What is defined as the water content at which soil changes from plastic to liquid state?

The water content at which soil transitions from a plastic state to a liquid state is known as the liquid limit. This is a critical point in soil mechanics because it signifies the moisture level at which the soil can no longer maintain its shape and begins to flow like a liquid.

In the context of soil classification and behavior, the liquid limit is an essential parameter as it helps in understanding the plasticity and potential for deformation of the soil when subjected to various loads or moisture conditions. Knowing the liquid limit allows engineers and geotechnical professionals to evaluate the suitability of the soil for construction and other applications.

The other options refer to different soil conditions. The plastic limit is the moisture content at which soil changes from a semi-solid to a plastic state. The compaction limit refers to the maximum density a soil can achieve when compacted, while the plasticity index is the numerical difference between the liquid limit and plastic limit, indicating the range of moisture contents over which the soil exhibits plastic behavior.