Super Powered Water: Solubility
Ask Science discusses water’s ability to dissolve in Part 3 of this series on the amazing superpowers of water.Â
Lee Falin, PhD
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Super Powered Water: Solubility
One of water’s nicknames is “the universal solvent.” This simply means that a lot of things dissolve in water. If you’ve been following along with my series on water, you might have guessed that the reason behind water’s ability to be an amazing solvent is its ability to form hydrogen bonds.Â
Check out Part 1 and Part 2 of the water series before you continue reading or listening..
Just to quickly refresh your memomy, what’s most important to remember is that water molecules are formed when one oxygen atom forms covalent bonds with two hydrogen atoms. The oxygen atom shares one electron with each hydrogen atom, which in turn share one electron each with oxygen, forming a water molecule.Â
However, since these shared electrons tend to spend more time on the oxygen side of the molecule, this leaves oxygen with a partial negative charge and the hydrogen atoms with partial positive charges. These partial charges allow water molecules to form hydrogen bonds or polar bonds with one another. These bonds are the source of water’s amazing powers, including its ability to dissolve so many things.
Let’s Mix Things Up
First, what does it mean to dissolve anyway? If I drop some salt or sugar into my water, does it disappear? When we dissolve something into water, we’re taking the molecules of that substance and mixing them into the molecules of water. If we want to be really scientific, we’d say that the thing were dissolving is called the solute and thing we’re dissolving it into is the solvent. Together, the solute and the solvent make what’s called a solution. Something that can be dissolved in water is said to be water-soluble.
A solution sounds a lot like a mixture, but it’s a bit different. If you pour a bag of chocolate chips into some cookie dough, the chocolate chips aren’t chemically bonded to the cookie dough. (If they are, you might want to double-check that recipe). When we combine two things together that aren’t chemically bonded, we call this a mixture.Â
One Lump or Two?
When you pour some sugar into a glass of water, the bits of sugar are actually crystals made up of tightly-arranged sugar molecules. Since parts of the sugar molecules are polar (meaning they have a charge), the water molecules can form hydrogen bonds with them, and they do so with a vengeance.
In fact, several water molecules can simultaneously form hydrogen bonds with a single sugar molecule, surrounding it with a sort of watery shield called a hydration shell. This shield of water molecules prevents the sugar from bonding with other sugar molecules, and therefore the sugar stays dissolved in the water.Â
The same thing happens when you dissolve salt into water, however since salt is made up of sodium and chlorine ions held together by ionic bonds, the water molecules pull those ions apart and form hydration shells around the individual ions, preventing them from reforming into salt crystals.
Time to Clean Up
This amazing ability of water to dissolve so many things is why we tend to use it to wash the dishes. If there’s one chore that’s universally beloved in my home by children and parents alike, it’s washing the dishes – probably because we love seeing water’s super solvent abilities in action. Okay, so maybe that’s a bit of an exaggeration.
However, if you’ve ever tried to wash dishes in plain water, you might have noticed that things like grease and oil tend to not come off very well. That’s because these substances are non-polar, meaning they don’t have any charges that water molecules can form hydrogen bonds with. As a consequence, they don’t dissolve, or are insoluble in water. We sometimes call these types of compounds hydrophobic which literally means “water fearing.”Â
Fortunately, some wise person in the annals of history invented dishwashing detergent (or “washing up liquid” as it’s called here in the UK). Dishwashing detergent has special chemicals called surfactants in them which have a hydrophobic side and a hydrophilic side (a side that fears water and a side that loves water).Â
When mixed into water, surfactant molecules tend to form little balls called micelle. The outside of these balls have the water loving side of the molecule, while the water fearing side sticks to the inside of the sphere. Surfactants help us with our washing up because the water fearing parts of the molecules can form bonds with oils, which then get pulled into the center of the micelle.Â
Since water molecules are able to form hydrogen bonds with the surface of the micelle, the oil molecules are now considered to be soluble. So when you rinse the soap off of an oily dish, the soap grabs the oil, the water grabs the soap, and it all goes down the drain together.
Do Try This at Home
A side effect of how surfactants work is that they tend to disrupt surface tension, making water spread out more than it normally would. A classic experiment is to take a bowl full of water and sprinkle some pepper on top. Then dip your finger into some dishwashing liquid and lightly tap the center of the water. When you do, the surface tension is broken, and the water molecules fly apart, taking the pepper with them.Â
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Soapsuds image, oatsy40 at Flickr. CC BY 2.0. Dish washing image courtesy of Shutterstock.
