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How Does Water Freeze 12 August 2012 16:34 How does water freeze 12 August 2012 16:34 Water is an unique substance that consists of many unusual properties. The two elements making up water are Hydrogen and Oxygen. There are two Hydrogen atoms per Oxygen atom. Hence the Molecular Formula of water is H2O. This makes water a polar compound and soluble with many substances. As hydrogen and oxygen are both non-metals, covalent bonding is used to form the compound. In covalent bonding the more electronegative (ability to attract electrons) atom has a slightly negative charge and the less electronegative is slightly positively charged. Polar molecules are attracted to one another by dipole interaction. Hydrogen has an electronegativity of 2.1 and Oxygen is 3.5. Therefore the difference in electronegativity is 1.4. The negative end of one molecule of water is attracted to the positive end of another. This results in hydrogen bonding. Hydrogen bonding occurs when the hydrogen bonds with a highly electronegative element e.g. O, F, N. The Hydrogen (slightly positive charge) is attracted to the lone pair of electrons in the nearby atom of Oxygen (the highly electronegative element). This Hydrogen bond has about 5% of the strength of a standard covalent bond. Hydrogen bonds are the strongest of all intermolecular forces. Hydrogen bonding in water is the only reason why it has such a high specific heat. Specific heat can be defined as the amount of heat required to change a unit mass ( such as a mole) of a substance by one degree in temperature. Hydrogen bonding weakens as the temperature rises, therefore much of the energy is used into breaking hydrogen bonds instead of raising the temperature. This causes water to have a higher heat capacity, In fact it is the second highest among all the heteroatomic species (after ammonia). According to Josh Willis, ofNASA's Jet Propulsion Laboratory, the oceans absorb one thousand times more heat than the atmosphere (air) and are holding 80 to 90% of the heat of global warming. Pasted from <http://en.wikipedia.org/wiki/Properties_of_water#Heat_capacity_and_heats_of_vaporization_and_fusion> Pasted from <http://en.wikipedia.org/wiki/Properties_of_water#Heat_capacity_and_heats_of_vaporization_and_fusion> Pasted from <http://www.madsci.org/posts/archives/jul2000/963870691.Ch.r.html> Pasted from <http://scienceworld.wolfram.com/physics/SpecificHeat.html> Ice is water frozen into the solid state. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions. The addition of other materials such as soil may further alter the appearance. The molecules in solid ice may be arranged in different ways, called phases, depending on the temperature and pressure. Usually ice is the phase known as ice Ih, which is the most abundant of the Inroduction Page 1 temperature and pressure. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. The most common phase transition to ice Ih occurs when liquid water is cooled below 0°C (273.15K, 32°F) at standard atmospheric pressure. It can also deposit from vapour with no intervening liquid phase, such as in the formation of frost. As a naturally occurring crystalline inorganic solid with an ordered structure, ice is considered a mineral.[1] It possesses a regular crystalline structure based on the molecule of water, which consists of a single oxygen atom covalently bonded to two hydrogen atoms, or H-O-H. However, many of the physical properties of water and ice are controlled by the formation of hydrogen bonds between adjacent oxygen and hydrogen atoms. It is a weak bond, but is critical in controlling the structure of both water and ice. An unusual property of ice frozen at atmospheric pressure is that the solid is approximately 8.3% less dense than liquid water. The density of ice is 0.9167 g/cm³ at 0 °C, whereas water has a density of 0.9998 g/cm³ at the same temperature. Liquid water is densest, essentially 1.00 g/cm³, at 4 °C and becomes less dense as the water molecules begin to form the hexagonal crystals[2] of ice as the freezing point is reached. This is due to hydrogen bonding dominating the intermolecular forces, which results in a packing of molecules less compact in the solid. Density of ice increases slightly with decreasing temperature and has a value of 0.9340 g/cm³ at −180 °C (93 K).