The Amazing Chemistry of Water

A: Let’s start with why water is so extraordinary on the molecular level. Its molecule forms a V-shape, where the oxygen sits at the vertex and the hydrogens are at the tips. But the key is those polar covalent bonds—it means the oxygen hogs more of the electrons, carrying a slight negative charge, while each hydrogen is slightly positive.

B: So you’re saying there’s an uneven ‘pull’ within the molecule? That’s how you get the delta-minus on oxygen and delta-plus on hydrogens, right?

A: Exactly. That internal polarity lets the molecules attract each other through hydrogen bonds—those are much weaker than actual covalent bonds, but together, en masse, they give water its signature properties. If you picture Figure 3.2, it’s like a molecular dance: each water molecule is constantly swapping partners as hydrogen bonds break and reform trillions of times per second.

B: Wait—if these bonds are so fleeting, how do they add up to anything important?

A: Great question! Even though each hydrogen bond is only about one-twentieth as strong as a covalent bond, the sheer number and constant re-forming creates powerful effects. Cohesion, for instance—water molecules stick together, giving us surface tension. That’s how a spider in Figure 3.3 can literally walk on water.

B: And the same stickiness helps plants pull water up through their roots, right? Something to do with cohesion and adhesion?

A: You’ve got it. Cohesion holds water molecules together in columns, while adhesion makes them cling to the cell walls—both are shown clearly in Figure 3.4. When water evaporates from the leaves, it tugs on the next drop below, and so on, pulling water up even the tallest trees.

B: Okay, so those are ‘sticky’ powers. But I always hear about water’s ability to stabilize temperature. How does hydrogen bonding fit into that?

A: Here’s the trick: much of the heat that enters water first breaks hydrogen bonds before speeding up the molecules. That’s why water has a high specific heat—1 cal per gram per degree Celsius. If you look at Figure 3.5, you’ll see coastal areas stay cooler in summer and warmer in winter compared to inland towns, thanks to water’s ability to absorb and release heat so slowly.

B: So oceans and even the water in our bodies buffer us against wild temperature changes… that’s pretty remarkable.

A: Exactly. And let’s not forget evaporation. Water has a high heat of vaporization—about 580 calories per gram at 25°C. That’s why sweating cools you off; the hottest, fastest molecules escape, lowering your body’s temperature. But in humid weather, sweat doesn’t evaporate as easily, so cooling is less effective.

B: And that explains why steam burns hurt so much: all that heat energy is released when vapor condenses back into liquid on your skin, right?

A: Spot on. Now, another quirk—ice floats. Solid water is actually less dense than liquid. As water freezes, hydrogen bonds push the molecules apart, creating a lattice. This keeps lakes from freezing solid, preserving life underneath.

B: One more thing: water dissolves so much! Is that because of the polarity too?

A: Absolutely. Water’s polar nature lets it form hydration shells around ions—like NaCl dissolving, as Figure 3.8 shows. But it also dissolves many polar molecules, like sugar. Hydrophilic substances interact well with water, while hydrophobic ones, like oil, are repelled. That distinction is vital for everything from cell membranes to how towels dry us off.

B: And when we talk about concentrations, we use moles and molarity—so one mole means 6.02 x 10²³ molecules. For sucrose, that’s 342 grams per mole. Right?

A: Perfect recall. And finally, water autoionizes, releasing H+ and OH− ions. That’s the springboard for acids, bases, and the entire pH scale. Buffers like carbonic acid can stabilize blood pH, while excess CO₂ can acidify oceans—ultimately affecting organisms’ ability to build shells, as Figure 3.12 illustrates.

B: So from sticky surfaces to ocean chemistry, water’s hydrogen bonds literally shape the world as we know it. That’s kind of... mind-blowing.