What Makes Water So Special?

by Carson

You use water every day — it’s essential for keeping yourself alive. In fact, our bodies are about 60% water. But other than that, water is a very special chemical. Let’s explore why in this article.

1. Water is a Universal Solvent

Water is well-known for its impressive ability to dissolve substances; it’s known as the universal solvent, as it can dissolve more substances than any other liquid. That’s mainly because water is a strongly polar molecule — one side of the water molecule (the hydrogen side) is positively charged, while the other (the oxygen side) is negatively charged. Thus, it can dissolve other molecules with the same polar property. Essentially, the electrostatic charges of the polar water molecules can split the polar solute molecules apart into ions and keep them in place through the electrostatic attracting force of opposite charges.

Why Is Water a Polar Molecule?

So why is water a polar molecule? That’s because of the difference in electronegativity between the hydrogen and oxygen atoms. In other words, the ability to attract electrons for these two molecules are different. Specifically, oxygen is better at this than hydrogen, so the shared electrons between the hydrogen and the oxygen atoms (in the covalent bond) are closer to the oxygen atom than the hydrogen atom. This results in the hydrogen side of the molecule being slightly positively charged, as only the nucleus is exposed.

On the other hand, there are 4 electrons on the oxygen atom that are completely uninvolved in the covalent bond. Those electrons are called lone pairs, and are not shared to other atoms in a molecule. Lone pairs tend to be in the direction opposite to the covalent bond, so this makes the oxygen side of the atom slightly negatively charged. With a positive and a negative charge, a dipole is formed between the two sides, making water a polar molecule.

An illustration of the polarity of the water molecule
The electrons are pulled away from the hydrogen atom, causing a slightly positive charge at the hydrogen atom
The free electrons cause a slightly negative charge at the oxygen atom
This makes the water molecule polar

Likewise, if you add any other combination of atoms with different electronegativities, you will likely get a polar compound, assuming that the molecular structure isn’t rotationally symmetric. Other examples of polar molecules include ammonia (NH3), carbon monoxide (CO), and ionic compounds such as sodium chloride (NaCl).

2. Water Expands Upon Freezing

Have you ever wondered why ice floats on water? The ability of water to act as a solvent is already amazing, but there is one more rare property of water: it actually expands upon freezing. Intuitively, as a substance heats up, its density decreases, as the molecules can find more room to move around as the temperature increases. The opposite is true when a substance contracts when cooling down. Water follows this rule for most temperatures — but when it undergoes phase transitions at its freezing point, it does not.

The density of water as a function of temperature
Note the peak in density at about 4 ºC, and note the difference in densities between the solid and liquid form of water
Image credit: Klaus-Dieter Keller, created with QtiPlot, Font: Liberation Sans, CC BY-SA 3.0, via Wikimedia Commons (https://commons.wikimedia.org/w/index.php?curid=19093965)

Specifically, the maximum density of water occurs at about 4 ºC. That’s because of the difference between the solid phase and the liquid phase of water. The water molecules are arranged in a hexagonal crystal in the solid phase. The hydrogen bonds bind the water molecules together through electrostatic attractions between the positive hydrogen atom and the negative oxygen atom in another molecule.

In this specific case, it produces a porous crystal structure that is slightly less dense than liquid water. Therefore, when the temperature exceeds that of the melting point, the water molecules can break away from the crystal structure and become (on average) slightly closer to each other in the liquid form. This crystal dissipation continues until about 4 ºC, when water reaches its maximum density. After that, thermal expansion decreases the density of liquid water again, as the temperature increases. The opposite is obviously also true, which means ice is less dense than liquid water.

The hexagonal crystal structure of water
Oxygen atoms are in red, and the hydrogen atoms are in white
The white dotted lines denote the hydrogen bonds between the hydrogen and oxygen atoms
Image credit: Adam001d, CC BY-SA 3.0, via Wikimedia Commons (https://commons.wikimedia.org/wiki/File:Ice_hexagonal_crystal.png)


In this article, we’ve explored two properties of water that make this chemical so special — it has an impressive ability to dissolve other chemicals, and its solid form is less dense than its liquid form, which is rare among the set of chemicals. If you have any suggestions or improvements to this article, please leave your ideas in the comments section below.

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