Why does hot water clean dishes better than cold?

[bunny]

My niece asked me - I confidently began answering, then realised i didnt really have a clue.

[Splendour]

Actually heat kills germs bunny. Utensils in particular go into our mouths and the typical human mouth has 30+ bacterias in it plus cold germs, etc.

A dishwasher is particularly good because the intensity of the heat will kill more germs i.e. sterilization occurs.

[Borodog]

I doubt that a dishwasher is hot enough to kill many bacteria. It's not like it's an autoclave.

Higher internal energy means that it is easier for the water molecules to penetrate food particles on the dishes and break them up. That's my guess, anyway.

[pzhon]

My guess is that the primary reason is that substances dissolve faster at higher temperatures. It probably helps that the surface tension of hot water is lower than the surface tension of cold water. Water from condensation can also have a significant cleaning effect in a dishwasher with an ineffective spray, and there is a lot more condensation when the water is hot.

[Cueballmania]

Solubility generally goes up as temperature rises. Water is polar and dissolves polar molecules. Soap is part polar part non-polar so it dissolves polar grease and washes away with water.

[Daddys_Visa]

I suspect it has a lot to do with dissolving the food particles and residue. Much of the grime on plates is water soluble, and as you know, things dissolve quicker in warmer water. Also most of the greasy residue will liquify at the higher temperatures, making it easier to rinse off.

[vhawk01]

[ QUOTE ]
I suspect it has a lot to do with dissolving the food particles and residue. Much of the grime on plates is water soluble, and as you know, things dissolve quicker in warmer water. Also most of the greasy residue will liquify at the higher temperatures, making it easier to rinse off.

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Most of the grime on plates is NOT water soluble. Most of it is fat and grease.

[hexag1]

ill take a stab:
The kinetic energy of all those hot water molecules is going to dislodge many more dirt particles off of the surface of dishes, when compared with the effect of cold water molecules.
Come to think of it, isn't temperature a measure of the root mean square momentum of the particles?

[furyshade]

[ QUOTE ]
ill take a stab:
The kinetic energy of all those hot water molecules is going to dislodge many more dirt particles off of the surface of dishes, when compared with the effect of cold water molecules.
Come to think of it, isn't temperature a measure of the root mean square momentum of the particles?

[/ QUOTE ]

well, all those fats were probably hot at one point and liquid/gelatinous. if you reheat them they will probably go from the solid state to a more gelatinous one and be easier to get rid of

[bunny]

Thanks all. I figured the higher energy thing had to be part of it, but wondered if there was a chemical component as well. (I speculated that maybe water and detergent mix better at higher temperatures based on brief experimentation).

[madnak]

The faster molecules bounce around, the more room there is for mixing, the less likely they are to clump together, the more force they have when they collide (and the more likely they are to knock one another loose), etc. And by adding energy to the system, you can make bonds break and increase the likelihood of reactions that might help the cleaning process.

[Nielsio]

Think about this: when your food is hot and moist after it is cooked, it doesn't stick to your plate amirite? So it cools, dries and it sticks. How could we possibly get it off? I know: hot water. BAM.

[furyshade]

[ QUOTE ]
Think about this: when your food is hot and moist after it is cooked, it doesn't stick to your plate amirite? So it cools, dries and it sticks. How could we possibly get it off? I know: hot water. BAM.

[/ QUOTE ]

you dun stole my word-thoughts

[Lestat]

[ QUOTE ]
Actually heat kills germs bunny. Utensils in particular go into our mouths and the typical human mouth has 30+ bacterias in it plus cold germs, etc.

A dishwasher is particularly good because the intensity of the heat will kill more germs i.e. sterilization occurs.

[/ QUOTE ]

Heat kills germs AND dissolves food substances better than cold or moderate temperatures.

[tshort]

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That's my guess, anyway.

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Borodog,

You are going to consult some colleagues on this, right? SMP needs a definitive answer!

[Neuge]

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That's my guess, anyway.

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Borodog,

You are going to consult some colleagues on this, right? SMP needs a definitive answer!

[/ QUOTE ]
Believe it or not, SMP has a bigger authority on this than Borodog. /images/graemlins/smile.gif (not denigrating Boro, it's just that chemical engineers, in general, can answer this type of question better than astrophysicists)

There are 3 major reasons hot water works better than cold water in this situation, and all of them have been mentioned in this thread.

1. The biggest factor is that hotter water allows higher concentrations of detergent. That allows more surface interactions between the solid food, detergent and hot water.

2. Hotter water allows more water penetration into the dried food-stuffs on the plate (which again facilitates the surface interactions).

3. Diffusion rates are higher in hotter water. This means detergent molecules that do dissolve a lipid molecule from the dish surface are more easily transported to the bulk liquid.

[Jetboy2]

Hi,

Try a simple experiment with your niece. It'll be fun, and she'll love it. And you didn't say the age of your niece, but I'm assuming pretty young, considering the question.

Take 4 glass containers. Put hot tap water in 2 of them and cold tap water in the other 2.

Now for the other ingredients. 2 equal pats of butter/margarine and 2 teaspoons of sugar.

So, put 1 pat of the butter into one of the "hot water" containers and 1 teaspoon of sugar into the other "hot water" container. Likewise, put the same into the "cold water" containers. See what happens.

(You could also add 2 more containers where you add detergent to both the hot and cold water.)

Anyway, little kids usually don't really care about explanations of molecular energy levels and whatnot. And I think that if you do this experiment with her; she'll no doubt, figure out the idea of why hot water works better for washing dishes.

The folks here can surely provide the physics/chemistry of how it works....

jb

[Borodog]

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[ QUOTE ]
[ QUOTE ]
That's my guess, anyway.

[/ QUOTE ]

Borodog,

You are going to consult some colleagues on this, right? SMP needs a definitive answer!

[/ QUOTE ]
Believe it or not, SMP has a bigger authority on this than Borodog. /images/graemlins/smile.gif (not denigrating Boro, it's just that chemical engineers, in general, can answer this type of question better than astrophysicists)

[/ QUOTE ]

Abso-freaking-lutely.

There's a reason why I went into astrophysics and not dish soap.

[Jetboy2]

You guys are so very smart, yet failed to help the original poster with the question.

You did manage to kiss each other's ass though....

[Jetboy2]

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Heat kills germs AND dissolves food substances better than cold or moderate temperatures.

[/ QUOTE ]

This is partially correct. Enough heat kills germs. Less heat can actually "grow" germs!

"Heat" is relative.

[Lestat]

But isn't a big part of it that heat dissolves better than cold? Germs aside, if you want to rinse off a plate (even before you put it into a dishwasher), you're better off using hot water, rather than cold. That's the only point I was trying to make.