Why do lemon slices float?

I filled some jugs today with cold water and added some ice and also some lemon and lime slices. We are curious to learn why the lemon slices float, yet the lime slices sink. Would you be able to answer this contradiction at all?

The answer to your nice little conundrum is not as simple as you might suppose. The reason lemon slices float and lime slices sink is due to their different buoyancies, based on their respective densities.

The fleshy pulp that makes up the main part of either fruit is of very similar density to water as it is composed of 98% water, so the difference lies in the skin and pith, which is denser in lemons. Incidentally, the citrus oil found in the skins of both is found in comparable quantities in both fruits, so the fact that a fat will float on water is not what is providing the buoyancy, as some have incorrectly suggested.

The layer of white pith found on a lemon is much thicker than that found on a lime, and in fact this contains more air, providing the buoyancy. So it’s the air in the lemon pith that makes the slices buoyant, the much smaller amount in the lime slices not giving rise to this effect.

If any budding scientists out there wish to prove this, cut off the rind and pith from two equal-sized pieces of sliced lemon and lime and place them in a jug. They will act the same, sinking to the bottom.