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.
… 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.
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.
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?
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.
When water is warmer, the hydrogen bonds form and break quickly.
However, as water cools, the hydrogen bonds last longer because the water molecules aren’t bouncing around like crazy (remember our brief lesson on how particles behave?). These hydrogen bonds create a lattice-like structure (image 5), which is actually less dense than the molecules when they are liquid water.
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