Solids dissolve better in hot water because hot water has more space between the molecules to accommodate them. Gasses dissolve worse in hot water for the same reason: the extra space allows the gas to escape.

When a solid dissolves in a liquid, it basically becomes liquid. This is kind of like melting - the solid becomes a liquid. For this to happen, the chemical bonds holding the solid together have to break. Breaking the bonds takes energy - this can come from heat, so when a substance melts it cools the area around it. The same happens when a solid dissolves - it cools the liquid around it.

However, there is a big difference between melting and dissolving. When a solid dissolves in a liquid, it becomes part of the liquid by bonding with the liquid molecules. This creates new bonds and making these new bonds releases energy as heat. This refunds some of the heat used to break up the solid in the first place.

For most solids, the heat refund is less than was paid to break the solid up. This means that dissolving costs energy and eventually you reach a balance where no more will dissolve. If you add more heat, there is more energy available and the balance points moves so that more ends up dissolved.

For some solids, the refund is actually more than was paid. In this case, adding heat reduces solubility bexause the energy balance point moves the opposite way.

The same thing happens with gases. They have super weak bonds when gases, and the bonds when dissolved are stronger, so you get an energy refund when they dissolve. Putting more energy in moves the balance point to reduce solubility.

ELI5: Almost everything is more soluble in hot water than cold water. Including Soaps. The soap is what cleans the germs off. More soap in the water = better germ cleaning.

Soaps help water remove things that don't usually mix with water - like oils, fats and germs. The soap has one end that like water and one end that likes fat/oil/"not water". The soap surrounds the germs and the water surrounds the soap

Beyond ELI5:

A few interesting examples occur of things are NOT more soluble in hot water than cold water: triethylamine is soluble in water below 19C only. Nicotine is soluble in water below 61C and insoluble above that (weirdly, it becomes soluble again above 210C in pressurised containers). Some polymers show similar behaviour. It's called a lower critical solution temperature

The explanation for this comes down to a Gibbs Free Energy change which is too advanced for this sub.

Other posts are addressing your main question, but I want to take a sec to correct your conclusion about germs.

The germs are not being dissolved in hot water. Bacteria will not break apart in any water that would be safe to touch. The soap helps to break down fats and oils on your skin so they can be rinsed off easier, and the scrubbing action of your hands does the work of actually getting things off. You can just as safely wash with cold water, but most people find warm to slightly hot water more comfortable.

Dissolving is molecules of water hitting the solid particle and taking away pieces of it. Molecules of water are constantly moving around. Temperature is the measure of average molecule speed. So hot water has faster molecules. Faster molecules means that they hit the solid harder and more often. Therefore they dissolve it more quickly.

>However, there is a big difference between melting and dissolving. When a solid dissolves in a liquid, it becomes part of the liquid by bonding with the liquid molecules. This creates new bonds and making these new bonds releases energy as heat. This refunds some of the heat used to break up the solid in the first place.

This is just not correct at all.

Let's take a simple example of salt being dissolved in water.

It's kind of true that the bonds that make up the crystal lattice for salt are broken as part of the dissolving process. This occurs mostly because the sodium and chlorine are already highly ionized (the electron being shared between them is spending most of it's time around chlorine, and we tend to short hand this by calling it an Na+ and Cl- ionic bond), so separating them is relatively easy if you have something with sufficient ionic attraction (like the partial charge that water carries due to the strong electron attraction of the oxygen atom in H2O).

But new bonds are NOT created. There is hydrogen bonding, but there's no chemist in the world who would say that hydrogen bonds are a form of chemical bonding. It's a dipole-dipole attraction, similar to a Van Der Waals force but much much stronger.

Secondly, this process doesn't release energy. The process of dissolving salt in water is endothermic - that is, energy is absorbed in the process of dissolving salt into water, not released, so both the salt and the water will get slightly cooler as a result of the salt dissolving.

> But new bonds are NOT created. There is hydrogen bonding, but there's no chemist in the world who would say that hydrogen bonds are a form of chemical bonding.

They didn't say "chemical bond", but just "bond". Just read what you wrote yourself again, even you used "hydrogen bonding", and indeed, there are many types of bonds and all of them involve some energetic changes. It doesn't matter if hydrogen bonds, ionic bonds, chemical bonds, or magical bonds.

> Secondly, this process doesn't release energy.

They didn't claim it does. Even in what you quoted they say "This refunds some of the heat used to break up the solid in the first place", implying that in total, energy is still spent. And it is correct that the break-up requires energy to overcome the ionic bonds, yet some other salts release quite a bit of energy when dissolved in water, exactly due to the reason they gave.

