Why does washing some clothes in hot water make them shrink?

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Why does washing some clothes in hot water make them shrink?

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When the clothing is being made, everything is stretched out, mostly via heat and pressure. Applying heat without the pressure makes them loosen up again, which unstretches the clothing, making it shrink

The long organic molecule chains that make up cottons and other fabric textiles form long thin strands of cellulose fibers. These fibers are also hydrophilic which means they love to attract and hold onto water. When you get them wet in the washer, this lubricates between the fibers and so they’ll tend to pack together more densly. Heat also relaxes these molecules to the fibers will get more bendy for a while and they compact together more easily. Also the mechanical tumbling combined with the water and the heat also physically compresses the fibers a bit. Add all of it up and the fibers making up your clothes are just now physically more dense, the material literally got smaller.

Just hang your woolens and shrinkables up to air and drip dry.

What shrinks the most is stuff like sheep wool. That’s because those are long protein molecules which get destroyed by heat as protein has a tendency to do so above about 40° Celsius. They loose their 3D alignment which isn’t reversible and therefore shrink, never to be the same again.
That’s probably the only thing I learned in chemistry class 😂