diffusion osmosis
Age Group
16 17
Expected time
1 period
The leaking bag.PDF
beaker symbol

The leaking bag

What’s the mystery? Two beakers will be displayed at the front of the room. Both look identical in that they both contain a plastic zip-lock bag with a starch solution inside. The zip-lock bag is in a clear solution. What the students don’t know is that while the solution in one beaker is just water, the second beaker contains iodine.In beaker two, the colour of the starch inside the zip-lock bag will change. Students need to figure out why this is happening.
Domain(s) Biology
Subdomain keywords Diffusion and osmosis.
Age group 16 to 17 years old.
Expected time for the mystery Approximate time for teacher preparation: 20 min.Approximate time in classroom: one 40-min lesson.
Safety/supervision Normal lab safety precautions apply. Care should be taken when using glassware. Care should also be taken when spraying the aerosol: it is necessary to ensure that it is not used too close to any of the students.Disclaimer: the authors of this teaching material will not be held responsible for any injury or damage to persons or properties that might occur in its use.
Preparation and list of materials Starch solution x 2, iodine, zip-lock bag x 2, beaker x 2, water, examples of impermeable (glass tiles), permeable (coffee/tea filters), and semi-permeable (Visking tubing) membranes, and concentrated solutions (e.g. sugar solutions).
Learning objectives Students will learn about osmosis and different types of membranes.