Solubility in water -- High school chemistry

[Barclay]

Q1. Why do simple molecular substances tend to be insoluble in water?

Q2. Molecular substances are often soluble in organic solvents.

Homework Equations

The Attempt at a Solution

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A1. Water molecules have strong intermolecular attractions between them. In order for a substance to dissolved, the intermolecular attractions between water molecules have to be broken to that the dissolving molecules can fit between them. Many simple covalent molecules lack polarity of sufficient size (in their own molecule) to break the intermolecular attractions between the water molecules. Therefore they do not dissolve

A2. I don't know why. Please help

 

[DrClaude]

Q2. Molecular substances are often soluble in organic solvents.

Is that really how the question is phrased? Because "molecular substance" is meaningless.

 

[Barclay]

Is that really how the question is phrased? Because "molecular substance" is meaningless.

Hello, the topic is "Simple Molecular Strucures". The book says "Molecular substances tend to be insoluble in water unless they react with it". Then it goes onto talking about how "attractions between the water molecules must be broken so that the dissolving molecules can fit in between them"

Then it goes onto say "Molecular substances are often soluble in organic solvents" ... then says "In this case, the intermolecular attractions between the two different types of molecule are much the same as the pure substances". This explanation does not really tell me much. I want to know what's happening moelcularly (High School level).

 

[DrClaude]

This is horribly written. I feel for you having to learn from that material!

I'll try to keep it simple, but don't hesitate to ask questions if any of this goes over your head. Water is a kind of molecule that is polar, meaning that the electric charges are not evenly distributed in the molecule. In particular the oxygen is slightly negative and the hydrogens are slightly positive. This makes the molecules stick together quite easliy, and for a molecule to readily dissolve in water, it needs to also be polar, so it can nicely place itself between water molecules. One example is ethanol, the alcohol molecule we can drink (but not if you're still in high school ). This is also why salt dissolves in water, at it splits into Na⁺ and Cl^-, and the polar water molecules can easily accommodate these charged atoms (ions). Conversely, non-polar molecules would "break" the links between the water molecules, and this is not favorable. This is why oil and water don't mix.

Organic solvents are mostly non-polar. They have good interaction with other non-polar molecules, so the latter will dissolve in the organic solvent.

Note that there are also molecules that are a mixture of both. For instance, soap molecules have one end that interacts favorably with water (it is hydrophilic), but not the other end (it is hydrophobic). The latter end will, for instance, stick to grease, enveloping the grease molecules, with the other end sticking out. As that other end is hydrophilic, the entire structure will dissolve in water, and this is why soap helps us clean.

Rule of thumb: in chemistry, like attracts like.

 

[Barclay]

Water is a kind of molecule that is polar, meaning that the electric charges are not evenly distributed in the molecule. In particular the oxygen is slightly negative and the hydrogens are slightly positive. This makes the molecules stick together quite easliy, and for a molecule to readily dissolve in water, it needs to also be polar, so it can nicely place itself between water molecules. One example is ethanol, the alcohol molecule we can drink (but not if you're still in high school ). This is also why salt dissolves in water, at it splits into Na⁺ and Cl^-, and the polar water molecules can easily accommodate these charged atoms (ions). Conversely, non-polar molecules would "break" the links between the water molecules, and this is not favorable. This is why oil and water don't mix.

Organic solvents are mostly non-polar. They have good interaction with other non-polar molecules, so the latter will dissolve in the organic solvent.

Note that there are also molecules that are a mixture of both. For instance, soap molecules have one end that interacts favorably with water (it is hydrophilic), but not the other end (it is hydrophobic). The latter end will, for instance, stick to grease, enveloping the grease molecules, with the other end sticking out. As that other end is hydrophilic, the entire structure will dissolve in water, and this is why soap helps us clean.

Rule of thumb: in chemistry, like attracts like.

Thanks for the explanation. I've understood the polarity bit and I said something about this on my first post: Water molecules have strong intermolecular attractions between them. In order for a substance to dissolved, the intermolecular attractions between water molecules have to be broken to that the dissolving molecules can fit between them. Many simple covalent molecules lack polarity of sufficient size (in their own molecule) to break the intermolecular attractions between the water molecules. Therefore they do not dissolve

Is that RED HIGHLIGHTED bit correct?

The bit I don't understand is why non-polar compounds are soluble in organic solvents.

Is that really how the question is phrased? Because "molecular substance" is meaningless.

I just realized what you meant by this : the author of my book should have written "non-polar molecules" and not "molecular substances".

 

[DrClaude]

Many simple covalent molecules lack polarity of sufficient size (in their own molecule) to break the intermolecular attractions between the water molecules. Therefore they do not dissolve

Yes, that is a correct way to say it.

The bit I don't understand is why non-polar compounds are soluble in organic solvents.

That's why I added the rule of thumb. It is difficult to explain without going into technical details. Basically, if molecules are similar, they will have overall attractive interactions. If a molecule is similar to a solvent molecule, it will readily dissolve in that solvent.

I just realized what you meant by this : the author of my book should have written "non-polar molecules" and not "molecular substances".

That would indeed have been much better.