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PCC BIO 100IN - Water - Chemistry and Properties

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Biology 100 Instructor Deborah Bird Water: Chemistry and Properties Reading 1 | Page WATER: Chemistry and Properties INTRODUCTION – THE CHEMISTRY OF WATER Water is the medium that makes life as we know it possible here on Earth. Almost 3/4 of the Earth's surface is covered by water. Most cells are surrounded by water and 70% to 95% of a cell's internal environment is water. We can think of ourselves as “lovely bags of water”. Water is so common that we often see it as nothing special. In fact, it is the makeup of the water molecule and its corresponding unique properties that support life on Earth. Water is formed by covalent bonding, the sharing of electrons, but in this case the sharing of the electrons is not equal and charges build up on the molecule. The oxygen atom can be called an electron bully in the sense it will not share electrons equally in the water molecule. It tends to hoard the electrons. The oxygen atom has tendency to have a negative charge while the two oxygen atoms tend to have a positive charge. (See the drawing) to put this phenomena in more scientific terms: the molecule as a whole is electrically neutral, but there is an imbalance in the distribution of electrons. The oxygen atom is much more electronegative than the two hydrogen atoms with which it shares electrons. Oxygen draws the shared electrons closer to itself, giving the molecule an asymmetric charge. This is why the hydrogen atoms are not aligned at right angles to the oxygen. The charges are called dipole moments and are represented thusly, the negative charge is δ- and the positive charge is indicated by δ+. This unequal distribution of charge is called polarity. Water, like other polar molecules, behaves a little bit like a magnet, with a + end and a – end. The positively charged portion of one water molecule can attract the negatively charged portion of another water molecule, forming a hydrogen bond. Each water molecule can form several hydrogen bonds simultaneously. Remember that H-bonds are weak individually, but because there are so many of them, water molecules “stick” together. The attraction of water for itself isBiology 100 Instructor Deborah Bird Water: Chemistry and Properties Reading 2 | Page " Water (H2O) Other names Dihydrogen monoxide Oxidane Hydroxylic acid Hydrogen Hydroxide R-718 Identifiers Properties Molecular Formula H2O Molar mass 18.01528(33) g/mol Appearance white solid or almost colorless, transparent, with a slight hint of blue, crystalline solid or liquid Density 1000 kg/m3, liquid (4 °C) 917 kg/m3, solid (Lighter as solid) Melting point 0 C 32 F Boiling point 100 °C, 212 °F Refractive index 1.33 Specific heat 1.0 called cohesion. (Water loves other water molecules.) . Water can also form hydrogen bonds with other polar or charged materials, an attraction referred to as adhesion. (This is why waterBiology 100 Instructor Deborah Bird Water: Chemistry and Properties Reading 3 | Page - - - - are the hydrogen bonds between a oxygen (red) and a hydrogen (white) of water molecules will stick to your skin when you get out of the shower. Your skin has charged areas that water can adhere.) Two other important properties of water, surface tension and capillary action, are results of these attractions. The Properties Created by the Chemistry At the interface between water and air, the water molecules are arranged in an orderly pattern; they are H-bonded to each other on three sides. Since the surface molecules are bonded on three sides instead of four, like the molecules in the bulk of the fluid, water acts as if it is coated with a film. It is this surface tension (the tough "skin" formed on the surface of the water) that also causes water to bead-up on a hard, non-wettable surface. Surface tension allows us to skip rocks on water and it allows small organisms like a water strider to "walk on water." Capillary action forces liquids into small pores. This is how water moves through dry soil. The forces of adhesion and cohesion are both necessary to explain capillary action. For example, water clings to glass by adhesion and this attraction pulls the water up the sides of a glass tube. Since water is H-bonded to itself, the molecules moving up the tube pull other water molecules up along with them. Movement stops when the weight of the water column is greater than the force of capillary action. The ability of water to maintain a continuous column is derived from cohesive properties. These same emergent properties of water are in part responsible for moving fluids from the ground to the top of a tree as well as surrounding your body cells with a protective moisture coating. Water and Temperature The specific heat of any substance is the amount of heat required to raise, or lower, the temperature of 1 g of the substance 1 oC and is measured in calories /g /oC. Since a calorie is defined as the heat required to raise 1 g of water 1 oC, the specific heat of water (by definition) is 1. 0. (The nutritional calorie is actually a kilocalorie which equals 1000 calories.1 kilocalorie can raise the temperature 1 degree C of 1000 ml. of water.)Biology 100 Instructor Deborah Bird Water: Chemistry and Properties Reading 4 | Page Compared to other substances, water has a high specific heat. Water is used as the Surface tension prevents the paper clip from submerging and from overflowing the blue glass. standard measurement by which all other materials are compared. Water resists temperature change. It takes a large amount of heat to break the hydrogen bonds of water. There is heat recorded by a thermometer when you are heating water and heat that is not recorded. The heat that is taken up by hydrogen bonds is called the latent heat of vaporization. When water is heated, most of the heat is used to break hydrogen bonds and not much is left over to actually raise the temperature of the water (increase the kinetic energy of the water). Hydrogen bonds make it difficult for water molecules to escape the liquid state and are responsible for water's high heat of vaporization. Water is the standard with a specific heat of 1.0 by which other materials are compared. Notice in the listing of metals below, some metals are above 1.0 and most others are below 1.0. The closer the number is to 1 the closer the material is to


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