Turmeric is a yellow-colored spice commonly used in cooking, and it is also known for its medicinal properties. When turmeric stains occur on fabrics or surfaces, they can be quite stubborn to remove. However, when washed with soap, the turmeric stains typically turn into a reddish color.

The reason behind this color change lies in the chemical composition of turmeric. Turmeric contains a compound called curcumin, which is responsible for its vibrant yellow color. Curcumin is a natural dye that has a tendency to bind with fabrics and surfaces, causing stains.

When soap is applied to a turmeric stain, it acts as a surfactant. Surfactants are compounds that lower the surface tension between liquids and solids, helping to remove dirt and stains. Soap molecules have a hydrophilic (water-loving) head and a hydrophobic (water-repelling) tail. The hydrophobic tails attach to the oily curcumin molecules, while the hydrophilic heads are attracted to water.

As the soap is rinsed off during the washing process, it carries the curcumin molecules away from the stained area. However, some traces of curcumin may still remain on the surface. When exposed to air, curcumin undergoes oxidation, leading to a color change. This oxidation process causes the turmeric stain to turn from yellow to reddish-brown or even dark red.

This color change occurs due to the chemical reactions of curcumin with oxygen in the air. Oxygen molecules interact with the double bonds present in the curcumin structure, leading to the formation of new compounds. These new compounds have a different chemical structure and absorb light differently, resulting in a change in color.

In conclusion, turmeric stains turn into a reddish color when washed with soap due to the oxidation of curcumin, the yellow pigment present in turmeric. The soap helps to remove the curcumin molecules from the stained area, but some traces may still remain, leading to a color change when exposed to air.