Does it float?

Kim and Philip are making soup. They chop up the vegetables and put them in a pot of water. Kim, look! The carrots and potatoes sink while the onions, broccoli, and mushrooms stay on the surface. Let’s see!

Take a piece of carrot and of mushroom that are the same size. Do they feel any different? The carrot is heavier than the mushroom. Is that why it sinks? Well, it is not exactly about how heavy it is.

It’s more about how tightly packed its mass is in the space it takes up — its density. Everything that has mass and takes up space, has density. The more mass packed in a certain space, the higher the density. Less mass in the same space means lower density. Vegetables, such as potatoes or carrots, sink, because they have a density higher than water.

Onions, mushrooms and broccoli have a lower density, so they float. That’s cool. I thought it was because the vegetables had gone bad! Oh, yeah, I’ve heard that fresh eggs sink, and spoiled eggs float! Is that because of their density too?

Exactly! A fresh egg has a higher density than water, so it sinks. But as an egg gets older, a little pocket of air forms on the inside of the shell. The gas has a lower density and lifts the egg up. But, you can make a fresh egg float too!

All you need to do is add salt to the water. When salt dissolves in water, it increases the water’s density. At some point, salty water has a higher density than a fresh egg. It might take quite a lot of salt, but eventually the egg will float! Whether something sinks or floats can depend on its shape so you can sometimes make things float if you change their shape!

Let’s try it! Take two identical pieces of aluminium foil. Roll one piece of the aluminium foil into a tight ball and put it in water. The ball sinks to the bottom, because the density of aluminium is higher than density of water. Now take the other piece of foil and fold the edges so it resembles a tray or a boat.

Place it gently on the surface. It floats! You can even add more weight by placing a few coins or other small objects inside the aluminium boat. Test how many coins you’d have to add before the boat sinks. The area that comes into contact with water is much larger for the boat, than for the ball of foil.

So, the weight of the boat is distributed over a wider surface. Because of its shape, it seems like the aluminium boat has a lower density than it actually does. This allows the boat to float. For the same reason, heavy cargo ships made of steel are able to stay afloat and travel across the seas! Whether something sinks or floats, depends on its density compared to the liquid it’s placed in, and on the surface area that touches the liquid.

Let’s make floating potato boats for our soup! I’d rather add more salt to make them float. That sounds like a really bad idea…