Using specimens A, B (WATER), C (SODIUM CARBONATE).and D, carry out an experiment to show that soil is made up of different particle sizes (a) (i) Describe ...

The experiment uses specimen A (a soil sample), specimen B (water), specimen C (sodium carbonate, a deflocculating/dispersing agent) and specimen D (a tall transparent measuring cylinder or bottle). The principle is sedimentation: when soil is shaken in water and allowed to settle, the particles settle out in layers according to their size and weight.

(a)(i) Procedure:

1. Place a known quantity of the dry soil (specimen A) into the transparent cylinder (specimen D).
2. Add water (specimen B) until the cylinder is about three-quarters full.
3. Add a small quantity of sodium carbonate (specimen C) to help disperse the particles.
4. Cover the mouth of the cylinder and shake the mixture vigorously so that all the soil is thoroughly suspended in the water.
5. Place the cylinder on a table and allow it to stand undisturbed for several hours (or overnight).
6. Observe and record the layers of particles that settle at the bottom.

(ii) Observations and results: The soil particles settle in distinct layers. The heaviest and largest particles (gravel and coarse sand) settle first at the bottom, followed by fine sand, then silt, and finally the finest clay particles form the topmost layer of the sediment. Very fine organic matter (humus) floats on the surface of the water, and the water above remains slightly cloudy. This layering shows that the soil is made up of particles of different sizes.

(b) Functions of the specimens:

• Specimen B (water): it acts as the medium in which the soil particles are suspended so that they can separate and settle out according to their sizes.
• Specimen C (sodium carbonate): it acts as a deflocculating (dispersing) agent that separates the clumped soil particles from one another, so that each particle settles freely according to its true size.

(c) Three precautions:

1. Shake the mixture thoroughly and uniformly so that all the soil is properly suspended.
2. Place the cylinder on a firm, level surface and do not disturb or shake it while the particles are settling.
3. Read the levels of the settled layers at eye level to avoid parallax error.