0:00:14.502,0:00:16.560
How come some insects are able to walk

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on the surface of a pond,

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but you quickly sink to the bottom

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when you try to walk on water?

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And why do lakes freeze from the top down in winter?

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In a word, the answer to all these questions is polarity.

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Water is a simple molecule

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made up of one oxygen atom and two hydrogen atoms,

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but it is essential to life.

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In fact, water makes up approximately 60%

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of the adult human being's body weight.

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The polarity within those water molecules

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gives this common substance the properties

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that make it unique and life-sustaining.

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Polarity refers to the unequal sharing

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of electrons within a molecule.

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For water, the bonding between the oxygen atom

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and two hydrogen atoms within a single water molecule

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is like a tug-of-war between a big, strong football player

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and a cute little toddler.

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Oxygen is a larger atom,

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with more protons in its nucleus than hydrogen.

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These positive charges are like a person's physical strength.

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They're able to attract

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the negatively charged electrons in the bond,

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just like a strong individual is able to overpower

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a weaker individual in a tug of war.

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So, oxygen is able to attract

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more than its fair share of electrons.

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Because hydrogen is smaller and has less strength,

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or fewer protons,

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it loses the tug of war

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and attracts fewer than its fair share of electrons.

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So, the oxygen in water behaves as though it's negative,

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and the hydrogens behave as though they're positive.

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The bonds within a single water molecule

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are called polar covalent bonds.

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Covalent means that the electrons are shared.

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But, as we just learned,

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polar means that these electrons are not shared equally.

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In water, the oxygen acts negative

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and the hydrogens act positive.

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Since negative and positive attract,

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that oxygen is attracted to the hydrogen atoms

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in neighboring water molecules.

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A special type of bond forms between water molecules,

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known as a hydrogen bond.

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Hydrogen bonds don't just happen in water either.

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They can form between a water molecule

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and different substances that are polar or ionic.

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Water's ability to stick to itself is called cohesion,

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while water's ability to stick to other substances

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is called adhesion.

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Now, think back to the initial questions.

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First, why are some insects able to walk on water?

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Surface tension due to hydrogen bonding

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creates a thin film on the surface of water

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that gives enough resistance

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for super-light insects to walk on.

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You can't walk on it

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because the hydrogen bonds aren't strong enough

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to hold you up.

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Why does ice float on top of liquid water?

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For most other substances,

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the solid state is more dense than the liquid state,

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but that is not the case for water!

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Hydrogen bonds keep water molecules

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farther apart in frozen water than in liquid water.

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The farther apart the molecules are,

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the less dense that solid is.

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So ice is about 9% less dense than water,

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which means it floats on the top.

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That's why lakes freeze from the top down

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and aquatic life is able to survive

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through a cold winter every year.

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It is the polarity of the water molecule

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and the resulting hydrogen bonding

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that account for water's unique properties.

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So, the reason that water is so special,

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from inside your cells

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to the world's oceans,

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is simply because it is a polar molecule.