Super Powered Water: Solubility

Everyday Einstein discusses water's ability to dissolve in Part 3 of this series on the amazing superpowers of water. 

Lee Falin, PhD,
November 30, 2013
Episode #078

Page 2 of 2

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. 

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.” 

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). 

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. 

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. 

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Soapsuds image, oatsy40 at Flickr. CC BY 2.0. Dish washing image courtesy of Shutterstock.