How To Determine Solubility - Online Tutor, Practice Problems & Exam Prep

The principle of solubility, "like dissolves like," emphasizes the importance of polarity in determining whether substances will mix. Polar molecules, such as water and ethanol, dissolve in each other due to their similar net dipoles. Understanding the polarity of various organic compounds, like pyridine and DMSO, is crucial for predicting their solubility in aqueous solutions. Drawing Lewis structures can help identify molecular dipoles, guiding conclusions about miscibility. This knowledge is foundational for organic chemistry, influencing reactions and interactions among different functional groups.

Understanding “like dissolves like”.

So I went ahead and took the luxury of pre-drawing this little diagram for you. It's very scientific. And, I thought that it would help you guys understand the rule of solubility, which is just that, like dissolves like. Alright? So you might have heard this in lab before, and all it means is this, this has to do with polarity. Okay? So remember that we learned how to figure out if molecules have a net dipole? That's all there is to it. We're going to figure out, okay, what we're dissolving or, yeah, what we're dissolving, is it going into something that has the same polarity as itself? So this is, you know, a very common scene from Friday night, maybe this was a few nights ago for you. You're pouring a bottle and you're pouring it into a cup of water, and you've got your ethanol little molecules spilling out, and they go into the water molecules, which are right down here. And you're not thinking for a second that they're gonna split apart and you're gonna have like alcohol on the top and water on the bottom. That would be really weird. In fact, what happens is that they just mix together and you can't even tell the difference, except you can tell the difference the next morning. You know that obviously you weren't just drinking water. So that has to do with the fact that both have the same polarity or similar polarity. Think about it. Water, let's just expand this a little bit. Water has a net dipole. Right? We said that the net dipole was pretty strong. Well, in the same way, ethanol also has a net dipole. Okay? So in this case, I drew it so that the net dipole would face the other direction, but it doesn't matter. It doesn't really matter what direction it's facing because obviously I could rotate that alcohol. The important thing is that they both have a strong net dipole. Since they're both polar, they're going to dissolve into each other. Okay? So that's the entire concept between like dissolves like.

Introducing common solvents and other molecules in organic chemistry.

I'm going to use this opportunity to teach you about some really weird molecules that you're going to see for the rest of organic chemistry, and maybe this will give you an opportunity to get a little bit more familiar with them. Alright? So let me go ahead and introduce these molecules first. We've got Pyridine over here. He's like an all-star. Pyridine looks like a benzene ring. I know we haven't been over that yet, but that's a benzene ring with a nitrogen. Alright. Very commonly used in reactions. Then on top, we've got DMSO. DMSO looks a lot like acetone except that it has a sulfur instead of the carbon. Okay? Then we've got THF. It's called Tetrahydrofuran. You don't need to know the full name, but it's basically just an oxygen in a 5-membered ring. Then we've got carbon tetrachloride. That one is a carbon with 4 chlorines all around. The polarity is going to be interesting for that one. And then we've got basically just a sulfur compound, basically that's called a thiol group. And then we've got nitrogen, which has basically 3 carbon groups around it. Okay? And for these 2, we're going to wind up having to draw Lewis structures to figure out what the solubility is. So what I want you guys to do is go through all of these and tell me if you would expect them to dissolve in an aqueous or be miscible in an aqueous solution. Okay? Now what aqueous means, this is tricky, it's not that tricky, but aqueous just means water. Okay? So what I want you guys to do is think about the polarity of water. Think about whether it is polar or apolar, and then look for dipoles on these 6 molecules. Once you think you've found if they have a dipole or not, a net dipole or not, then you can draw your conclusion: is this soluble or is this not going to be soluble? Alright? Now for the last 2, like I said, you are going to have to draw proper Lewis structures in order to know if it has its dipole or not. Okay? Because it can get a little tricky if you don't draw the Lewis structure.

Would you expect the following molecule to be miscible in an aqueous solution? 

• A. Miscible
• B. Immiscible

Would you expect the following molecule to be miscible in an aqueous solution? 

• A. Miscible
• B. Immiscible

Would you expect the following molecule to be miscible in an aqueous solution? 

• A. Miscible
• B. Immiscible

Would you expect the following molecule to be miscible in an aqueous solution? 

• A. Miscible
• B. Immiscible

Would you expect the following molecule to be miscible in an aqueous solution? 

• A. Miscible
• B. Immiscible

Would you expect the following molecule to be miscible in an aqueous solution? 

• A. Miscible
• B. Immiscible

Here’s what students ask on this topic:

What does 'like dissolves like' mean in terms of solubility?

The phrase 'like dissolves like' refers to the principle that substances with similar polarity will dissolve in each other. Polar molecules, which have a net dipole moment, will dissolve in other polar substances. Conversely, nonpolar molecules, which lack a net dipole moment, will dissolve in other nonpolar substances. For example, water (a polar molecule) will dissolve ethanol (another polar molecule) because both have similar polarities. This principle is crucial for predicting solubility in various solvents.

How do you determine the polarity of a molecule?

To determine the polarity of a molecule, you need to consider its molecular geometry and the electronegativity of its atoms. First, draw the Lewis structure to visualize the arrangement of atoms. Then, assess the electronegativity differences between bonded atoms. If there is a significant difference, the bond is polar. Finally, consider the molecule's shape to see if the dipoles cancel out or create a net dipole moment. If the dipoles do not cancel, the molecule is polar; otherwise, it is nonpolar.

Why is it important to draw Lewis structures when determining solubility?

Drawing Lewis structures is important for determining solubility because it helps visualize the arrangement of atoms and the distribution of electrons in a molecule. This visualization allows you to identify the presence and direction of dipole moments, which are crucial for assessing the molecule's polarity. Understanding the polarity helps predict whether the molecule will dissolve in a given solvent, based on the 'like dissolves like' principle.

What is the solubility of pyridine in water?

Pyridine is a polar molecule due to the presence of a nitrogen atom with a lone pair of electrons, which creates a net dipole moment. Because water is also polar, pyridine is soluble in water. The similar polarities of pyridine and water allow them to interact and dissolve in each other, following the 'like dissolves like' principle.

How does the polarity of DMSO affect its solubility in water?

DMSO (dimethyl sulfoxide) is a polar molecule due to the presence of a highly electronegative oxygen atom bonded to sulfur, creating a significant dipole moment. Because water is also polar, DMSO is highly soluble in water. The similar polarities of DMSO and water allow them to mix well, adhering to the 'like dissolves like' principle.