). Divide each prepared solution among three separate cups, about 30 mL per cup, and put one strip in each cup. This gives three independent replicate cups at each concentration and about the same solution volume per strip.
Before soaking, blot every strip with the same light method and record its starting mass to 0.1 g and length. Label all nine cups with both concentration and replicate number. Cover them and soak all samples for 24 hours at the same room temperature. The changed variable is salt concentration; use the same potato source, strip dimensions, solution volume per strip, container type, temperature, soak time, and blotting method. An adult should handle the knife or prepare the strips, and used samples must not return to food.
10 // Calculate and interpret the change
For each strip, calculate mass change = final mass − starting mass, then percentage change = mass change ÷ starting mass × 100. Keep all nine rows. For each concentration, calculate the mean and range of its three percentage changes and show the individual points alongside the mean. Positive values mean the strip gained mass; negative values mean it lost mass. A graph of mean percentage change against salt concentration may estimate where change is near zero for this potato and procedure. The tested concentrations bracket a possible crossing only if measured means fall on opposite sides of zero; they do not locate an exact or universal value.
Osmosis is net water movement across a selectively permeable cell membrane in response to a water-concentration or water-potential difference; the solutes do not all cross as freely as water. A more concentrated surrounding solution may be associated with net water movement out of potato cells, while a more dilute solution may permit net movement into them. Connect that model cautiously to measured tissue mass: a strip contains many cells and cut surfaces, so mass does not reveal what every cell did. Avoid saying salt mechanically 'pulls' water.
11 // Check for confounds and improve the graph
Check that all nine pieces came from comparable parts of the potato, that replicate labels stayed with the correct cups, and that the same volume and soak time were used throughout. Blot each piece with the same number of gentle contacts: surface liquid can distort a small mass difference. Record a torn or unusual strip and its reason; do not discard a result only because it disagrees with the expected trend.
Concentration is ordered numeric data, so a scatter or line graph can show all replicate points and the mean at each concentration. Label the salt units as grams per 100 mL final solution and the response as percentage mass change. A wider range warns that the condition was less consistent in these samples. A follow-up can add more concentrations near the zero crossing, or test potato type or soak duration, but change one factor at a time. Limit conclusions to the potato, sample dimensions, solution preparation, temperature, and timing tested.
12 // Find the concentration where potato tissue changes little
A potato strip that gains mass in dilute salt water and loses mass in concentrated salt water suggests water moved between the tissue and its surroundings. If you test several concentrations, graph percent mass change against salt concentration. The point where the trend crosses zero is an estimate of the concentration at which there was little net mass change for this potato and procedure; it is not a universal value for every potato.
Use strips cut from similar parts of the same potato, with equal dimensions and starting masses where possible. Peel consistently, blot with the same number of gentle contacts, and weigh on the same balance. Potato cells vary by location and storage, and surface liquid can overwhelm a small mass change. Record any torn or unusually thin strip and repeat the condition rather than treating a single odd result as a biological rule.
13 // Questions people ask
Why might a potato strip lose mass in salt water?
Different water potentials across potato-cell membranes can lead to net water movement out of the tissue, lowering a strip's mass. Salt does not mechanically pull water, and the direction or size of a class result should be checked against repeated measurements.
Why might the potato strip in distilled water gain mass?
Distilled water generally has higher water potential than potato tissue, so net water movement into cells may increase the strip's mass. Measure all three replicates instead of assuming every strip changes by the same amount.
How is the potato osmosis lab different from the osmosis egg lab?
Both can demonstrate net water movement across a selectively permeable membrane, but the egg uses one large exposed membrane around a cell while potato mass is a tissue-level measure across many cells. A mass or firmness change alone does not prove plasmolysis in individual potato cells; that requires cellular observation.
14 // Keep going
Can you explain the investigation?
Potato osmosis lab · Student Data Sheet
Question: How does salt concentration relate to potato-strip mass change after a 24-hour soak?
Hypothesis: If a potato strip soaks in a higher-concentration salt solution, then it will lose more mass, because differences in water potential can cause net water movement across selectively permeable cell membranes; the measured tissue response depends on the potato and test conditions.
Materials
- Potato
- distilled water
- measured salt solutions
- cups
- cork borer or knife
- ruler
- balance
- paper towels
- goggles
Safety before starting
- A responsible adult should cut the potato on a stable cutting board with the knife moving away from hands. Students can handle the prepared strips instead.
- Do not eat experimental potato pieces. Wash hands and tools after handling raw food, and wipe up water spills promptly.
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.
Variables
Independent: Salt concentration of the soaking solution · Dependent: Potato-strip mass change (primary); length (supporting observation)
Keep constant: same potato and cutter · matched strip dimensions · same solution volume per strip · same soak time and room temperature · same blotting method
Procedure
- Cut identical strips: Cut potato strips of the same size using a cutter or ruler, from the same potato where possible.
- Weigh each strip: Blot dry and record the starting mass and length of each strip before soaking.
- Mix and divide the solutions: Prepare 100 mL final volume of each condition: 0 g, 5 g or 10 g salt per 100 mL. Divide each solution evenly among three labeled cups so every strip has its own replicate cup.
- Soak nine strips for 24 hours: Place one matched strip in each of nine cups: three replicate cups per concentration. Cover them and leave all samples at room temperature for 24 hours.
- Compare three replicates: Blot every strip with the same method, reweigh and remeasure, then calculate each percentage change. Compare the three values, their mean and range for each concentration.
Record your observations
| Solution / replicate | Start mass (g) | After 24 h (g) | Change (g) | % change | Length (cm), start → final |
|---|---|---|---|---|---|
Formula: Change = final − start; % change = change ÷ start × 100. Compare the three replicate values, mean and range.
Answer key and expected pattern
A strip in distilled water may gain mass while strips in salt solutions may lose mass, but compare the class's measurements instead of assuming exact values or an ordering. Differences in water potential can cause net water movement across cell membranes; three replicate strips help show variation but do not establish a universal result.
A salt-soaked strip may feel less firm if its cells lose water pressure, but a strip-level observation does not show that every cell changed equally or prove plasmolysis in individual cells.
Remember: Water crosses cell membranes in response to water potential. Use strip measurements to compare the three solutions; repeat before generalizing.