Eli5 Why does it matter what temperature the water is when we do laundry?

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I’ve been a “throw everything in and wash on cold” person forever, but I started to notice certain things such as towels that typically require hot or warm water didn’t feel right anymore. Once I switched back they came out feeling soft and fluffy as usual. Why does it matter?

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22 Answers

Anonymous 0 Comments

Different fabrics like different things, that’s about it. Towels usually hold onto more bacteria, odor, etc. So hot water works better to clean them. A cotton T-shirt doesn’t benefit much from hot water, and might even shrink form it, so cold is fine to use.

Anonymous 0 Comments

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Anonymous 0 Comments

And a lot of detergents are biological ie have enzymes in them which require a certain temperature to work. Too cold or too hot makes the enzymes inactive

Anonymous 0 Comments

I use hot for sanitizing and killing bacteria, and cold for regular washes. Also you should periodically wash hot to help keep the inside of your washing machine clean, apparently.

Anonymous 0 Comments

Try to dissolve sugar in ice cold water or clean an oily pan with ice water, it will be difficult. Try the same with warm/hot water and things go much easier. How well soaps works is related to temperature. Too cold and they don’t work as well. Modern detergents though are typically happy with warm water, doesn’t need to be scalding hot. Modern washing machines often mix hot and cold to reach a desired temp, that may mean mixing some hot with cold even if you choose cold or mixing some cold with hot if you choose hot.

Anonymous 0 Comments

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Anonymous 0 Comments

Separating dirt from clothing requires energy. You either get it from chemicals (detergent), mechanical ( agitating), and/or heat (water temperature).

Ideally you have all 3. There’s not really a scenario where cold water cleans better than warm water, it’s just a marketing thing. If you do the math out on the cost to heat water for laundry…it’s basically trivial (on the scale of a few dollars a year). But “buy our more expensive specialty cold water detergent to save money on water heating!”

Anonymous 0 Comments

Some clothes are made of fabric that don’t like hot water. It’s kinda like play dough, if you leave it out it gets all dry and hard and crusty. If you wash some clothes in hot water they get all shriveled and messed up. Sometimes they even melt! Now it normally isn’t a big deal if a little gets on them (like how a little dry play dough isn’t a big deal), but if it’s washed in it you can’t use it anymore.

Anonymous 0 Comments

A machine washer salesperson told me that your sweat attaches to the clothes at your body temperature (your clothes are a bit cooler than your internal body temp, but they’d get fairly close).

Like imagine your armpits, they’d be close to the ~37-38 degrees C of your body.

So, they argued that you want to at least match the temperature at which those stains were set in. So a wash at 40 degrees C makes some intutive sense.

[This is not quite an explanation, like *why* do you need to match the temperature that the stain was set? Is that true? What is the chemistry of that? But it does at least feel like it makes sense.]

Aside from that, generally, most things dissolve more easily at high temperatures (gasses don’t, but gasses are not attached to your clothes).

So if there is any grease or oil or stain, most of the time warm water will dissolve it better. It may also dissolve the detergent better.

Anonymous 0 Comments

Temperature causes molecules (the tiny bits that make up everything) to move faster. The higher the temperature, the faster they move. This helps them to dislodge anything water soluble (ie: can dissolve in water) in the clothes.

Oils are not water soluble. This is where detergent comes in. It acts like a hook, pulling the oil out from the clothes. The faster the water molecules agitate – ie: the higher the temperature – the faster that works.

Many detergents are biological now days. They work via enzymes, which act kind of similar to detergents (this is a massive oversimplification). Enzyme-based detergents don’t need as high temperatures to be effective, but they do need some temperature. Between 30-40 degrees celsius is optimal for them usually. Much higher and the enzymes are denatured – ie: destroyed – and lower they don’t really work.

Result: A cold wash isn’t going to reliably wash your clothes properly. With modern bio detergent, 30 degrees Celsius should generally be fine, 40 would be more inefficient but more effective sometimes, and above that you don’t want bio detergent.

Very high temperatures can also be used to sterilize clothes. Not really relevant to most people’s situation, but a 90 degree Celsius wash will kill most germs. It will also destroy the enzymes in biological detergent, so you’d need to use non-bio detergent, and it’s really energy inefficient. If you don’t specifically need sterilized clothes (eg: from working with bodily fluids in a hospital), then it’s really not necessary.