Osmosis and Diffusion in an Egg Lab: Measure Water Movement

Cells and Transport Teaching Resources · High school biology; AP extensions where noted.

A shell-less egg can model net water movement across a barrier. Its remaining shell membranes are not a living cell’s plasma membrane, and mass change alone does not identify every substance that crossed. Use this investigation to practice measurement and evaluate a model’s limits.

Objectives and preparation

Students will predict water movement, measure mass change consistently, calculate percentage change, and distinguish observations from explanations. Prerequisites: osmosis, tonicity, variables, and balance use. Teacher preparation takes several days; allow brief observation sessions over an additional day or more. Pilot the timing with the available eggs and refrigeration before class.

Materials and safety

Per group: two intact eggs or access to a class set, labeled lidded containers, household vinegar, distilled water, plain corn syrup, a balance readable to 0.1 g, a weighing boat, disposable towels, and a scoop or spoon. Use equal exposure times and enough liquid to cover each egg.

Raw eggs can carry Salmonella even with clean shells. Prefer pasteurized shell eggs when available; they still require safe handling. Follow school approval, allergy, eye-protection, and sanitation procedures. Keep containers in a designated refrigerator at 4°C/40°F or below, labeled “experiment—do not eat,” within secondary containment and separated from food. Never taste materials. Wash hands and clean contaminated surfaces and balance trays. Do not seal a gas-producing vinegar reaction in an airtight container. The teacher manages preparation, disposal, and any damaged eggs. Use a teacher demonstration or a non-egg alternative if safe refrigeration or allergy controls are unavailable.

Teacher preparation

  1. Cover eggs with vinegar in loosely covered containers under the approved refrigerated protocol. Allow the calcium-carbonate shell to dissolve; replace vinegar if needed. Cold conditions may slow the reaction.
  2. When shells are removed, gently rinse the intact membranes. Discard ruptured or suspect eggs. The shell-removal reaction is preparation, not the measured osmosis treatment.
  3. Assign intact eggs to water and syrup groups. Use several eggs per treatment across the class. Record the shell-less starting mass immediately before treatment.

Student procedure

  1. Predict each treatment’s mass-change direction and explain your assumptions. Label egg identity, treatment, and start time.
  2. Gently blot each shell-less egg using the same method and duration. Weigh in a tared boat and record starting mass in grams.
  3. Place one egg in distilled water and the other in syrup. Keep temperature, exposure time, container conditions, and liquid volume consistent across groups. Refrigerate between measurements.
  4. After the teacher-selected interval, lift with a spoon, remove surface liquid consistently, and reweigh. For syrup, the teacher should standardize any brief rinse because rinsing can change results.
  5. Record final mass, time, appearance, and any membrane damage. Do not silently omit a leaking egg; document it as an invalid replicate.
  6. Clean up under teacher direction and wash hands.

Record and calculate

Use columns for egg ID, treatment, starting mass (g), final mass (g), exposure time (h), temperature, observations, and percentage mass change. Calculate: 100 × (final mass − starting mass) ÷ starting mass. An egg changing from 60.0 g to 66.0 g has a +10.0% change. Compare replicate values and report variability as well as a mean.

Analysis

  1. Did the mass-change direction match the prediction? Use measurements, not just appearance.
  2. Why is the shell-less starting mass a better baseline than the intact egg mass?
  3. How might syrup left on the surface, a damaged membrane, or different blotting affect the result?
  4. Why are separate parallel treatments easier to compare than sending one egg through water and syrup in sequence?
  5. What would you need to measure to show that a particular solute diffused across the membrane?

Teaching note: Water commonly increases egg mass and concentrated syrup commonly decreases it, but do not alter data to match that pattern. Water movement is an interpretation supported by the treatment and observations, not proof that only water moved. An AP extension can examine water potential and explain why pressure and permeability limit simple concentration comparisons.

Continue the unit

Sources and further reading