Physics of Water

Why is water wet? Many people will answer this question by simply saying, "Because it is." The physical properties of water are fundamental to life and nature on Earth, and are often accepted as simple truths. Water is so common on Earth that its physical characteristics have a large impact on the physics of Earth in general. (Physics is the study of matter and energy, and of interactions between the two.) Water covers almost three quarters of the planet's surface. It is the only natural chemical substance that exists as a liquid, solid (ice), and gas (water vapor) within Earth's normal temperature range. Water is liquid in a range critical for biological life (0–100°C, 32–212°F), and liquid water is present almost everywhere on Earth. Water's ability to absorb heat regulates Earth's climate and weather.

Phase changes

Matter exists in three states, or phases: solid, liquid, and gas. (Matter is anything that has mass and takes up space). Substances like water change from one phase to another at specific temperatures and pressures. Add heat (or pressure), and a substance begins to change from a solid to a liquid at its melting point. Add more heat, and the substance will begin to evaporate, to turn from liquid to gas, at its boiling point. Remove heat (or pressure), and a substance will condense from gas to liquid and then freeze from liquid to solid. In water's solid phase, its molecules are bound together in a rigid framework called a crystal. (A molecule is a collection of two or more atoms held together by chemical bonds and an atom is the smallest unit of an element.) A water molecule, known by its chemical symbol H₂O, is a group of two hydrogen atoms and one oxygen atom. In liquid water, the molecules are still attached, but less strongly so, and they can move more freely. The molecules in water vapor are completely detached from one another and mingle with other types of atoms and molecules in air.

Phase changes either use or release heat energy. Melting and boiling are endothermic phase changes; they absorb heat. The opposite processes, freezing and condensing, release heat and are called exothermic phase changes. A block of ice, sitting in the sun, warms to the melting point of water (0°C or 32°F at sea level on Earth) and then begins to melt. The ice stays at exactly 32°F until it has completely melted. In this endothermic process, the heat is breaking the bonds between molecules within the ice crystal. Once the ice has melted, the resulting liquid water begins to absorb heat and the water temperature rises. When water temperature reaches the boiling point, 212°F (100°C), the chemical bonds between molecules break, and the water evaporates into its gas phase. Again, the liquid water stays at exactly 212°F (100°C) until the phase change is complete. During exothermic reactions, the temperature likewise remains the same until enough heat has been released for the phase change—melting for example—to become complete.

Pressure changes can also cause phase changes. Moun-taineers in the Himalayas can have trouble cooking their food because water boils at a lower temperature at high altitude (air pressure decreases at high altitude). For this reason, they sometimes carry pressure cookers that raise the temperature in the pot by trapping steam and raising the pressure. Cake mixes have special high-altitude cooking instructions printed on the box. Ice-skaters can slide across the ice because the pressure of their skate blades temporarily melts the ice and forms a slippery film of liquid water.

Buoyancy: Archimedes and the King's Crown

Archimedes, a mathematician who was born in 287 b.c., first explained the principles of liquid displacement and buoyancy. Legend tells that the king of Syracuse on the island of Sicily asked Archimedes to find out if his beautiful crown was made of solid gold. The king suspected that the crown's maker had stolen some of the gold by substituting silver inside the crown. Archimedes considered the king's problem while bathing. He knew that silver is less dense than gold, so a part-silver crown would weigh less than a gold crown the same size. As he pondered ways to measure the volume (amount of space occupied) of the irregularly shaped crown, he noticed that the water level in the pool rose when he got into the bath, and fell when he got out of the bath. He put the crown in a basin filled with a measured amount of water. By calculating the amount of water displaced by the crown, he correctly estimated its volume. (To this day, pastry chefs use Archimedes' displacement method to measure butter or shortening.) Then he placed the crown on a scale and measured it against a pile of gold blocks of the same volume. Sadly for the deceitful craftsman, the scales tipped to the gold blocks instead of balancing the crown.

Archimedes' principle of buoyancy states that a completely submerged object displaces a volume of fluid equal to its own volume. The upward force placed on the object, called the buoyant force, is equal to the weight of the displaced fluid. If the object weighs more (has a greater mass) than the fluid it displaces, it sinks. If it weighs less, the buoyant force pushes it upward and it floats. Drop a steel ball in a pool, and it sinks because its density is greater than the density of water. A wooden ball the same size will float. A steel-hulled ship floats because it is hollow and the air contributes to the total mass. Add cargo, and the ship floats lower in the water. Add too much cargo, and it sinks. A submarine submerges by filling its tanks with water, and surfaces by filling the tanks with air. Archimedes' principle of buoyancy also correctly predicts that the tip of an iceberg is about one-eighth of a floating ice block. Ice is less dense than water, so icebergs float. However, they float with the majority of their volume below the water line.

