What type of liquid does pH paper test for?

pH paper is specifically designed to test the acidity or alkalinity of water-based liquids. This ability arises because pH paper contains indicators that react with hydrogen ions in solution. Water, being a universal solvent, also helps to dissolve and distribute these ions, allowing for an accurate measurement of pH levels within these types of liquids.

In contrast, hydrocarbon solvents, oils, and fats do not dissolve water-soluble indicators effectively, making them unsuitable for pH measurement. Airborne particles do not have any inherent pH value that can be tested using pH paper since it does not involve a liquid medium where pH can be quantified. Therefore, water-based liquids are the correct focus for pH paper testing, confirming the validity of the chosen answer.