ELI5: Why does hot water take dirt off dishes so much easier than cold water?

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Heating water increases molecular activity. Warm liquids always dissolve things faster than cold. Hot liquids even faster. The agents that bond food to dishes are usually starches, sugars and oils. In hot water starch bonds relax, sugar crystal liquefy and oils move from solids back to liquids. Edit: Not all liquids and not all “things”. I was referring to stuff in the average household setting!


1 Hot stuff have their atoms move around faster. This results in them hitting the dirt harder, resulting in it dissolving faster. 2 For fairly complicated reasons solids dissolve better into hotter liquids (and gas the reverse dissolve better in cold liquids). 3 Additionally when you have certain solids to clean (e.g. fats) they tend to become more liquid like as they are heated up resulting in them not sticking to the surface as hard and are easier to wash away. Theses three factors together result in dirt being taken off much better by hot liquids.


To understand this, we need to talk about molecules. Molecules are the smallest thing that’s still the same thing. You can divide a cup of water in half, and the two halves will still be water. If you cut a water molecule in half, you don’t get two even-smaller waters. Molecules are tiny. Molecules in liquids and some solids tend to slightly stick to each other. Sometimes one molecule will have parts of it that are friendly with one kind of other molecule and a different part that’s friends with a completely different kind. Soap is like that. It’s friends with both water and fat. Let’s say that there’s a plate (ceramic) molecule and their friend, a fat molecule. They’re holding hands but you want to break them up. Should you send them to a ballroom (cold water) or a mosh pit (hot water)? People move a lot faster and more violently in the mosh pit, so that’s the best bet. In the same way, the molecules that make up fat are pulled away by hot water, whose molecules are vibrating faster than cold water’s. Water isn’t very friendly with fat though, so hot water alone doesn’t work great. It works even better if fat’s clingy, moshing friend, the soap molecule, grabs a hold of both the moshers and fat and pulls them away into the crowd.


Follow up question: Have you been eating dirt?


Sinner’s Circle. Heat, detergent, time, and agitation all play a role in cleaning. Adding to any of them increases cleaning effectiveness. The other answers in here talk to the specifics of heat, but it’s an interesting topic to read up on, and that search term should give good information.