Soil acidity and alkalinity (pH concept check): Which statement correctly identifies soil reaction based on pH value, and also reflects typical geotechnical terminology used in site investigations?

Introduction / Context:
Soil pH is a measure of acidity or alkalinity and is widely reported in geotechnical, environmental, and agricultural investigations. Understanding pH helps anticipate corrosion potential for buried steel, sulfate attack risk on concrete, and the behavior of clay minerals. The pH scale is logarithmic and centered at 7 under standard conditions.

Given Data / Assumptions:

• pH < 7 indicates acidic reaction; pH = 7 is neutral; pH > 7 indicates alkaline (basic) reaction.
• Terminology follows standard aqueous chemistry conventions applied to soils.
• No special buffering or temperature effects are assumed.

Concept / Approach:
The pH value is defined as negative log of hydrogen ion activity. In soils, pore-fluid chemistry and mineralogy control pH. Acidic soils (pH < 7) may increase corrosion of metallic elements and alter the durability of cementitious materials. Alkaline soils (pH > 7), often influenced by carbonates or sodium, may indicate potential for swelling in certain clay mineral assemblages and can affect concrete chemistry differently than acidic soils.

Step-by-Step Solution:

Verification / Alternative check:
Field kits and laboratory tests (e.g., slurry pH) confirm soil reaction. Geotechnical reports typically list pH alongside sulfate and chloride contents for durability assessments.

Why Other Options Are Wrong:

• Option A: Incorrect; pH > 7 is alkaline, not acidic.
• Option B: Correct description of acidity but not the only correct statement asked; the best single identification for pH > 7 is option C.
• Option D: Irrelevant; presence of cement-manufacturing chemicals is not a selection criterion.
• Option E: Incorrect; pH = 7 is neutral, not alkaline.

Common Pitfalls:
Confusing neutral (7) with alkaline; overlooking that pH is logarithmic, so one unit change represents a tenfold change in hydrogen ion activity.

Final Answer:
A soil having pH greater than 7 is an alkaline (basic) soil