Index Properties MCQ with answer and solution

Concept

Dispersing Agent Correction in Hydrometer Analysis

Formula

R = Rh + Cm - Cd ± Ct, where R is the corrected hydrometer reading, Rh is the observed reading, Cm is the meniscus correction (+), Cd is the dispersing agent/deflocculating agent correction (-), and Ct is the temperature correction (±).

Step-by-step solution

1. Identify the formula for corrected hydrometer reading: R = Rh + Cm - Cd ± Ct. 2. Observe that the dispersing agent correction (Cd) enters the equation with a minus sign (-Cd) to account for increased fluid density. 3. Conclude that the dispersing agent correction is always negative (Option B).

Explanation

When a dispersing agent is added to the soil-water suspension, it increases the density of the liquid. This increased density makes the hydrometer float higher than it normally would, resulting in a higher observed reading than the actual value. Therefore, a negative correction (subtraction) must always be applied to get the correct hydrometer reading.

Shortcut

Remember: Dispersing agent increases fluid density, so the hydrometer reads higher, requiring a subtraction. Shortcut rule: Dispersing = Subtract (Always Negative).

Common mistake

Many students confuse dispersing agent correction with meniscus correction (which is always positive) or temperature correction (which can be either positive or negative), leading them to incorrectly choose "Positive" or "Can be either positive or negative" instead of "Negative".