Today while washing curtains, my mother dipped them in hot water(containing detergent) for an hour and then washed them.When i asked her that why she done that,her answer was simple(it will make the cleaning easier and effective). It seems to me that there is a scientific phenomena is lying behind it,but i don't know what it is.

The question i m asking is why hot water is more effective than cold water in cleaning (cloths,curtains,bedsheets etc).

I shall be thankful to have your answers.

up vote 3 down vote accepted

Many physical and chemical reactions are affected by temperature. Usually higher temperature causes the reaction to occur faster, and lower temperature will slow it down. Water solubility (the ability of substances to go into solution in water, i.e. bond with water molecules) is a reaction that is affected by temperature for many substances. Changes in temperature may increase or decrease the solubility of a certain substance in water, depending on the substance properties.

Since high temperature will cause water molecules to move around relatively faster, it will also cause the hydrogen bonds between them to form and break at a higher rate (that is why hot water vaporizes more quickly than cold water). In addition, the high temperature increases the water's energy, and this energy may be used to break bonds between dirt and curtains for which colder water will not have enough energy. This means that the detergent/water solution will be able to dissolve (form bonds with) the dirt faster and better, because the bonds between the detergent and water and the bonds between the detergent and dirt will form faster and will be strong enough to prevent the dirt from reforming the bonds with the curtains. As mentioned earlier, these bonds will also break faster, but the presence of a detergent helps to stabilize the solution and keep the dirt dissolved in the water. Once the dirt is dissolved in the water, the curtains are clean, and the dirty water can be easily discarded.

In summary, hot water helps the cleaning process (i.e. moving the dirt from the curtains to the water) because it can dissolve the dirt faster and break strong bonds (up to a limit, of course) of dirt with the curtains.

It is worth mentioning that between the detergent and the hot water, the detergent contributes more to the dissolution of dirt in the water than does hot water alone (that is the same reason it is better to wash your hands with soap and water than with water alone). However, for optimal results, using both detergent and hot water will clean the curtains better (and faster).

Here are some images I found online that may help clarify this point:

Effect of temperature on solubility of different salts Mechanism of action of detergents (surfactants)

Detergent breaks surface tension by disrupting the bond between water molecules. This is because detergent has a polar and non-polar heads on its molecules.
The hydrogen and oxygen molecules in water form hydrogen bonds, which form a type of skin on the surface of the water. This skin is strong enough to support the weight of very light objects. That is why ants seem to walk on the surface of a body of water and why a needle carefully placed on top of the water will float.
Detergent molecules have a polar head on one edge that attracts water and a non-polar head on the other edge that repels water. These opposing heads interfere with the hydrogen bonds in the water molecules and weaken the strength of the skin on the surface, breaking the tension.
enter image description here

I found this table which shows the value of surface tension at different temperatures. Its clear from this table that hot water is more effective when used with detergent.
Source(data table): https://srd.nist.gov/JPCRD/jpcrd231.pdf

  • Most water soluble substances are more soluble at higher temperatures. Hot water will be more effective for getting rid of these types of substances. – Zhe Oct 26 '16 at 15:02

Because the bicarbonates of mg and ca decomposes respectively into their products on adding hot water. Then water can produce effective lather easily

Hot water has faster moving molecules which bombards the impurity which helps to clean.

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