GEF

Unit 2  |  Water Science and the Water Cycle 42 GROWYOUR KNOWLEDGE Precipitation is water that falls from the atmosphere in the form of rain or snow. Some areas on Earth get a lot of precipitation, while other areas get very little. In this activity, research the precipitation where you live, and answer the questions below. You might start by contacting a local weather service, or visit the website for the National Oceanic and Atmospheric Administration (noaa.gov.) 1. What is the average annual amount of precipitation where you live? 2. How much of the precipitation is rain and how much is snow? 3. Has the annual amount of precipitation increased or decreased over the past ten years? 4. How might changes in the amount of precipitation affect people and other organisms living in your area? Essential Concept Water is an essential part of life and Earth’s functions. 2.1  Water Physics and Chemistry Plants and animals cannot live without water. In fact, the human body is 55 to 78 percent water. But what specific properties make water so indispensable to life? A basic under- standing of water physics and chemistry is a good first step in understanding water sustainability. Water is called H 2 O because a water molecule consists of two hydrogen (H) atoms bonded to a single oxygen (O) atom. The charges within the molecule are unequal—the oxygen end is slightly negative and the hydrogen end is slightly positive. These opposing charges create polarity in the water molecule. Since opposite charges attract, the oxygen atom in one mol- ecule attracts the hydrogen atoms in another, so that water molecules tend to stick together by hydrogen bonding. MOLECULE A group of two or more atoms held together by chemical bonds. POLARITY A separation of electrical charges within a molecule. The molecular structure of water consists of two hydrogen atoms attached to a single oxygen atom.