What effect does a small percentage of ammonia have when mixed with oil under reduced pressure?

When a small percentage of ammonia is mixed with oil under reduced pressure, it can lead to the formation of foam. This is primarily due to ammonia's ability to act as a surfactant, which lowers the surface tension between the oil and air. Under reduced pressure, the volatility of ammonia allows it to interact more readily with the oil, potentially stabilizing air bubbles and leading to a foaming action.

Foaming may cause changes in the oil's physical properties, but the primary effect in this scenario is the creation of bubbles and an increase in the volume of the mixture due to trapped air. This characteristic can be particularly significant in various industrial applications where the physical properties of oils need to be managed carefully.

Other options, such as causing the oil to solidify or evaporate, are inconsistent with the chemical and physical behaviors of ammonia and oil in a reduced pressure environment. Similarly, while ammonia can influence viscosity, its primary action in this context focuses more on the creation of foam rather than enhancing viscosity substantially.