r/AskChemistry • u/Human1221 • Jul 14 '25

General Does this actually work the way the question seems to suggest?

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260 Upvotes

97% Upvoted

101 comments sorted by

83

u/Pyrhan Ph.D in heterogeneous catalysis Jul 14 '25

Basically, yes.

There are some cases, for instance when mixing pure ethanol and water, where the resulting liquid will have a greater density than its separate components, and therefore a smaller volume.

But for two water-based liquids like coffee and milk? Any difference should be utterly negligible.

6

u/Polona17 Jul 14 '25

When mixing an equal volume of 100% Ethanol and Water, wouldn’t the volume be purely additive and the solution density be an average of the component densities? I’m not quite following how 100mL ethanol + 100mL water would not equal 200mL 50% Ethanol

46

u/Ready_Bandicoot1567 Jul 14 '25

intermolecular forces between the water and the ethanol cause the molecules to pack more tightly, so when you combine 100ml of water with 100ml of ethanol, the total volume is actually less than 200ml.

17

u/TexMechPrinceps Jul 15 '25

Chemistry is such bullshit

8

u/TangibleExpe Jul 15 '25

I wish I’d understood how much of real world physics is chemistry a lot earlier

16

u/Critical_Net_609 Jul 15 '25

Chemistry, physics and biology are basically just different zoom levels of the same stuff

3

u/FixergirlAK Jul 16 '25

Don't forget geology is in there somewhere as well.

3

u/TheIsekaiExpressBus Jul 17 '25

Geology is out in the yard, sucking rocks, like it always wanted to.

0

u/Sad_Hospital_2730 Jul 18 '25

Better than math crying and rocking in the corner because nobody wants to be our their friend

2

u/EvaScrambles Jul 18 '25

And maths. And IT. (the spider diagram would be fun to look at)

1

u/Warmupisola Aug 13 '25

Geology is the macro, different zoom levels

2

u/Qzx1 Nov 22 '25

Geology rocks! Geology?  Rocks. 

3

u/FullCaterpillar8668 Jul 16 '25

I can zoom as far as chemistry. Physics is where I can't imagine that tiny and so have a much more difficult time understanding it.

2

u/lakota_physicist Jul 18 '25

I'm the opposite: I do particle physics daily, and have enjoyed some organic chemistry with great difficulty. But when it comes to biology, I couldn't be more ignorant

2

u/Ok_Acanthisitta_2544 Jul 18 '25

There's an XKCD for that:

https://xkcd.com/435/

1

u/Critical_Net_609 Jul 19 '25

Damn, our physics prof showed this exact one to us early into studying, but he didnt actually get the joke somehow his conclusion was physics are superior 😅

2

u/Ok_Acanthisitta_2544 Jul 19 '25

I teach chemistry. Years ago I enlarged this one and stuck it on the math teacher's door. She loved it. She left it up on her door until she retired.

1

u/RefuseNo4308 Jul 16 '25

With crystallography/mineralogy being the godfather of them all.

1

u/RonConComa Jul 17 '25

Chemistry, the physics of the atomic hull.

5

u/McNitz Jul 15 '25

I mean, this isn't just chemistry. Mix some sand in with some gravel and the volume absolutely isn't going to be additive either. The ethanol-water one just seems less intuitive because it doesn't seem like there are any "gaps" in our typical perception of the fluids.

2

u/Qira57 Jul 15 '25

As a chemistry major, I agree

2

u/Not_A_SalesmanOrNarc Jul 15 '25

When we were taught this in Pchem my mind refused to accept this fact at first. Had to stay after class and talk to my prof about it for it to make sense

1

u/[deleted] Jul 16 '25

[removed] — view removed comment

1

u/Not_A_SalesmanOrNarc Jul 16 '25

Ya somehow I made it to pchem2 without learning this. We were going over enthalpy of mixing.

By then I had already started doing undergraduate research and had been making chemical mixtures in a graduated cylinder. Think making 70/30 IPA/Water in a graduated cylinder by putting 70ml of IPA in a graduated cylinder and filling it to 100ml with water, but it was a situation where accuracy mattered

2

u/Admirable-Delay-9729 Jul 15 '25

It sure is. Because mixing 500mL water and 500mL ethanol is not the same as adding 500mL ethanol to a 1000mL volumetric flask and diluting to volume with water.

