easily use. Most liquid freshwater is found under the Earth’s surface as groundwater, while
the rest is found in lakes, rivers, and streams, and water vapor in the sky.
Figure 13.2: Earth’s water is mostly in the oceans. Fresh water is only 3% of all the Earth’s
water, and most of that is in the form of ice. ( 3 )
The Water Cycle
Water is a special substance. It is abundant on Earth and frequently appears as a gas, liquid,
and solid. It is one of the few substances on Earth that is frequently found in all three phases
of matter. Moreover, it can readily cycle through the globe: the same molecule can travel
through many different regions on Earth.
Three States of Water
Part of the reason that water is unique is because of its melting point and boiling point. Un-
der normal atmospheric conditions, water freezes at 0oC (32oF) and boils at 100oC (212oF).
Because of our Earth’s position in the solar system, Earth’s temperature varies from far
below the melting point of water to well above that melting point. Even though water does
not boil at normal temperatures, it often becomes gaseouswater vaporby evaporating.