To obtain the liquid limit of a soil, a graph is plotted between:

Detailed Explanation

The liquid limit is determined using Casagrande's standard grooving-tool apparatus: a groove of fixed dimensions is cut into a soil pat inside a brass cup, and the cup is repeatedly dropped from a fixed height by a hand crank until the groove closes along a specified length. This is repeated at several different water contents, each producing a different number of blows to close the groove.

Why "water content and number of blows" Is Correct

Plotting the water content of each trial (Y-axis, ordinary/linear scale) against the corresponding number of blows (X-axis, logarithmic scale) produces a straight line called the flow curve. The liquid limit is read directly off this curve as the water content corresponding to exactly 25 blows — the standard reference point.

Water content is plotted against the number of blows (log scale) for several trials; the resulting straight "flow curve" gives the liquid limit as the water content corresponding to 25 blows.

Key Point

The flow curve always uses number of blows on a logarithmic X-axis and water content on a linear Y-axis — reversing these axes is a common mistake to watch for in diagram-based questions.

Key Concepts for Students

The slope of the flow curve is used to compute the flow index, I_f — a measure of how sensitively the soil's consistency responds to changes in water content.
25 blows is the standard reference point used to define the liquid limit — trials are typically run to bracket this value (some giving more, some fewer blows) rather than trying to hit exactly 25 blows in a single trial.
Because the flow curve is plotted on semi-log axes, it should come out as a straight (or very nearly straight) line for a consistent soil sample — a curved or scattered plot usually indicates test error.
The flow index (from the flow curve's slope) and the toughness index (I_p/I_f) are directly related quantities that recur together in several later questions on this topic.