>They didn't say "chemical bond"

Yes they did:

...it becomes part of the liquid by bonding with the liquid molecules.

Nothing is "bonding with the liquid molecules" - that's not happening at all.

>even you used "hydrogen bonding"

Which is decidedly NOT "bonding with the liquid molecules" exactly like I stated.

>It doesn't matter if hydrogen bonds, ionic bonds, chemical bonds, or magical bonds.

Yes it does, these are all WILDELY different things.

If you don't understand chemistry don't pretend to.

>They didn't claim it does.

Did you not read their post?

making these new bonds releases energy as heat

When you can actually read what they posted and my reply, get back to me.

Random question... Does breaking stuff from solid make stuff slower aka colder? I know nacl+ water makes it colder but is it always like that

At a basic level heat is just molecules moving around more. The colder it is, the slower water molecules fly around. Water molecules either bind or breakdown other molecules through typical chemical processes. When water comes in contact with glucose it binds with it and your Hot Chocolate mix starts to become one fluid instead of water and a powder. So the more the molecules are moving around the more likely they are to come into contact with the thing you are dissolving. Fats also undergo chemical changes when they get hot so they may become fluid and mix better in water.

No it’s not always like that. During dissolving the bonds between the pieces of the solid are broken but new bonds between water molecules and ions are formed. Breaking bonds requires energy, and making bonds releases energy. Therefore, if the pieces of solid are more inclined to bonding with water than with other pieces of the same solid, then overall energy is released and the temperature rises (e.g. sugar). Otherwise, energy is consumed and the temperature falls (e.g. table salt).

Maybe some examples for lesser solubility at higher temperatures that are salts:

• many calcium salts such as its carbonate (gypsum), sulfate (chalk), phosphate, etc.;
• sodium sulfate has its highest solubility at 33°C, it falls off in both directions;
• unlike what many believe, table salt's solubility increases almost not with temperature; only the speed of dissolving does.

It is also interesting to note that some salts produce a lot of heat (e.g. NaOH a.k.a. caustic soda) or cold (e.g. ammonium nitrate a.k.a. the stuff that blew up Beirut, or urea). This and solubility are related, but all combinations can happen.

Are you seriously saying that "hydrogen bonding" is not "bonding""? Because that is clearly silly.

And yes, they said the new bonds (the hydrogen ones, duh!) release energy; after more(!) energy was used to break ionic bonds.

You are just trying to be pedantic and fail at it.

I'm saying it's not "bonding to a molecule" if you disagree you need to take introductory chemistry.

Further, I just want to point out, hydrogen bonding only applies to ionic substances being dissolved in specific solvents. So many things dissolve without hydrogen bonding at all.

You clearly don't understand the basics of chemistry.

Well, I will leave my expertise of chemistry to the other readers (and my post history), but you clearly can only do insults (and down-votes, it seems; surely a sign of a healthy adult scientific discussion!).

Liquids have a fixed volume regardless of temperature, space isn't really the issue. The space between molecules in cold water is pretty much the same as in hot water, what's different is the average speed and energy of the molecules moving around in the water. Gases escape from hot water more quickly because gases don't 'want' to be dissolved in water, so all it takes is a little bit of energy to push them out of the liquid and into the air. Since hot water has more energy in it, collisions with dissolved gases happen more often and with more power, so it's relatively easy for the gas to bubble out of solution

Other answers are good so I wont re explain them, but not everything's solubility increases with temperature. Calcium carbonate, for example, is more soluble at cold temperatures because it creates scales at higher temperatures.

>you clearly can only do insults

Says person who referred to my posts as "pedantic" because they corrected incorrect statements.

The only difference is - all of my posts have contained content that directly supports my position by describing how chemistry actually works. This post of yours doesn't, it JUST serves to insult me.

>and down-votes

Yes, because you haven't down voted any of my posts, right? *Eye roll*

If you want to have a "healthy adult scientific discussion" I'm happy to do that.

But saying "hydrogen bonding is a form of bonding with molecules" isn't an adult scientific discussion. It's a blatant misrepresentation of actual chemistry. A hydrogen bond - as I stated in my very first post - is a dipole-dipole interaction. It's an inter-molecular force (a force between two separate molecules, not a bond) similar to Van Der Waals forces but many magnitudes greater in strength.

Further, hydrogen bonding doesn't even apply to the way many things dissolve. So even if we could ignore chemistry and say that hydrogen bonding is a form of bonding to a molecule - which it's decidedly not - that STILL wouldn't make the post I replied to correct, because it only applies to a subset of things that dissolve.

Organic molecules, for example, can dissolve through Van Der Waals forces or/specifically London dispersion forces.