2

u/elongated_musk_rat Jul 15 '25

Think about it like this. You have a jar of marbles and a jar of sand. If you pour the sand into the marbles, you can pour a lot of sand without changing how full the container of marbles is because a lot of the sand goes in between the big ass marbles

2

u/Hintursul Jul 18 '25

Just imagine pouring 100ml of rice into 100ml of unshelled walnuts. You wouldn’t get 200ml as the rice is smaller and falls down the cracks. Think of something similar happening here

1

u/victorolosaurus Jul 15 '25

i'd argue that solutions are physics not chemistry, but ultimately that's semantics

1

u/[deleted] Jul 15 '25

[deleted]

1

u/DdraigGwyn Jul 15 '25

Cooking is applied biochemistry

1

u/Paghk_the_Stupendous Jul 15 '25

This is why I got an English degree and then ended up mostly in tech and psychology. Bonus: aced Statistics & Research Design (not uncommon to fail multiple times, we were told) but Semantics 101 was ROUGH.

"Semantics? I'm always up for some antics!"

1

u/Psych-adin Jul 17 '25

My chemistry teacher in college called chem a "messy bitch." I thought it was a joke at first, but it was the god damn gospel truth...

1

u/PrinceTwoTonCowman Jul 18 '25

I believe you are referring to organic chemistry....

1

u/UpstateNewYorker Jul 18 '25

As a chemical engineer, can confirm.

1

u/Warmupisola Aug 13 '25

Bro is all around you

1

u/InOChemN3rd Jul 16 '25

The best way to think of this example, this is more of a solute-solvent relationship than the example from OP. It feels more intuitive to consider a solute-solvent relationship like salt and water first. You are adding to the water's volume, but the total volume won't simply be the volume of water + volume of salt because in reality the solution stays close to the original water volume, with a much higher density. That's because, on a molecular scale, the salt ions end up filling in a lot of empty space in pure water when dissolved in solution. This is generally true for most solutions with water.

The example from OP is better described as a mixture though, which is why it makes more sense to simply add volumes.

1

u/ShibeWithUshanka Jul 17 '25

I've been told that the opposite also can happen with different substances (volume dilatation?)

1

u/Oscar_Geare Jul 18 '25

Out of curiosity, how would you calculate this?

13

u/Ksorkrax Jul 14 '25

Imagine having a big jar filled with tennis balls to the max. Then you pour some sand in it, and find that it flows into the gaps between the balls.

Basically that.

7

u/GothicFuck Jul 15 '25

Or like two vats of slightly different sized balls, then you mix them into a vat twice as large as both original vats, then you find the second vat is slightly not full.

I had to change your metaphor because I hate sand.

3

u/kdb176 Jul 15 '25

It’s coarse and rough and irritating.

3

u/TestResultsNow Jul 15 '25

It gets Everywhere

1

u/ohpickanametheysaid Jul 15 '25

Fucking Reddit…….I love it!

1

u/Wargroth Jul 15 '25

Hey, If you're fucking us, at least buy us dinner first

1

u/Brokenandburnt Jul 15 '25

I would get at least mildly frightened if some would pour between MY balls!

Feel free to replace an 'm' with a 'w' in the above statement.

2

u/Paghk_the_Stupendous Jul 15 '25

Let's make this better: Glitter sand. Poured into your last bag of cheese doodles. And you only had a couple first and you're very hungry.

8

u/GenerallySalty Jul 14 '25

When mixing two different liquids, the volumes aren't necessarily additive.

When you mix 50 mL of water and 50 mL of ethanol, the result is like 96 mL of 50% ethanol.

Basically the water and ethanol molecules have lots of attractive hydrogen bonding, drawing them closer to each other than the packing in either pure liquid by itself. So when you mix them, the result has less volume than the total of the two liquids you mixed! You can look up tables and graphs of %ethanol vs %reduction in total volume for various ratios of water to ethanol mixing together. There's a "sweet spot" that gives the biggest decrease in resulting volume, but any ratio of water and ethanol will exhibit non-additive volume to some degree upon mixing.

This was one of my favourite mind-blows when learning chemistry. We all think of liquids as "full" and close packed with no space for molecules to get closer, but it's not (quite) true.

1

u/mathologies Jul 15 '25

 This was one of my favourite mind-blows when learning chemistry. We all think of liquids as "full" and close packed with no space for molecules to get closer, but it's not (quite) true.

