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Solutions, Solutes, and Solvents

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  1. 0:33 What is a Solution?
  2. 1:02 Molecular Interactions in an…
  3. 1:50 Hydrophilic Molecules
  4. 3:19 Hydrophobic Molecules
  5. 4:49 Lesson Summary
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Taught by

Greg Chin

Oh no! Your friend Ben just drank chili oil on a dare, and now his mouth is burning. Should he drink the ice water or vegetable oil to cool his mouth? Quick. Watch this lesson if you aren't sure.

So poor Ben just drank a shot of hot chili oil on a dare and his mouth isn't feeling too great at this point. You're warning him as he's reaching for that tall glass of water to cool his mouth but he just doesn't believe you. Do you think you can convince him that water really isn't going to help him that much? Before we start talking to Ben about why he shouldn't be drinking the water, we better explain to him about solutions first.

What is a Solution?

So a solution is just a mixture in which one substance is dissolved in another. For example, if I took some salt and dissolved it in water, that would give us a salt solution.

Now to define things more precisely, let's call the substance that's doing the dissolving the solvent and let's call the substance that is getting dissolved the solute.

Molecular Interactions in an Aqueous Environment

It's appropriate that we talk about water because of the importance of water to biology. Because of this importance, scientists characterize the behavior of molecules in a water or aqueous environment. We've talked about molecules in terms of being polar or non-polar molecules, and whether or not a molecule is polar or non-polar defines how it's going to behave in this water environment.

Hydrophilic Molecules

Molecules which are polar are going to be able to form hydrogen bonds with the water molecules. That means that we refer to these molecules as being hydrophilic. Let's take these roots and define them.

'Hydro' refers to water and 'phil' refers to love. So taken together, this literally means 'water loving.' These molecules appear to be water loving because they have polarity, and the partial charges on their atoms allow them to interact with water molecules.

Diagram of hydrophilic molecules
Diagram of Hydrophilic Molecules

So here is an ammonia atom and here is a water molecule. Let's draw a couple of water molecules. Because of the polarity in charge, the partial positive charge on the hydrogen can interact with the partial negative on the oxygen atom and the same goes for the nitrogen atom. Because these molecules can interact, they're said to love each other.

Hydrophobic Molecules

Let's now thing about the non-polar molecules. Non-polar molecules can't interact with water because they lack polarity. So we refer to them as being hydrophobic. Recall that we said that 'hydro' refers to water. 'Phobic,' as you may know, refers to fear. We can take this literally as meaning 'water fearing.'

What this means is if we have our water molecules and we have something like oil, the water molecules are preferentially going to interact with each other and their not going to want to interact with this oil over here. The oil is just going to continue to interact with itself to the exclusion of water.

Other types of hydrophobic molecules that are going to be relevant to biology that we'll be discussing are also things like fats and hydrocarbons.

Unfortunately, what this means for poor Ben, is that if he tries to drink lots of water, that's not going to do anything to knock that hot chili oil off of his tongue; he's going to need to drink something that's hydrophobic, and ironically, more oil that doesn't have any chili in it would probably help him, and ice water is not going to help him at all.

Lesson Summary

So in summary, we learned a little bit about solutions. A solution is a mixture in which one substance is dissolved into another substance. We've learned that a solution is composed of a solvent and a solute. A solvent is the substance in which a solute is dissolved. A solute is the substance which is dissolved in the solvent.

Water is the main solvent in biological processes. Molecules which can form hydrogen bonds with water are called hydrophilic. The ability to interact with water means that hydrophilic substances can be dissolved in water.

Molecules that cannot form bonds with water are called hydrophobic. The failure to interact with water means that hydrophobic substances cannot be dissolved by water but would be dissolved by a non-polar solvent such as oil. So the lesson for Ben is to maybe consider drinking a little bit of canola oil.

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