Science Buddies: "Ask an Expert"

My son has completed the "Now You See It, Now You Don't - how acidic groundwater makes rocks disappear" experiment. The changes in mass over time indicate that the experiment went "as expected", as the mass decreased for the rocks in the vinegar solutions. However, his PH values are confusing and we are concerned they are not correct. We helped him make the solutions so feel that the start of those was correct/according to instruction. However, we did not monitor the pH testing process as closely.

The PH strips used had a large range (0 - 14).
They don't indicate a significant pH difference in the solutions by day 4. On day 0, the solutions with various vinegar concentrations were a pH of 3 and increased to a pH of 5 by the end (day 4). Were the strips not sensitive enough for this experiment? Or did we make a mistake somewhere along the way? Any help/thoughts would be wonderful. Unfortunately we do not have the materials or the time to conduct the experiment again from the start.


[Administrator note: Project: https://www.sciencebuddies.org/science- ... -disappear ]

Hello there,

Apologies for the delayed response. I think the pH values are not unreasonable, and I think it unlikely that different pH strips would have gotten you different results. A pH increase from 3 to 5 is significant! Note that pH is on a logarithmic scale, so a +2 on the pH scale is a 100x decrease in H+ ion concentration. (This helpful chart https://www.sciencebuddies.org/science- ... e-ph-scale notes that this difference is like going from orange juice to black coffee in terms of acidity.) Though it depends on how much limestone mass was lost in each solution, I would consider it reasonable.