Filter paper riddle

We carried out the well-known experiment where a copper rod is placed in a silver nitrate solution, AgNO3(aq). We measured the reduction in mass of the copper and the mass of silver liberated. We filtered off the solid silver, washed it with distilled water and placed it, still on the filter paper, to dry in the oven. At the same time we placed a number of filter papers of exactly the same type in the same oven, so that we could weigh them afterwards, and so deduct the mass of the paper to calculate the mass of the silver.

Burned filter paper.However, we had an unexpected surprise. On opening the oven, we found that the paper on which the silver had rested, had been burnt round the edge (see photo) while all the clean, unused papers, in the same oven, were unaffected.

Take a moment and consider the answer to the riddle: "Why did the paper burn?"

After you give it some thought, go to page 13 for a discussion on this result.

[It is interesting to note that you can see this same reaction in the photo on the front cover and on page 11.]