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Why Does Ice Float?

Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

Water has some unique properties that cause it to behave differently than many other substances. This lesson will explain one such property and explain why ice is less dense than liquid water.

Life Mysteries

Ahhhh.... it's a hot summer day and you are enjoying a tall glass of ice water. As you take in the sun and the blue sky, you are probably wondering why in the world the ice in your glass is floating on the water. I mean, ice is a solid. Water is a liquid. Shouldn't the solid sink? And, as you start to ponder this, you start to think about all of the other life mysteries you don't have answers to. What is the meaning of life? Does space ever end? What will become of the human race? Oh no! Now you won't be able to sleep at night! Okay, okay, calm down. At least one of these mysteries can be answered fairly simply.

Density

Let's start with a quick lesson on density, which is a measure of how much 'stuff' is within a specific volume. And 'stuff' refers to particles, like atoms and molecules. These particles are constantly moving and bouncing around. When something is hot, the particles bounce around more and tend to take up more space. When something is cold, the particles bounce around less and take up less space.

Image 1. Particles moving at different temperatures.
temp

If you look at image 1, you can see that the object making up the hot particles is less dense because there is less 'stuff' in a given volume compared to the cold and the warm particles.

If you look at image 1 again, you can also imagine that the 'cold' substance is a solid, like ice, the 'warm' substance is a liquid, like water, and the 'hot' substance is a gas, like water vapor. So you can image that a solid is more dense than a liquid, which is more dense than a gas. Right? And you may also imagine that less dense objects can float on more dense objects, just like you can see in image 2. And you'd be correct.

Image 2. Oil is less dense than water so oil floats on water.
oils

But not with water. The solid ice floats on the liquid water, as you can see in your glass of ice water. Hmmmm.... What's happening?

Hydrogen Bonding

What if I were to tell you that water becomes less dense as it freezes? Yep. Water is weird. Remember, when substances get colder, the particles move less and clump closer together, thus making them denser. So that's the opposite of what happens to water.

This 'weirdness' is due to hydrogen bonding, or a bond that forms when a hydrogen atom attaches to an electron-hungry atom. Wait? Electron hungry? Yep. Those electron-hungry atoms are called electronegative and it just means they really want another atom's electrons. In water's case, the electron-hungry atom is oxygen.

Let's take a look at the chemical makeup of water in image 3. Each water molecule is made up of two hydrogen atoms and one oxygen atom. You can think of it as a mouse with the 'ears' being hydrogen and the 'head' being oxygen.

Image 3. Water is made up of one oxygen, or the red ball, and two hydrogen, or the grey balls.
water

When you get a bunch of mice heads, oops, I mean water molecules, together a hydrogen bond will form between the hydrogen of one water and the oxygen of another water. Check out image 4.

Image 4. The grey dashed lines are hydrogen bonds.
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