I found this lab in the Modern Chemistry curriculum and liked that it was a ‘real’ world application of chemistry. The lab is called “What’s so special about bottled drinking water?” and is part of the online resources if you buy the Modern Chemistry curriculum. Students sample 5 different brands of bottled water – we had distilled water, pure drinking water, sparkling mineral water, spring water and a few with added electrolytes.
Students took a good look at the labeling of the bottles, noting the method of purification, the amount of dissolved solids, marketing claims, etc. The actual lab involved using a very small amount (0.1 grams) of sodium polyacrylate (water absorbing chemical used in diapers) in a cup and determining how many mililiters of bottled water it could absorb. The amount of water absorbed is determined by the amount of mineral salts in the water. The more ‘pure’ the water, the more the sodium polyacrylate will absorb.
The lab starts with putting 0.1 g sodium polyacrylate (which looks like a pinch of salt) in a plastic cup and 100 ml of the bottled water being tested in a graduated cylinder. Students then used a pipette to drip water from the graduated cylinder into the cup with the sodium polyacrylate until it no longer absorbed water. This was the difficult part of this experiment – when do you stop? When has it actually stopped absorbing water? Its not a clear cut transition so students just tried to be consistent and when the water soaked gel became more gooey than solid (jello like) they stopped.
They recorded the amount of water left in the graduated cylinder and figured out the amount of water absorbed by the sodium polyacrylate and recorded it in the data table.
Students were to plot the data, volume of water absorbed (ml) as a function of mineral salts in the water (ppm or mg/L), but only two of the bottles actually had those numbers on the label. I did happen to have an instrument used to measure ppm so we used that to get a number for each water sample. Unfortunately I don’t think we went far enough with the water because the graph of our data is very scattered and the students noticed that as they let the cups sit there the gel continued to absorb the water. It would be nice if there was a clearer way to determine a cut off but even though our final results may not be the best, I think the students learned a lot just having to take a closer look at the labeling and marketing claims. Sodium polyacrylate is also very impressive to see at work, absorbing many times its weight in water.