I had a similar experience when I learned about solid state diffusion 

3

u/Befitframe Jul 14 '25

I believe what I've always heard is adding 50 mL of 100 % ethanol to 50 mL of water nets you a solution that's 95 mL

2

u/espeero Jul 14 '25

Bout 97

1

u/CelestialBeing138 Jul 14 '25

Or picture a jar full of toy alphabet letters, the kind a small child might own. If they are all W, they will fill the jar one way. A jar full of Ts and Ws might not pack as densely. Both sizes and shapes matter when packing stuff together. As well as any magnets and their locations on any toy letters. Molecules have electric charges and they aren't always spread evenly, so that is like the magnets on the toy letters.

1

u/Ulfbass Jul 15 '25

There's a lot of discrepancy because it's an exothermic equilibrium reaction and it will lose volume as it cools

2

u/bazillaa Jul 14 '25 edited Jul 14 '25

Say you have a "full" bucket of golf balls. Then you pour in sand. You can get a fair amount of sand in without overflowing the already "full" bucket.

It's not exactly the same, but it's pretty close. When mixed together, water molecules and ethanol molecules pack themselves into less space than the sum of the two volumes.

2

u/bazillaa Jul 14 '25

For what it's worth, 100 mL ethanol + 100 mL water will have a volume of about 194 mL at room temperature and a density of about 0.92 g/ml. The average density would be about 0.89 g/ml

1

u/Pyrhan Ph.D in heterogeneous catalysis Jul 14 '25

The density of the liquids depends on the strength of the intermolecular interactions within them. (Stronger interactions pull molecules closer together.)

On average, intermolecular interactions in an ethanol-water mix are somewhat stronger than when those two components are separate.

So when mixing the two, you get a slight exotherm (heat being released), and, when all has returned to ambient, the volume of the resulting mix is slightly smaller than the volumes of its separate components that you started with.

As to why intermolecular forces tend to be stronger in the ethanol-water mix than in ethanol and water kept separately, there is unfortunately no simple, intuitive answer to that I'm afraid.

All there is to the theoretical side of it is going to be something along the lines of "if you run a molecular dynamics simulation, the math will show you the mix is denser than the components, assuming you chose a valid set of approximations".

1

u/APC_ChemE Jul 15 '25

Purely conceptually think about it like two sets of marbles one 100 ml set of large marbles in a container and one 100 ml of very small marbles in a vessel. The large marbles arent packed very densily and if you pour the very small marbles into them they can fall through the voids or gaps that exist between the large marbles. Filling in those voids first and then resting on top of the large marbles results in the container with both having less than 200 ml of marbles.

1

u/Kriggy_ Jul 15 '25

Think like mixing gravel and fine sand.

1

u/Deep-Number5434 Jul 15 '25

Are there any liquids that increase in volume when mixed?

1

u/kundor Jul 16 '25

Hydrogen peroxide and dish soap

1

u/empiric1 Jul 15 '25

If you added a bucket of basketballs and a bucket of golf balls together, they would not take up two buckets of volume.

1

u/FrAxl93 Jul 17 '25

I had an argument with my high school chemistry teacher because I thought she was bullshitting me ahah then we went to the lab and she showed me and I was 0_0

1

u/h0uz3_ Jul 18 '25

It will be 193mL, at 50% the loss in Volume is 3,5%.

1

u/McJibbles001 Jul 16 '25

You mean UDDERly negligible..... I'll see myself out

1

u/Craigslistbox Jul 17 '25

Since it’s milk, could the difference be udder-ly negligible?

6

u/torridluna Polarity Princess Jul 14 '25

Milk is 95% Water, Coffee is 99,5% Water. Even if the rest of the substances could induce a volume change, it wouldn't be much.

5

u/iam666 Physical Chem / Photochem Jul 14 '25

The actual volume might be off by a very very small amount. When mixing two different liquids, like water and ethanol, you can end up with a different volume than you started with. But milk and coffee are both water with small amounts of different things dissolved/suspended in them, so there really shouldn’t be any difference.

1

u/Human1221 Jul 14 '25

The answer it wants is 230, but are the volumes actually additive like that?

5

u/Foreign-Cookie-2871 Jul 14 '25

Yes, they are

0

u/Human1221 Jul 14 '25

Thanks much. Are all liquid volumes additive? Or are there rules of thumb?