Liquid water

Molecules in liquid water stick together. It takes a lot of heat energy to break the electrical attractions, called hydrogen bonds, between water molecules. Because of this, water has a high specific heat; it can absorb a lot of heat energy before it changes temperature. In general, heating raises the temperature in a liquid by making its molecules move faster in relationship to one another. In water, some of the heat energy is used to break the hydrogen bonds between molecules. When water cools, hydrogen bonds reform, and heat is released. Because of its high specific heat, liquid water can store a lot of energy, a property that has significant consequences for Earth's climate and biological life.

The oceans, Earth's massive reservoirs of liquid water, store and distribute heat energy from the sun. They absorb intense sunlight during the daytime and summer, and then release it slowly during the night and wintertime in the form of ocean currents. These currents carry stored heat from the tropics near the equator (an imaginary line around the Earth between the North and South Poles) toward the North and South Poles. Coastal and wet climates are usually milder than inland or arid (dry) climates. Water temperature stays relatively constant in the oceans, creating a stable environment for marine (ocean) ecosystems (communities of living organisms). Water protects organisms from temperature changes. Humans, who are composed mainly of water, can survive extreme hot and cold partly because water's high specific heat maintains human body temperatures at around 98.6°F (37°C).

Solid water: ice

Ice floats. Most liquids contract (draw together) as they cool and reach their maximum density (mass per unit of volume) as a solid. Water is different. It contracts until it reaches about 39°F (4°C), and then it expands until its molecules have all frozen into water's crystalline form at 32°F (0°C). So, cold water sinks, but ice floats. Water is the only natural substance on Earth that is less dense as a solid than as a liquid. If not for this property of water, bodies of water would freeze from the bottom up, ice cubes would sink in a water glass, and there would be no such thing as an iceberg (chunks of floating ice).

Gaseous water: water vapor

Water's phase transformation from liquid to gas occurs when molecules in liquid water escape and rise to mingle with other types of molecules and atoms in the atmosphere (mass of air surrounding Earth). Boiling occurs when the temperature within a volume of liquid reaches the point at which all the molecules are vibrating too rapidly to stay bonded to each other. Bubbles of gas escape, and eventually the liquid is gone. Water molecules also enter the gaseous phase by evaporation from the water surface. Water molecules in liquid water are constantly moving. Even at low temperatures, a percentage of the less-confined surface molecules move enough to break their bonds to their neighbors and escape into the atmosphere. Water from Earth's oceans, lakes, and rivers enters the atmosphere by evaporation and, fortunately for life on Earth, not by boiling.

It takes a lot of heat to break the hydrogen bonds in liquid water and to form water vapor. And, water vapor molecules' attraction to one another causes them to condense easily into liquid water droplets. Water prefers to be liquid. Water evaporating from the surfaces of Earth's oceans and other water reservoirs transfers heat into the atmosphere. When water vapor condenses into liquid droplets in clouds, heat is released and the air stays warm. Water vapor is an important greenhouse gas in Earth's atmosphere. A greenhouse traps heat in the atmosphere. Natural greenhouse gases in the atmosphere keep Earth warm, but not too warm. Water vapor's phase changes from liquid to gas and back to liquid act to trap incoming solar energy.

Laurie Duncan, Ph.D.

For More Information

Books

Farndon, John. Water (Science Experiments). Salt Lake City: Benchmark Books, 2000.

Rose, Calvin. An Introduction to the Environmental Physics of Soil, Water and Watersheds. New York: Cambridge University Press, 2004.

Websites

"Archimedes of Syracuse, the Father of Buoyancy." Utah State University Junior Engineering.http://www.engineering.usu.edu/jrestate/workshops/buoyancy/buoyancy.php (accessed on August 1, 2003).

"Causes of Color, Why Is the Ocean Blue?" Web Exhibits.http://webexhibits.org/causesofcolor/5.html (accessed on April 6, 2004).

"Icebergs." Solcomhouse.http://www.solcomhouse.com/iceberg.htm (accessed on April 16, 2004).

"An Ocean of Sound." Oceanlink.http://oceanlink.island.net/oinfo/acoustics/acoustics.html (accessed on December 4, 2003).

"Physical Properties of Water." PhysicalGeography.net.http://www.physicalgeography.net/fundamentals/8a.html (accessed on February 16, 2004).