Introduction:
When a yellow curry stain on a white shirt turns red after being washed with soap, it can be attributed to a chemical reaction between the curry and the soap. This change in color is due to the nature of the curry stains and the components present in both the curry and the soap.

Explanation:
1. Curry Stains:
Curry stains typically contain pigments that give curry its characteristic color. These pigments are organic compounds known as curcuminoids, with the main pigment being curcumin. Curcumin is a yellow pigment that is soluble in oils and fats.

2. Soap and its Components:
Soap is composed of molecules called surfactants. Surfactants have a hydrophilic (water-loving) head and a hydrophobic (water-repelling) tail. The hydrophobic tail tends to dissolve in oils and fats, while the hydrophilic head interacts with water.

3. Chemical Reaction:
When the curry stain comes into contact with soap during the washing process, a chemical reaction occurs. This reaction involves the interaction between the curcuminoids in the curry stain and the surfactants in the soap.

4. Emulsification:
The hydrophobic tail of the surfactant molecules in the soap binds to the curcuminoids in the curry stain, forming an emulsion. An emulsion is a mixture of two immiscible substances, in this case, the curcuminoids and the soap.

5. Dispersal of Pigments:
During the agitation and rinsing process, the emulsion created by the soap helps to disperse the curcuminoids more evenly throughout the fabric. This dispersion causes a change in the way light interacts with the pigments, resulting in a change in color perception.

6. Color Change:
The dispersed curcuminoids, which were initially yellow, can appear red or reddish-brown when more evenly distributed on the fabric. This change in color perception occurs due to factors such as the concentration of curcuminoids, the thickness of the stain, and the interaction of light with the dispersed pigments.

Conclusion:
The change in color from yellow to red when a yellow curry stain on a white shirt is washed with soap is primarily due to the emulsification and dispersion of curcuminoids in the curry stain. The interaction between the curcuminoids and the surfactants in the soap leads to a change in color perception, resulting in the stained area appearing red after washing.