Water as a Solvent — Overview & Importance

(Video) Water, the Universal Solvent

by Mr. McAughan

In this video, Mr. McAughan teaches you about the most important solvent in the world, water. You'll learn how water's structure helps it dissolve other things and even see a diagram of how water pulls apart polar solutes.

I'm a huge coffee snob. I don't only enjoy it for the caffeine. Although, that certainly helps me write better early in the morning. I love the whole world of coffee. I can tell you my coffee's origin, tasting notes, and the best way to prepare it. I even roast all my coffee. I'm a bit off the deep end!

Coffee takes advantage of one of chemistry's most powerful principles. When I pour boiling water onto freshly ground coffee, I use water as a solvent. Coffees have thousands of different chemicals that create their tastes and aromas. But, how do they end up in my cup?

A Quick Refresher on Solubility

How do we know if something will dissolve? We check its solubility. In a solution, we have a solute and a solvent. The solute is the molecule that dissolves. The solvent does the dissolving. We can have many different solute and solvent combinations. The phase of matter is not important. To know if a substance is soluble or insoluble, we use "like dissolves like." So, a polar solvent dissolves a polar solute. But, a non-polar solvent would not dissolve a polar solute.

The 'like dissolves like" rule comes from the electrostatic interactions. Let's think about the different intermolecular forces. Non-polar molecules only experience dispersion forces. Polar covalent molecules have dipole-dipole interactions or hydrogen bonding. They can also interact with ions through ion-dipole interactions. To be soluble, we need to have similar intermolecular forces.

Let's imagine we try to dissolve oil in water. Because water is polar, it needs to balance its partial charges. It balances its polarity by hydrogen bonding with itself. But, non-polar oil cannot balance water's polarity. Dispersion forces create oil's dipoles. They are weak and temporary. So, the water interacts with itself. Oil will not dissolve in water. To get oil to dissolve in water, we need to create an emulsion.

The "like dissolves like" rule is a general guideline. For more detail, use the qualitative solubility rules.

Why is Water such a Great Solvent?

Let's talk more about water. It can dissolve more substances than any other liquid! So, we call it the universal solvent. You've already seen that water can't dissolve everything. We just looked at water and oil. So, don't get confused. Water dissolves a lot of substances.

Water is an excellent solvent because it has hydrogen bonds. Water's polarity allows it to balance other charges. For example, it offsets other polar molecules and ions. Remember, oxygen is exceedingly electronegative. So water's oxygen pulls the electrons towards it. The oxygen end has a partial negative charge. The hydrogen side is left partially positive.

Let's think about adding ammonia gas to water. What happens? Ammonia is polar with trigonal pyramidal VSEPR geometry. Its lone pair is on top of the electronegative nitrogen. So, the nitrogen hoards the electrons. Meanwhile, its hydrogens point down. We can imagine water's hydrogens interacting with ammonia's nitrogen. The partial positive balances the partial negative. Ammonia's hydrogens would interact with water's oxygen. Again, the partial charges are balanced.

What happens if you add an ionic solid to water? Let's imagine sodium chloride. Ionic compounds form crystal lattices. So, the sodium and chloride ions distribute into unit cells. Water collides with the ions on the crystal's surface. The water offsets the ion's charge. So, it can leave the crystal lattice. Ions have complete charges because they lost or gained electrons. But, water is only partially charged. So, it takes multiple waters to balance the ion. The water molecules form a shell around the ion.

Chemistry in the World: Kidneys

Water is an essential biological solvent. Every day, your kidneys remove waste from your blood. Water dissolves that waste. If we didn't have water, excess waste would build up in our bodies.

The kidneys use water to dissolve excess ions. Let's say you eat a salty lunch. Now, you have too much sodium. Your body needs plenty of water to remove all that sodium. So, you'll feel thirsty after a salty meal.

The body also uses water to remove excess protein. Let's say you drink a protein shake after a hard workout. Most shakes have too much protein. Your body cannot handle that. So, the liver converts the extra protein to urea. Urea is a polar molecule. When urea is in the kidneys, it dissolves in water. Of course, urea is the root of urine. It has a light yellow color when dissolved.

Chemistry in the World: A Little More About Coffee!!!!!!

There are two types of coffee brewing: infusion and percolation. For infusion brewing, you pour boiling water over ground coffee and let it sit. For percolation, you continually add fresh water. Coffee's flavors come from polyphenols. Polyphenols are polar molecules that dissolve in water. You encounter them in organic chemistry. In immersion brewing, the water reaches equilibrium with all the different polyphenols. So, they often have a lot of flavors. In percolation, only the most soluble polyphenols dissolve. The most soluble compounds are acids. So, percolation highlights a coffee's acidity. You might hear a coffee snob, like me, say that a coffee has crisp apple notes. Its acidity reminds me of biting into a crisp granny smith apple on a fall day.