In general, iron does not float in water because it is heavy and more dense than water.
That being said, there might be times when you observe iron floating (like when it’s part of a ship!)
Let us explain.
Does Iron Float? (EXPLAINED)
Why Do Things Float Or Sink?
Here on Earth, we are constantly subject to forces acting upon us.
Gravity, for example, is a force that helps keep our feet planted on the surface of the planet.
When it comes to floating and sinking, an object will sink if the forces acting on that material are greater than the ones which would hold it up (buoyancy).
Usually, when something is quite heavy, there’s no question that it will sink once it enters water, as the weight combined with the force of gravity upon it will pull it down.
When something is light, it may also sink, though perhaps not as quickly, as the weight force on the lighter item would still be greater than what pushes it up.
The density of the material also plays a factor (if the item is less dense than water it will likely float).
Density generally means how tightly packed the atoms/molecules are in the substances.
The more tightly packed they are, the denser they are (and less likely to float).
Other factors in addition to weight and density play a factor.
The shape of the item, for example, makes a difference, especially if the shape of the item traps air in with the substance.
When there is air contained within the item, more downward force will be required to make the item sink.
Another factor is whether much of the material can touch the surface of the water at the same time (like a piece of aluminum foil).
A piece of aluminum foil will float almost indefinitely, until one of the edges catches or water gets put on top of the strip pushes it below the water’s surface.
This is the result of the “surface tension” of the water.
As you can see, there are many factors that could influence whether an item could float or will sink.
Why Doesn’t Iron Float In Water?
In general, iron doesn’t float because it is denser than water, and is sufficiently heavy that gravity pulls it down, easily overpowering any forces pushing in the opposite direction.
However, the shape of the piece of iron does matter.
Just like a piece of aluminum foil, a small piece of iron placed in water in a shape where a lot of the water can touch the surface can actually float.
For example, a small and skinny iron needle placed carefully on the surface of the might might float on top of the water (if you are able to place it there without breaking the surface tension of the water).
But if you were to take the same amount of iron but in a different shape, such as a square or a ball, it would sink.
A great hulking ship weighing thousands of pounds (made of iron) can float because of the way it is shaped and how it displaces the water.
If the same amount of iron were shaped into a ball and dropped into the sea, it would sink immediately.
If you were to grind a piece of iron down into think shavings, or even to powder, the resulting small pieces would likely float on the top of the water rather than sinking immediately.
Iron Floats In Other Liquids
Just because iron doesn’t float in water, doesn’t mean it can’t or won’t float in other liquids.
Iron should float in any liquid that is more dense than itself.
Mercury is a good example of a liquid that is more dense than iron.
If you put a piece of iron in mercury (depending on how much mercury you have and how much iron you have), you should be able to find a balance where the iron does float.
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See also: Can Iron Scratch Glass?