[3] The effect of expansion during freezing can be dramatic, and is a basic cause of freeze-thaw weathering of rock in nature. It is also a common cause of the flooding of houses when water pipes burst due to the pressure of expanding water when it freezes, then leak water after thawing. Pasted from <http://en.wikipedia.org/wiki/Ice> Any substance can exist in three different physical forms; solid, liquid and gas. Water is the only substance on earth that naturally occurs in all three states. Temperature reflects the amount of kinetic energy of molecules or more simply, the motion of the molecules. The faster the molecules are moving the higher their temperature. This is similar to how the faster you move, the hotter you get! When moving, the hydrogen bonds between the molecules of water can break. The water molecules in the liquid phase can move so fast that they break their hydrogen bonds and go from a liquid to a gas phase. If temperatures get hot enough, the molecules all move fast enough to break their hydrogen bonds and move into the gas phase. We see this happening when water boils! When liquid cools the molecules have less kinetic energy, slowing down and packing closer together. Volume is decreasing without mass changing so the water gets more dense. As seawater gets more dense, it sinks to the bottom of the ocean. This sinking of cold water at the poles creates global circulation of seawater. Credit NASA Inroduction Page 2 Credit NASA As water continues to cool the hydrogen bonds form and hold the water molecules in place as a solid. This solid form of water is ice. Ever notice that ice cubes float in your drink? That is because fresh water gets denser as it cools until it reaches about 4°C, (39°F). Below that temperature ice gets less dense as it cools. This happens because water expands as it freezes. If you have ever made ice cubes you have seen this happen. Since the mass remains the same, but the volume of the solid is greater, ice is less dense than liquid water ~ so it floats! Because of this, bodies of water such as lakes and bays freeze at the surface. This property allows fish living in lakes and ponds to survive the winter. The ice that freezes on the surface insulates the water below so it stays warmer! In general, it is extremely unusual for the solid phase of a substance to be less dense than its liquid phase, but lucky for aquatic life it is! Pasted from <http://new.coolclassroom.org/discoveries/view/water> Changes of Phase (or State) Heat and Temperature Energy Mechanics Contents Index Home The term 'change of phase' means the same thing as the term 'change of state'. There are four states, or phases, of matter. They are: ○ Solid ○ Liquid ○ Gas ○ Plasma We will not be discussing the plasma state here. When a substance changes from one state, or phase, of matter to another we say that it has undergone a change of state, or we say that it has undergone a change of phase. These changes of phase always occur with a change of heat. Heat, which is energy, either comes into the material during a change of phase or heat comes out of the material during this change. However, although the heat content of the material changes, the temperature does not. Here are the five changes of phase. They are diagrammed in the above animation and listed below. Description of Term for Phase Heat Movement Temperature Phase Change Change During Phase Change During Change Phase Change Solid to liquid Melting Heat goes into the None solid as it melts. Liquid to solid Freezing Heat leaves the None liquid as it freezes. Liquid to gas Vaporization, which Heat goes into the None includes boiling and liquid as it evaporation vaporizes. Gas to liquid Condensation Heat leaves the gas None as it condenses. Solid to gas Sublimation Heat goes into the None solid as it sublimates. So, how could there be a change in heat during a state change without a change in temperature? During a change in state the heat energy is used to change the bonding between the molecules. In the case of melting, added energy is used to break the bonds Inroduction Page 3 between the molecules. In the case of melting, added energy is used to break the bonds between the molecules. In the case of freezing, energy is subtracted as the molecules bond to one another. These energy exchanges are not changes in kinetic energy. They are changes in bonding energy between the molecules. If heat is coming into a substance during a phase change, then this energy is used to break the bonds between the molecules of the substance. The example we will use here is ice melting into water. Immediately after the molecular bonds in the ice are broken the molecules are moving at the same average speed as before, so their average kinetic energy remains the same, and, thus, their Kelvin temperature remains the same. Look at the following diagram and continue to read the text below it.
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