Cucumber Osmosis

Subject Area	Biology – Biochemistry and Osmosis
Age or Grade	Introductory Biology – Grades 9 +
Estimated Length	1.5 hours
Prerequisite knowledge/skills	Students should understand the concepts of concentration, osmosis, hypotonic and hypertonic solutions, and the scientific method.
Description of New Content	This activity is an extension of previously introduced content, and does not introduce any new material.
Goals	This laboratory requires students to apply their theoretical knowledge of osmosis to determine the relative concentrations of several different solutions. The scientific method is heavily emphasized by requiring students to design and carry out their own experiment with using given materials.
Materials Needed	Each group of students will require: - 9 slices of cucumber - Scale - 3 beakers of NaCl solution (different concentrations)
Procedure	Setup : - At the beginning of class, the teacher will present 3 (or more) 1 liter beakers filled with distilled water. Different amounts of salt will be added to each beaker, creating solutions with different concentrations of ions. - The beakers will be mixed up so that it is impossible to tell which contains what concentration of salt Experimental procedure: - The teacher will instruct the students to use cucumber slices and their knowledge of osmosis to determine the relative concentration of each solution compared to the others - Students will be given 3 beakers labeled A-C, each with ~ 100 ml of a different solution, and 9 slices of cucumber - Students must conduct three trials of an experiment using the salt water beakers, cucumber slices, and scale, and record all data. Closure: - At the end of the period, each student must turn in their experimental protocol, data, and discussion explaining how they have determined the relative concentrations of the solutions.
Evaluation	Students will be graded on the quality of their laboratory report, their experimental procedure, and on whether or not they correctly identified the concentrations of the solutions.
Extensions	Time permitting, the students can use salt and water to determine the osmolarity of cucumbers. This can be done by making several solutions of different concentrations, and observing the change in weight of cucumber slices that are placed in each solution for 10 minutes. The solution which results in the least change in weight (ie least osmotic movement of water in or out of the cucumber cells) is closest in concentration to the cucumber itself. Given enough time, it is thus possible to determine the concentration of a solution that is isotonic to the cucumber (ie containing the same concentration of solutes.)
References	“Modern Biology” Textbook, Wikipedia