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Why do we use hot water to wash clothes?

The basic answer is because heating causes molecules to move faster.

Temperature is just a measure of how quickly, molecules are moving. When a substance dissolves in water, its components, be they ions or molecules, form stronger attractions to water molecules than to each other and therefore can be pulled apart from each other and become surrounded by water molecules. In other words, the water molecules have to wedge themselves in between the units of the solute to bring it into solution The faster the water molecules are moving, the greater their kinetic energy and the greater the chance that they can blast themselves into the solute. Hot water will therefor remove water soluble stains more quickly. But not all stains are water soluble.

The molecules making up oily stains have a greater affinity for each other than for water. The water molecules cannot wedge themselves into the network of fatty molecules. They are repelled. The oil floats on water and oily stains do not dissolve. So we use detergents. These are molecules where one end has an affinity for oil, the other for water. The oil soluble ends anchor in the stain, the other end is loose in the water. As the water molecules move around, they tug at the detergent which in turn tugs at the fatty molecules in the stain, breaking the stain apart. At higher temperatures all  of this molecular dancing around occurs with greater speed and energy and the stain is removed from the fabric more readily.

In the case of detergents which contain enzymes that are added to break down protein stains, high temperature is not a plus. The enzymes themselves can degrade if the temperature is too high; that’s why enzyme detergents will have instructions to use lukewarm water. 

The increase in solubility with temperature does not always hold. Gases are less soluble at higher temperatures. This is because gas molecules in solution have very weal attraction for the solvent and they are volatile, that is they escape from a liquid easily. Since at a higher temperature they are moving more rapidly, they escape more easily from the surface of the solution. A cold carbonated beverage will dissolve more carbon dioxide than a warm one. Just heat a soda and watch the bubbles evolve. This is also what happens if you let a glass of cold water stand. The dissolved air comes out in the form of bubbles.

There is a limit to the increase of solubility of gases in water as the temperature drops. When the water freezes, the water molecules take on a crytal pattern and squeeze out the molecules of solute. This is why it is dangerous to put a bottle of beer in the freezer. A lot of carbon dioxide has been dissolved under pressure and as the liquid freezes it comes out of solution, possibly causing an explosion when you pick up the bottle. Then you’ll have beer stains all over you. But you know what to do, use hot water to wash out the stain.


@joeschwarcz

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