STEM – Blackcurrant Osmosis

Who doesn’t love Ribena? And chips? Okay, maybe not really together as such. I found this experiment through Practical Biology a few years ago, but have only just got around to testing it myself and then putting it into a STEM session. The Practical Biology/Nuffield site have lots of different experiments you can browse through, each with teacher and separate student guidance. I simplified the Blackcurrant Osmosis one for the time and age of the students.

It really is very simple and easy to set up. Also, it’s very visual for students to look at. Here are some of our potato chips after 15-20 minutes in different Ribena concentrations –

Blackcurrant Potatoes

It’s quite obvious which one is 0% Blackcurrant and which is 100%. It’s pretty clear even for the other concentrations. That wouldn’t happen with plain old sugar water, would it?

We did this with a group or KS3 who knew nothing about osmosis, nothing about concentration and had never done a practical quite like this before, but in the hour session they all did pretty well. I think this would be perfectly suited for an hour length lesson especially with the right background knowledge. I would simplify it for a repeat STEM session though and cut all of the chips to perfect size so that step could be cut out completely. I am pretty amazed at how well the year 7s did with making the different concentrations and, from the results, all of the concentrations were made pretty accurately. There was only one group with anomalous results that made no sense!

0% - 80% Strength

It’s all actually quite simple really – make 6 concentrations of blackcurrant squash (from 0% to 100%) (oh, and whilst Ribena works very well, so does the blackcurrant cordial we tried), put a pre-weighed potato chip into each concentration and leave for 15-20 minutes.

Floating Chips

(Note how the chips float with increasing squash concentration. Can you explain that?)

Then, remove each chip, pat them dry and weigh each one again. Then calculate the percentage change in mass.

0% - 100% Chips

The 0% blackcurrant was the only chip to not lose mass as water moved into it from the greater concentration outside of the potato cells. All of the others had water leave the chip via osmosis (to varying extents) as the water concentration was greater within the chips compared to the sugary Ribena.

For younger students, if you leave the chips in long enough they might get enough out of the experiment from just seeing the purple coloured chips. Can they explain it? And then with age/knowledge move on to the theory behind osmosis and the graphing example Nuffield gives.

I’d also like to try their example of altering this experiment to show the rate of water movement. Perhaps that would be better for understanding the basics behind osmosis for younger students.

8 Water Tricks That’ll Melt Your Mind – YouTube

8 Water Tricks That’ll Melt Your Mind – YouTube.

Water illusion – turn the arrows the other way with only a glass and water. What happens?

Pepper Trick – trough and water with pepper scattered on the surface, pop your finger in it and then a soapy finger! What happens?

Water and JD – shot of water on top of shot of JD, separated by a credit card. Introduce a small gap between the two glasses. What happens?

Fireproof balloon – balloon filled with water on a candle flame. What happens?

Laser Trick – point a laser through a bottle filled with water. What happens? Put a small hole in the bottle and let the water leak out, point the laser through the hole. What happens?

Glowing Water – using tonic water, what happens when you put a UV light on it and turn off the lights?

Water Suspension – bottle, some screen around the top and held in place by an elastic band, fill it with water. Turn upside down – What happens? Now put cocktail sticks in it (still upside down) – What happens?

Instant freeze – put a bottle of pure, distilled water in the freezer and wait for it to reach 32 degrees Fahrenheit. Bang the cold bottle of water on the table. What happens? With a repeated bottle, pour it out onto an ice cube. What happens?

If you don’t want to try them, watch the video!

I saw the instant freeze on a science show a while ago (with beer not water) and really wanted to know how I could use it in a school environment. I finally have the answer!!!! Now to try all of these!