What happens to the oil in the compressor when it is mixed with ammonia?

When oil is mixed with ammonia in the compressor, the correct phenomenon that occurs is that it foams when pressure is lowered. This foaming is primarily due to the low solubility of ammonia in oil at high pressures. As pressure decreases, ammonia that is dissolved in the oil can be released rapidly, leading to the formation of bubbles and consequently foam. This foaming can reduce the lubrication properties of the oil and potentially interfere with the compressor's operation.

The other options do not accurately describe the interaction between oil and ammonia in this context. For instance, the oil does not generally become less viscous or separate into layers due to the presence of ammonia in compressors; rather, the foaming is a more direct and observable effect in scenarios involving pressure changes. While ammonia can indeed be flammable, the typical mixing does not inherently increase the flammability of the oil, unless specific conditions or concentrations are met, which is not a direct consequence of the mixing process described.