Properties of Pure Water

Gerd Brunner

In this chapter, the role of water in general, the main properties of water at standard and hydrothermal conditions, the structure of water, and phase behavior are addressed first. Then, individual properties of water are presented. The thermodynamic, transport, electrical properties are discussed in detail. For thermodynamic properties, the fundamental thermodynamic equations are briefly reviewed, followed by the individual properties like volumetric properties (and P,V,T-behavior), heat capacity, enthalpy, and entropy. Transport properties that are discussed include viscosity, thermal conductivity, and diffusivity. Electrical properties that are covered are ion product, electrical conductivity, and dielectric constant. This chapter is concluded with a brief discussion of refractive index and surface tension. Various tables and diagrams, useful for understanding and first conceptual designs are included throughout the chapter.

2.1 Introduction

Water is the most abundant and ubiquitous compound on planet Earth. It is essential for life and is a major constituent of all known life-forms. Water determines our lives in many aspects such as nurturing, cleaning, and influencing the weather. Water occurs on earth in three states of aggregation: as vapor, as liquid, and as solid, which are a consequence of our planet's orientation to our sun. Water in the form of water vapor is the most important compound for controlling temperatures on earth. The water content of our atmosphere is variable between about 0.5 mol% in cold dry air and 3–4 mol% in humid tropical air [1]. The water cycle, that is, the amount of water transported in the atmosphere, is about 2500 times larger than the carbon cycle. In its liquid form, water comprises more than 96% of the oceans which cover our earth to about 71% with a depth of about 4200 m for more than half of the oceans [2]. Liquid water in fresh water lakes and rivers is a minor contribution of about 0.1% to total liquid water on our planet. Water as solid ice in glaciers and ice-caps around the poles contain only about 1.7% of all water [3] but plays an important role for our climate. Hydrothermal water occurs in nature in thermal baths, in geysers, and in volcanic outlets below water level (black smokers).

Water as the all-time and everywhere-available compound acts as a working fluid in many aspects, for example, from the use of its potential energy in ancient and modern machines to make mechanical and electrical energy available, as cooking fluid for preparing food, as solvent in the extraction of natural compounds, and not least of all, as processing fluid in power plants.

This chapter will concentrate on water at elevated temperatures and pressures, and on the applications of water at hydrothermal and supercritical conditions, as the title of the chapter states. The main area covered in this chapter is water and its applications at temperatures above T > 100 °C (373 K) up to the critical temperature of water (T = 374 °C, 647 K) and far beyond, and pressures above the saturation pressure of water up to around P = 200 MPa. Properties of water at ordinary or ambient conditions will be included for comparison.

The reader will get sufficient insight to understand or realize the effects that the properties of water and their dramatic changes exert on their environment and on other compounds. Data of properties of water will be provided to such an extent that the reader will be able to work with them without being forced to refer to other sources. This holds for understanding the effects and for using data for at least conceptual process design. It may be necessary to refer to fundamental treatises, to more extensive databases, or to other computational resources for detailed design purposes or for special effects. Therefore, the necessary and available sources will be addressed.

Pure water is a tasteless, odorless, colorless, clear liquid. It shimmers lightly blue in thick layers as does its solid form, that is, ice. Water in this chapter refers to the isotopic composition of water defined in the international standard called Vienna Standard Mean Ocean Water (VSMOW) [4]. Heavy water D2O, that is, where hydrogen is 100% deuterium, is beyond the scope of this chapter. For more information, see IAPWS (International Association for the Properties of Water and Steam) releases at http://iapws.org/.

Properties of water are summarized in Table 2.1. A detailed description of the individual properties follows below. The values of properties in Table 2.1 are the recommended values issued by IAPWS. This organization has over several decades dedicated its work to the precise measurement, correlation, and distribution of best values of the properties of water. The IAPWS releases and guidelines are the most reliable data available for the properties of water and should be used in scientific and industrial work. They can be accessed at http://iapws.org/.