This post came about because I was trying to prove to my friend that I could remove the shell of a raw egg and still keep its contents in tact. Because it takes a few days, I took pictures to send as proof and voila, not only to I have hard proof, but I have a neat little science demonstration to share!
Because of the inner membrane underneath the shell, an egg is a perfect tool when teaching students about selectively permeable membranes. This demonstration takes seconds to set up and can be left out for a few days for students to observe. Materials needed are: raw egg, vinegar, karo syrup.
Day 1: Take raw egg and cover with vinegar – the acetic acid will take about 24 hours to dissolve the shell which is made of calcium carbonate, giving off carbon dioxide bubbles as seen below. Sometimes more vinegar needs to be added if you really want all the white of the shell to disappear.
Day 2: Rinse off egg carefully under water and show students the difference in size next to a raw egg. Another thing to have handy is a scale if you want to have them follow the increase in mass.
Day 2 cont.: After students have made observations, a discussion can begin about osmosis and cells being placed in a hypotonic solution (solution where water content is higher than that of cell). Next, have students predict what will happen to vinegar-soaked egg once it’s placed in karo syrup overnight…
The karo syrup is a hypertonic solution (solution where water content is lower than cell), therefore, the permeable membrane of the egg allows water to leave the cell and move into the syrup. Here’s a pic of all three eggs: vinegar-soaked, syrup-soaked and raw. Easy and memorable!!! Have fun!