# Surface Tension - Using hot soapy water to wash clothes

• brotherbobby
In summary, the concept of surface tension explains why hot, soapy water is used for washing. To thoroughly wash clothing, water must be able to penetrate the small spaces between the fibers, but this is hindered by surface tension. This is why increasing the surface area of the water, through decreasing surface tension with heat and soap, helps with the washing process. Additionally, a sphere has the maximum surface area for a given volume, making it an effective shape for increasing surface area in this scenario.
brotherbobby

I mention the details in the book (verbatim) in the form of a paragraph in green below. Later I ask my questions in blue font for better reading.

"Surface tension also explains why hot, soapy water is used for washing. To wash clothing thoroughly, water must be forced through the tiny spaces between the fibers (Fig. 12.16). To do so requires increasing the surface area of the water, which is difficult to achieve because of surface tension. The job is made easier by increasing the temperature of the water and adding soap, both of which decrease the surface tension." (University Physics, 13th Edition)

1. Let's begin with the diagram. I know that the excess pressure within a (spherical) water molecule with a single surface is ##p=\frac{2T}{R}##. Assuming hot water and soap bring about a reduction in T, it will reduce p. In which direction is water (with soap) supposed to travel in the diagram? Shouldn't air and water be on one side and the clothing and its inner fibers with dirt on the other?
2. Let's see the text above in green from the book. To force water through the fiber spaces in the clothing, it says the surface area of water needs to be increased! Increased? Shouldn't the surface area of water be decreased so that the water, with the same volume, can move through the clothing fiber spaces?
I present my own explanation, highlighting crucial points in red.

In order to force the soapy water through the fiber spacings, their surface area need to be reduced. A sphere is known to have the maximum surface area for a given volume. A decrease in the surface tension of water using soap and heat effects a decline in its surface area with the same volume of water. This reduced size of the soap water helps it make its way into the clothing. (Please note that nowhere in my explanation have I used the concept of excess pressure).

A help would be welcome.

Some other terms are wettability, surfactants, hydrophilic, hydrophobic surfaces and molecules if you want to explore further.

brotherbobby said:
Let's begin with the diagram. I know that the excess pressure within a (spherical) water molecule with a single surface is p=2TR. Assuming hot water and soap bring about a reduction in T, it will reduce p. In which direction is water (with soap) supposed to travel in the diagram? Shouldn't air and water be on one side and the clothing and its inner fibers with dirt on the other?
The fabric, say a shirt, is usually dry when added to the wash. Being dry, air is surrounding all the fibres, which instead of the one layer shown would be of several layers, as threads are made up of long fibres twisted together, and weaved together to make the cloth. If soiled after wearing, the fabric can contain dirt and other extraneous particles within and around the fibres. To flush out these particles, water has to be able to penetrate into the small spaces between the fibres, enclose the particle and move it away from the fabric. Surface tension of the water hinders this process. the water may just "bead" on the surface and not penetrate into the "pores", and displace the air.

brotherbobby said:
Let's see the text above in green from the book. To force water through the fiber spaces in the clothing, it says the surface area of water needs to be increased!
...
A sphere is known to have the maximum surface area for a given volume

Why do you say that? ie a sphere has the maximum surface area per volume.
Compare for the same volume , the ratio of surface area / volume for a sphere, cube, or any other shape.

## 1. What is surface tension?

Surface tension is the force that causes the molecules on the surface of a liquid to stick together. It is what allows insects to walk on water and creates a "skin" on the surface of a liquid.

## 2. How does hot soapy water affect surface tension?

Hot soapy water reduces the surface tension of water by breaking the bonds between water molecules. This allows the soap to penetrate and lift dirt and stains from the fabric more easily.

## 3. Why is surface tension important in washing clothes?

Surface tension is important in washing clothes because it helps the soap and water to spread evenly on the fabric, allowing for better cleaning. It also helps to prevent the dirt and stains from reattaching to the fabric during the washing process.

## 4. Can using hot soapy water damage clothes?

Using hot soapy water can potentially damage delicate fabrics or cause colors to fade. It is important to check the care instructions on clothing labels and use the appropriate water temperature and detergent for the fabric.

## 5. Is hot soapy water the only way to remove stains from clothes?

No, there are other methods for removing stains from clothes such as using stain removers, pre-treating with laundry detergent, or using natural remedies like vinegar or lemon juice. However, hot soapy water is an effective and commonly used method for removing stains from clothes.

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