Something being dissolved can be endo or exothermic, there's no hard and fast rule about whether dissolving something will heat or cool a liquid. Saying it's one way or another just isn't accurate, it depends entirely on what you're dissolving.

>Liquids have a fixed volume regardless of temperature

https://www.researchgate.net/publication/344527033/figure/fig1/AS:944168385916929@1602118693482/Temperature-dependence-of-the-specific-volume-of-ice-and-water-plotted-based-on-Refs-15.ppm

> Something being dissolved can be endo or exothermic, there's no hard and fast rule about whether dissolving something will heat or cool a liquid. Saying it's one way or another just isn't accurate, it depends entirely on what you're dissolving.

Look, you get hinged on this again, yet the original post never said it is just one or another. It differentiated between the exothermic "reaction" of the formation of hydrogen bonds (or van-der-Waals forces or others, unmentioned ones) and the endothermic dissolution of the (often ionic, but also not explicitely mentioned) bonds in the solid. Those are two things, their sign is (almost always) pretty clear and opposite, and the heat/cold of dissolution comes from their (signed!) sum. Hence it can be either way, and this was mentioned in that post.

Yes, they left out some detail and might( have wrongly implied that the total is always negative, i.e. dissolution is endothermic, but this was not what you called out and this is ELI5, not a journal paper.

To add to this, unless the hand soap is anti microbial, hand washing generally doesn’t kill germs. As you said though it helps rinse them off your skin and temperature doesn’t make any difference beyond comfort unless it’s hot enough to really burn you.

Edit: I stand corrected, disrupting the membrane can kill some bacteria. I thought that was only true for some soaps and not all but I guess I was wrong.

How great is the temperature rise from sugar?

>Yes, they left out some detail and might( have wrongly implied that the total is always negative, i.e. dissolution is endothermic, but this was not what you called out and this is ELI5, not a journal paper.

Sure, you could argue that his statement was specifically saying that hydrogen bonding - or any other force being used to maintain the solvation of the solute - is exothermic.

But my point remains that this isn't a bond between molecules, and that is an ELI5 level topic.

There is no point in chemistry where anyone would ever suggest that solvation involves molecules bonding. No chemistry teacher would ever say that hydrogen bonds, Van Der Waals forces, or any other intermolecular force is "bonding with the liquid molecules" or that "this creates new bonds."

there are two things going on here and they aren't really related except that they have the same solution. most "dirts" are oils, waxes, and greases. heat softens them and lets the soapy water slip in between the dirt and your skin more easily. it also destroys germs because they don't have a skin. they have a fatty membrane surround, so soap and the mechanical action of washing your hands literally tears them apart.

That is wrong. Soap is very effective at disrupting cell membranes and destroys bacteria. The surface of cells, including many bacteria, is kind of like a bubble of fats. Not exactly days, but similar enough that soap does the same thing and dissolved it. Dissolve the surface of a cell and all the innards spill out. Now it’s not a living cell, it’s a collection of dead cell bits.

Soap will also detach many bacteria and let you wash them off, but soap itself is strongly antimicrobial. It doesn’t contain antibiotics and for most uses there’s no reason to add antibiotics. The same is true for alcohol hand sanitizer: it’s not antibiotic, but it’s lethal to cells.

Let me quote Wikipedia, but this is also in agreement with what I learned and sources are given there as well: '''The strength of chemical bonds varies considerably; there are "strong bonds" or "primary bonds" such as covalent, ionic and metallic bonds, and "weak bonds" or "secondary bonds" such as dipole–dipole interactions, the London dispersion force and hydrogen bonding.'''

So bonds are just that, bonds. Some are just stronger or more commonly encountered.

Also, dissolving salts involves their ionic bonds, so if we are pedantic, this part involves even more bonds. But I get some feeling that even ionic bonds are not "bonds" in your mind, only covalent ones?

Thats not true. Hydrogen bonding is an intermolecular force of attraction, as in between two molecules, and it certainly doesn't only apply to ionic substances being dissolved. The reason why water is a liquid at room temperature is because of hydrogen bonding between the positive hydrogen ends of one water molecule being attracted to the negative end of a second water molecule.

I think the point you were getting at was intramolecular vs intermolecular forces of attraction, and that in general intramolecular forces are considered to be true bonds, as they hold the atoms in different molecules and compounds together whereas intermolecular forces, not being nearly as strong as intra, are generally demoted to just being weak attractive forces, with hydrogen bonding being confusingly named as its not even a true bond by these metrics. This whole thread seems pretty semantic, 'bond' is just a strong word to describe an attractive force, at the end of the day and at its core a triple covalent bond is not that different from dispersion forces, they are just two ends of a spectrum representing how little/how much energy you need to break an attraction.