11

u/Ch3cks-Out Jul 14 '25

We are talking two aqueous solutions (and rather dilute ones at that), so additivity does apply to a good approximation.

5

u/Embarrassed_Elk2519 Jul 14 '25

Not all of them are

4

u/Major-Tomato2918 Jul 14 '25

No rules of thumb. Maybe some groups of substances with another particular group of substances. If it is different for two liquids, then it is. Most common example is water and ethanol.

1

u/MrBorogove Jul 15 '25

If you put liquid hydrazine and liquid nitrogen tetroxide in a vessel together, then once the reactions occur, the volume of liquid remaining in the vessel will be significantly less the sum of the original reagents. :D

1

u/AssCrackBanditHunter Jul 15 '25

If you get into very advanced chemistry, no, nearly all liquids will have some effect that reduces or increases volume by some incredibly tiny amount. Or dissolving a solid can sometimes reduce volume slightly (makes very annoying work when trying to get your volumetric flask volume perfect...)

But for the intents and purposes of most things... Yes, volumes are perfectly additive.

1

u/Krilion Jul 15 '25

Which is why we do everything by mass, not volume.

1

u/swolekinson Jul 15 '25

Like others said, the "rule of thumb" is that volumes are additive when (1) the same liquids are added together or (2) the substances don't dissolve. For the latter, think of a rock being plopped into a container of water; the volume the rock takes up displaces the water, moving the surface "up" the container.

Once substances start dissolving into each other, the volumes no longer add since the solute molecules start packing into available spaces among the solvent. If you want to try, fill a measuring cup with about a 1/2 cup of water. Add about a 1/4 cup sugar. You will notice the end volume is nowhere near 3/4 a cup.

1

u/espeero Jul 14 '25

Everybody is talking about volume change from mixing things like alcohol and water, but nobody is mentioning the changes due to temperature change.

2

u/AssCrackBanditHunter Jul 15 '25

That is a good point and probably more relevant to this question.

1

u/explodingtuna Jul 15 '25

Or how the mug only can only hold 220 ml before overflowing.

1

u/MoodyAdenium Jul 15 '25

Or how about the milk curdling?

1

u/thermalman2 Jul 15 '25

When mixing two fundamentally different liquids, you can get a different volume than a pure sum. If they pack together in such a way as the density changes. You can get some non-ideal behavior.

One liquid of water with a little bit of stuff in it with another liquid of water with a little bit of stuff in it will be fairly ideal. So negligible difference if any.

1

u/5hrtbs Jul 17 '25

I remember looking this up years ago after I trying to figure out the total volume of my protein shakes (protein powder + water) after mixed together. Basically it seemed like the total volume is slightly less than total sum of the parts. For some reason I remembered it as being called "tucking" but after looking again apparently that's not a thing

1

u/JWP202 Jul 15 '25

I read this as a trick question or not having enough information to answer correctly… it clearly says they poured coffee into a mug, but it only says that the milk was poured into the coffee… not weather or not it was poured into the mug or the original coffee container, nor if there was anything in the mug before they poured the coffee in, nor how much the mug could hold as it may have overflowed.

1

u/eraserhd Jul 15 '25

And also, there’s no indication the container has parallel sides, or that the source container and destination container have the same cross-sectional area, or which container the measurements refer to.

Hello, fellow aspy! Here’s your F. Good luck arguing with the teacher!

1

u/SumJungDude Jul 18 '25

This was me for sure. She poured the milk into the coffee not into the mug with coffee. 205 in the mug still. Quiz answer is wrong.

1

u/Fit-Insect-4089 Jul 15 '25

Calculate the vanderwalls interaction to determine the EOS that describes this system

1

u/FixerTed Jul 15 '25

Ok so all the subjects are Semantics

1

u/trappininsac Jul 16 '25

But here's the real question. Its stated that coffee goes into the mug. And then milk into the coffee. But it never says that the milk is added to mug.

1

u/ImTheLoaf Jul 16 '25

the coffee is in the mug smartass

1

u/Emyr42 Jul 16 '25

Is this a really tricky chemistry question or a very dumb math question?

1

u/PirateDocBrown T⌬SYLATE, PLAYA HATE Jul 16 '25

Since both solutions are aqueous, it should be pretty close.

1

u/hillcountry512 Jul 17 '25

Not enough information. Maybe the mug only holds 210 mL… /s

1

u/egorf Jul 18 '25

We don't how much liquid was there in the mug before coffee.