Materials & Prep
- Per group: 4 labeled cups containing 0.0 M, 0.2 M, 0.4 M, and 0.6 M sucrose solution; 4 equal-size potato cores; balance; paper towels; forceps; marker; and data sheet.
- Set up one teacher-pre-soaked set of four potato cores per group in the same concentrations at least 45 minutes before class. Weigh each core BEFORE it goes into solution and write that initial mass on its own label — a mass change can only be calculated from one core measured twice, so students need the number belonging to the core they will weigh. Keep these separate from students’ fresh cores.
- Project the class data table and post the lab rule: blot gently, then weigh. Confirm local procedures for food materials, solution spills, balance use, handwashing, and disposal. Potato cores are lab materials, not food.
Opening
0–7 minDisplayTwo identical-looking potato cores are shown: one has soaked in distilled water and one in concentrated sucrose solution. Ask: “After soaking, which core has more mass, and which has less? Where did that mass go or come from?” Students privately commit to a prediction and draw an arrow showing the predicted direction of water movement.
Turn and talkStudents recall from prior mass-measurement work why a core must be blotted before weighing. Listen for: surface liquid would change the measured mass without showing water that crossed the potato cell membrane.
ConfrontCollect a quick show of hands for the prediction that sugar moves into the potato and makes it heavier. Tell students they will use mass evidence to test whether the changing substance is sugar, water, or both.
Direct instruction
7–17 minExplainPotato cell membranes are selectively permeable. Water can move across them, while sucrose does not readily cross during this investigation. Osmosis is the net movement of water across a selectively permeable membrane from lower solute concentration to higher solute concentration.
AnalogyPicture a crowded elevator. People have more open space outside than inside, so movement into the elevator is more likely until the difference is reduced. Likewise, water has more available movement in the less concentrated solution and moves toward the more concentrated side.
ClarifyA mass change is an observation. “Water moved into the potato” is an inference supported by that observation and by membrane behavior. A heavier core supports net water movement into potato cells; a lighter core supports net water movement out.
Think aloudModel the 0.0 M condition. “My initial mass is 5.20 g. My final mass is 5.54 g. I calculate 5.54 minus 5.20, which is +0.34 g. The plus sign means the core gained mass. Since only water is expected to cross readily, my claim is that net water moved into the potato from the 0.0 M solution.” Emphasize the common error: students sometimes reverse subtraction or say the potato pulled sugar in simply because it gained mass.
Check for understandingProject: initial mass 4.80 g; final mass 4.55 g in 0.6 M sucrose. Students hold up one finger for “water moved into potato” or two for “water moved out,” then write the evidence phrase. Expected: two; the core lost 0.25 g, so net water moved out.
Guided practice
17–30 minModelDemonstrate balance use without completing a student measurement: check that the balance reads 0.00 g, place one blotted core in the center, wait for a stable reading, record to the displayed precision, remove the core, and recheck zero. Students, not the teacher, will operate their group balance.
StudentsIn groups of four, assign roles: balance operator, blotted-core handler, recorder, and concentration manager. Roles rotate after two measurements. Each group labels its four fresh cores by concentration, gently blots each one, measures and records initial mass, places each core into its matching cup, and records the start time.
CirculateCheck that students match core labels to cup concentrations and use a fresh blotting area between solutions. Ask: “What is the independent variable? What measurement will show a possible water movement?” Look for: sucrose concentration; change in potato-core mass.
ScaffoldProvide the data-table prompts: concentration, initial mass, final mass, mass change (final minus initial), observation, and water-movement inference. Post the sentence frame: “In the M solution, the potato core changed by g. This supports the claim that net water moved the potato because .”
Independent analysis and discussion
30–47 minStudentsRetrieve the pre-soaked cores. Each student operates the balance for at least one final-mass measurement; groups gently blot, measure, and record the final masses for all four concentrations. Students calculate mass change for each concentration by subtracting the initial mass written on that core’s own label — not the initial mass of the fresh core they set up, which is a different potato. Their own cores keep soaking and give a second data set to compare at the start of the next lesson.
WriteGroups add mass-change results to the projected class table, then create a quick graph with sucrose concentration on the x-axis and mass change on the y-axis. They identify the concentration closest to 0 g change as the condition in which there was little or no net water movement.
DiscussEach group prepares a claim–evidence–reasoning response: “How did increasing external solute concentration affect water movement across potato cell membranes?” Require at least two numerical data points as evidence and reasoning that connects mass change to net water movement.
ChallengeStudents ready for more identify why results may not be perfectly linear and explain why a core at approximately 0 g change does not mean water stopped moving in both directions.
SupportConfer with groups that have reversed their conclusion. Have them trace only one condition using arrows: “The core lost mass. What crossed the membrane readily? Which side must water have moved toward?”
Closing
47–55 minExit ticketProvide this new result: A potato core begins at 5.00 g and ends at 5.18 g after soaking in 0.1 M sucrose. Students write a claim about the direction of net water movement, cite the calculated mass change as evidence, and explain what this suggests about the relative solute concentrations inside the potato cells and in the 0.1 M solution.
ExpectedThe core gained 0.18 g, so net water moved into the potato. The 0.1 M solution was less concentrated in solute than the potato-cell contents. Collect tickets to identify students who still equate mass gain with sugar entering the cells.
StudentsDispose of materials and clean balances and benches according to the school’s lab procedures.