The density of mercury used in shrinkage limit apparatus is (approx).
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Detailed Explanation
The shrinkage limit test requires precisely measuring the volume of a dry soil pat, which is done by displacing mercury out of a container and weighing the displaced mercury. Mercury is used specifically because of its very high, well-known, and highly consistent density, and because it does not wet or adhere to the soil surface the way water would (which would introduce measurement error).
Why "13.6 g/cc" Is Correct
The density of mercury is a well-known physical constant, approximately 13.6 g/cc (equivalently about 13,600 kg/m³) — roughly 13.6 times denser than water. This large, precisely known density lets a small volume of displaced mercury be converted very accurately into the corresponding volume of the soil pat via a simple weight measurement.
| Substance | Approx. Density |
|---|---|
| Water | 1.0 g/cc |
| Mercury | 13.6 g/cc |
Key Point
13.6 g/cc is a physical constant worth memorizing outright for this test — it recurs any time a shrinkage-limit calculation needs to convert mercury weight into a volume.
Key Concepts for Students
- Mercury's high density means even a relatively heavy reading of displaced mercury corresponds to a comparatively small, precisely-measurable volume.
- Mercury does not wet the soil surface, unlike water, which avoids meniscus/surface-tension errors in the volume measurement.
- The same mercury-displacement principle is used twice in the shrinkage limit test: once to find the volume of the wet soil pat (at the shrinkage limit), and once to find the dry volume of the pat after oven drying.
- Because mercury is toxic, modern lab practice increasingly favors alternative volume-measurement techniques, though the classical method (and this density value) remains the standard textbook reference.