I appreciate that wikipedia may describe it like this, but that doesn't make it an accurate description used in chemistry.

This paper, for example, specifically looks at the differences between chemical and hydrogen bonding (specifically by look at bond strength).

>But I get some feeling that even ionic bonds are not "bonds" in your mind, only covalent ones?

I'd argue that the distinction is more categorical than physical.

All chemical bonds are covalent bonds, there's just a disparity in how much the electrons are actually shared.

You said "that's not true" but then discussed a bunch of stuff I never mentioned. I never mentioned whether or not hydrogen bonding only applies to dissolving substances. I never mentioned water being a liquid at room temperature.

But I will address this:

>they are just two ends of a spectrum representing how little/how much energy you need to break an attraction.

At a physical level I agree with you. But not at a categorical level. These things are categorically distinct when we talk about them because of the size of disparity in energy required.

Let a cup of salt water sit and salt will spontaneously crystallize out of the water within hours. Just through Brownian motion.

Stable molecules could take billions of years to change their structure, or longer. This is why we categorize "water" as it's own molecule, and "salt" as it's own molecule, but we don't categorize "salt water" as a molecule - we categorize it as a solution of 2 molecules.

Why do gases tend to have higher solubility in cold water?

The FDA has found that anti-microbial soap is equally as effective as regular soap. But the main anti-microbial ingredients can actually be harmful to the environment, so it's worse if you care about the environment.

It's different in a hospital setting, I think they have some more unique reasons for needing to use anti-microbial soap.

I was thinking of pressure then, oops, regardless it doesn't change the fact that average kinetic energy of the water molecules is the main driver of solubility

It is also the average kinetic energy of the water molecules that is a main driver of volume as well, so the concepts are not unrelated.

You have good answers above, but one thing I didn't see is a mention that how much something will dissolve in hot water and how fast it dissolves in hot water are two different things. They usually go together, but not always.

Most, but not all, things will dissolve more in hot water than cold. It's tough to explain why that is in li5, but has to do with the universe liking it when things can move around more (we call this higher entropy), and that's what happens when most things dissolve in water- they get to float around and mix. When things are hotter the universe apparently likes it even more when things can move around more, so more stuff dissolves. There are some things that dissolve even less in hot water because the water has to organize or be shaped a certain way to get the thing to dissolve (the water is inconvenienced and can't move around as much), but that doesn't happen too often. I don't know why the universe likes it when things move around more, and likes it even more at higher temperature, but that's what a math equation called the Gibbs Free Energy equation tells us, and it seems to work.

But with washing hands what may be more important is that anything that's going to happen in water will happen faster when it's hot. I think this is always true- apparently water even freezes faster when it's warmer, as long as it's still cold enough to freeze. As others have explained above this is because the water (is "water molecules" acceptable for ELI5?) is moving around faster when it's hot, and a hotter temperature is very much like having a fast forward setting on, and cold water like being in slow motion. Your hands would probably wash ok in cold water, but it'd take longer (unless youre using one of those soaps that's made to work best in colder or room temperature water).

Another effect is that hot water is a little bit thinner and sticks to itself less than cold water, but that's probably not the main reason it cleans better. It should help dirt and germs get "wet" and start to loosen up a little bit faster though.

Gas molecules are light and fast molecules that can easily be bounced away and leave the liquid altogether. At low temperatures molecule speed is lower, so the gas molecules are less prone to do that.

Temperature effects of dissolving sugar (and table salt) are very small.

this seems wrong, i feel like without hot water, i am smearing skin grease and oils around, not a lot but like, what your hands would generate in a few hours, and I assume germs can become infused with such congregations of detritus and with using cold water only, my hands feel waxy faster because they heat up to normalize after washing along with taking longer and probably not being as completely as effective as possible.

It doesn't have to be super hot water either, as long as it is above 80 degrees or so it becomes even more useful, below that and you might as well just suds up and wipe dry with a paper towel because you're not gonna convince your skin to let go of that microbial film.

Depends on how much sugar in how much water. But not a lot.

That’s what the soap is for. I guess that just rinsing in warm would be better than just rinsing in cold, but neither is particularly effective.

From the CDC: “Is it better to use warm water or cold water? Use your preferred water temperature – cold or warm – to wash your hands. Warm and cold water remove the same number of germs from your hands. The water helps create soap lather that removes germs from your skin when you wash your hands. Water itself does not usually kill germs; to kill germs, water would need to be hot enough to scald your hands.”

https://www.cdc.gov/handwashing/faqs.html#:~:text=Is%20it%20better%20to%20use,when%20you%20wash%20your%20hands.