Unit 2: Composition of Earth Chapter 3: Matter and Atomic Structure     Matter and Atomic Structure The polar nature of water makes possible a different kind of attraction between molecules: a hydrogen bond. This is not a chemical bond as is a covalent bond or an ionic bond, but rather an attraction between molecules. This type of bond occurs when the positive, hydrogen ends of the water molecules are attracted to the negative, oxygen ends of other water molecules. Hydrogen bonding gives water some unique properties. In fact, bonding often determines the properties of a substance and its state of matter under normal conditions at Earth's surface. The density of water in its different states of matter provides a good example of the unique properties of water. Solid water is less dense than liquid water. Usually, most solids have higher densities than their corresponding liquid forms. For example, solid silver (Ag) is denser than liquid silver, which is silver that has been heated to its melting point. Thus, solid silver sinks when it is placed in liquid silver. However, because the density of water decreases below 4°C, ice floats in liquid water. Hydrogen bonding holds the water molecules in an open, three-dimensional structure. All substances on Earth can be found in one physical state or another at room temperature and at standard atmospheric pressure-so-called normal conditions at Earth's surface. Whether a substance is a solid, liquid, or gas in its normal state depends upon the type of forces holding the substance together. LINK UP: Find out more information about water's unique properties. Cycles of Matter On Earth, water continually undergoes changes in state as it moves through the global water cycle. Any given water molecule spends most of its time in seawater. When it evaporates, it becomes water vapor, a gas in the atmosphere. Eventually, the water molecule is incorporated into a raindrop or snowflake, which falls to Earth's surface. It may be taken up by a plant's roots, flow by gravity into the underground water table or into a river or a stream, or become part of a glacier. A water molecule taken in by a plant may be released again into the atmosphere through the plant's leaves in the process of transpiration. A water molecule that falls into a river or stream will flow back to the ocean. A water molecule that becomes part of a glacier will remain until the glacier melts, and the cycle begins again. Other elements cycle through the atmosphere, hydrosphere, lithosphere, and biosphere of Earth and become part of many different compounds. Carbon atoms, for example, can be found in carbon dioxide gas, carbonic acid solutions, and solid calcium carbonate in limestone. Nitrogen atoms exist in nitrogen gas, in dissolved nitrates in soil and in the ocean, and as part of the proteins found in living organisms. In fact, all matter cycles, because matter cannot be created or destroyed. Strong Solids Earlier we mentioned that the normal state of a substance depends on the forces that bind it together. Many solids are composed of atoms joined by covalent bonds. The structures of solid molecular compounds are extremely strong. For example, diamond, the hardest substance on Earth, is a solid molecular compound consisting of carbon atoms that each have four valence electrons. The outermost energy levels of neighboring carbon atoms overlap in such a way that each level contains eight shared electrons. The great hardness of diamond illustrates how strong covalent bonding can be. Gases In contrast to diamonds, many molecules exist normally in a gaseous state under the temperature and pressure conditions that occur at Earth's surface. Carbon dioxide gas, for example, consists of CO2 molecules. Hydrogen gas occurs as H2 molecules. Most of the oxygen in the atmosphere occurs as O2, a molecule consisting of two oxygen atoms. Another forms of oxygen is O3 (ozone), a molecule consisting of three oxygen atoms. Although ozone plays a crucial role in shielding life on Earth from harmful UV rays, it is actually a poisonous gas. Activity When an ice cube is placed at room temperature, it warms up and some of the water molecules break away from the crystalline structure as a result of increased thermal vibrations. When all the water molecules break away, the solid ice becomes liquid water. However, the temperature of the melted ice will still be 0°C, the freezing point of water. Form a hypothesis that explains why this is the case. Then design and carry out an experiment to test your hypothesis.