How can the presence of lipids be tested using brown paper?

The presence of lipids can be tested using brown paper through the formation of a translucent spot on the paper. When a sample containing lipids is placed on brown paper, the lipids penetrate the paper fibers, causing the area of the paper where the lipid is present to become translucent or semi-transparent. This characteristic change indicates the presence of fats or oils, as they have a unique ability to leave a greasy mark due to their hydrophobic nature.

The test relies on the interaction between the lipids and the cellulose fibers in the paper. Unlike other substances, lipids do not evaporate quickly, leaving a persistent, visible mark on the paper. This method serves as a simple and effective qualitative test for lipids in various samples, such as food products.

The other potential outcomes, such as the paper turning yellow or changing to blue, do not relate to lipid testing. The changes in color are more commonly associated with different types of chemical reactions or tests unrelated to lipids. The tearing of the paper does not provide any relevant information regarding the presence of lipids. Thus, becoming translucent is the clear indicator in this context.