The glass in the picture above appears to be floating on water. How can a solid float on water? Actually, nothing is floating on the water; rather this picture depicts some of the most unique properties of water. What appears to be a glass floating on water is really water splashing upward in response to an item being dropped into the water. Water’s unique properties of adhesion, surface tension, and its viscosity are what contribute to this illusionary picture. These properties and others will be discussed in this chapter.
• Describe the distribution of Earth’s water.
• Identify water’s unique properties that support life on Earth.
• Define acids, bases, and pH.
• Explain why water is essential for life.
INTRODUCTION Water, like carbon, has a special role in living things. It is needed by all known forms of life. All organisms on Earth are made up mostly of water, thus water is the biological medium of life here. Water is a simple molecule, containing just three atoms, two hydrogen and one oxygen. Nonetheless, water’s structure gives it unique properties that help explain why it is vital to all living organisms. In fact, without water, life would not be possible. This simple fact is why scientists are constantly looking for water on other planets - the presence of water could indicate the presence of life.
WATER, WATER EVERYWHERE Water is a common chemical substance on planet Earth. In fact, Earth is sometimes called the ‘‘water planet” because almost 75% of its surface is covered with water. If you look at Figure 3.1, you will see where Earth’s water is found. The term water generally refers to its liquid state, and water is a liquid over a wide range of temperatures and pressures on Earth. However, water also occurs on Earth as a solid (ice) and as a gas (water vapor).