Soil moisture terminology: The portion of soil moisture that is driven off by oven heat (standard water-content determination) is called:

Introduction / Context:
Water in soils exists as gravitational, capillary, hygroscopic, and chemically bound water. Standard water content (moisture content) testing dries a specimen in an oven to remove removable moisture without altering mineral structure. Understanding which fraction is removed clarifies what “water content” actually measures in practice.

Given Data / Assumptions:

• Oven drying at about 105–110°C for 24 hours.
• Goal: remove the physically held moisture.
• No dehydroxylation of minerals (no structural water loss).

Concept / Approach:

Oven drying removes free and capillary water as well as most adsorbed (hygroscopic) water. In classical terminology, the “moisture driven off by heat” that defines gravimetric water content corresponds to the removable (non-chemically bound) water, often referred to in soils texts as hygroscopic and capillary water. Among the listed terms, “hygroscopic water” best names the portion emphasized as removable by heating that distinguishes it from chemically bound water.

Step-by-Step Solution:

Verification / Alternative check:

Standards (e.g., oven-dry method) target removable moisture and avoid higher temperatures that would drive off lattice water.

Why Other Options Are Wrong:

“Free/gravity water” is only part of the removable moisture; “capillary water” is also removable but not the sole component; “none” is incorrect.

Common Pitfalls:

Confusing chemically bound (structural) water with oven-removable water; using too high temperature and altering the clay minerals.

Final Answer:

Hygroscopic water