Swimming citrus

It started last December. I put some orange wedges and cranberries into a glass pitcher of water and put it into the refrigerator for a special Christmas lunch. Their dad dropped the boys off, and when the three of us sat down to eat, something miraculous happened. My boys bypassed juice options to drink the fancy water! Amazing!

Now I keep the pitcher filled with water in the frig – and every few days I’ll cut up different fruit and put it into the water. No infuser – nothing fancy. Just fruit.

One day I was cutting up limes, lemons, and oranges. I began to wonder about whether the citrus fruit would sink or float.

Step one in science is to limit variables, where possible. I happen to have a digital kitchen scale, so mass was the easiest variable for me to calculate. I cut three pieces of citrus until they each weighed the same amount, which happened to be 12 grams.

Now that you have your idea about this, let’s see what happened.

With pieces of the same weight, the lemon and orange floated, but the lime sank.

Why does this happen? The scientific concepts at work are buoyancy and density, which can be challenging to understand. Here’s what made the most sense to me – imagine there’s a glass filled to the top with water, and I drop a lemon in. The lemon will displace an amount of water equal to the volume of the slice of lemon, and the water will spill out of the glass. I can collect that water and find the mass of the water.

In this case, volume is the same for both the lemon and the amount of water that overflows. If the mass of the water pushed aside is greater than the mass of the lemon, the lemon will float. If the amount of water the lemon displaces weighs less than the lemon, the lemon will sink.

So, why does the lemon and orange float? Because the volume of water they displace weighs more than the lemon and orange. The lime sinks because the volume of water it displaces weighs less than the lime. Density is mass divided by volume, so another way to say this is that the lime is more dense than the lemon or the orange. Why is that so?

Citrus fruit is mostly water, and the fruit part of all three fruits is about the same. If you take a closer side by side look at the lime, lemon, and orange, you can see that the lime has less “pith” around the edge and a thinner skin. That difference makes the lime more compact or more dense. The mass of the volume of water pushed aside is less than the mass of the lime, so the lime sinks.

All inspired by a pitcher of water!