WATER AND HYDROGEN

This chapter covers two intimately related topics: water (the most abundant compound on Earth's surface) and hydrogen (the simplest and most abundant element in the universe). Understanding water's sources, purification, and properties provides the foundation for understanding hydrogen's preparation, properties, and uses. The chapter concludes with the basics of reduction and oxidation (redox) reactions, which are fundamental to understanding chemical changes involving hydrogen and many other reactions.

1. SOURCES AND PURIFICATION OF WATER

Water is essential for life and for countless industrial and domestic processes. Its sources and quality vary greatly.

1.1 Natural Sources of Water

Water on Earth is classified based on its location and salt content. The two main categories are surface water and underground water.

1.1.1 Surface Water

• Rainwater: The purest natural form of water, formed by condensation of atmospheric water vapor. It is essentially distilled water by nature's process. However, as it falls through the atmosphere, it dissolves gases (oxygen, carbon dioxide, nitrogen oxides) and may pick up dust and microorganisms. It is soft water (contains no dissolved minerals).
• River and stream water: Contains dissolved minerals from the soil and rocks (calcium, magnesium bicarbonates, sulfates, chlorides), suspended solids (sand, clay, silt), organic matter (decaying leaves, aquatic organisms), and microorganisms. The composition varies with season and human activities upstream.