Water Activity

Synonyms

Available water; Free water

Keywords

Water, vapor pressure, temperature, water availability

Definition

Water activity, a _w, is a dimensionless parameter defined as the vapor pressure of water in a system (p) divided by that of pure water at the same temperature (p _o). It correlates with the proportion of water available for biological or chemical reactions and is related to the energy status of the water molecules in a given system (pure distilled water has a _w = 1).

Overview

a _w measures how efficiently the water present in a system, including for example microbial cells, can take part in a reaction or in a physical process and is often also referred to as the “free” or “available water” in a system.

For an ideal solution,

$$ {a_{\text{w}}} = \frac{{{n_1}}}{{{n_1} + {n_2}}} = \frac{p}{{{p_0}}} $$

where n ₁ refers to moles of water, n ₂ to moles of solute, and p, p ₀ refer to the vapor pressure of solution or solvent, respectively.

a _wis temperature dependent since it is...