water properties notes .pdf

properties of water -Study notes Water is not only 60 to 70% of the bodies of organisms, but it also has some unusual chemical properties that make it very good at supporting life. These properties are important to biology on many different levels, from cells to organisms to ecosystems. Polarity of...

properties of water -Study notes Water is not only 60 to 70% of the bodies of organisms, but it also has some unusual chemical properties that make it very good at supporting life. These properties are important to biology on many different levels, from cells to organisms to ecosystems. Polarity of water molecules A water molecule consists of two hydrogen atoms bonded to an oxygen atom, and its overall structure is bent. This is because the oxygen atom, in addition to forming bonds with the hydrogen atoms, also carries two pairs of unshared electrons. All of the electron pairs—shared and unshared—repel each other. Because oxygen is more electronegative—electron-greedy—than hydrogen, the O atom hogs electrons and keeps them away from the H atoms. This gives the oxygen end of the water molecule a partial negative charge, while the hydrogen end has a partial positive charge. Water is classified as a polar molecule because of its polar covalent bonds and its bent shape. Hydrogen bonding of water molecules Thanks to their polarity, water molecules happily attract each other. The plus end of one—a hydrogen atom—associates with the minus end of another—an oxygen atom. These attractions are an example of hydrogen bonds, weak interactions that form between a hydrogen with a partial positive charge and a more electronegative atom, such as oxygen. The hydrogen atoms involved in hydrogen bonding must be attached to electronegative atoms, such as O, N or F. NOTE: a molecule is polar if its has 0-H, F-H or N-H Water molecules forming hydrogen bonds with one another. The partial negative charge on the O of one molecule can form a hydrogen bond with the partial positive charge on the hydrogens of other molecules. Water molecules are also attracted to other polar molecules and to ions. A charged or polar substance that interacts with and dissolves in water is said to be hydrophilic: hydro means "water," and philic means "loving." In contrast, nonpolar molecules like oils and fats do not interact well with water. They separate from it rather than dissolve in it and are called hydrophobic: phobic means "fearing." Cohesion of water Have you ever dropped water on a coin? Before it overflows, the water forms a dome-like shape above the coin surface. This dome-like shape forms due to the water molecules’ cohesive properties, or their tendency to stick to one another. Cohesion refers to the attraction of molecules for other molecules of the same kind, and water molecules have strong cohesive forces thanks to their ability to form hydrogen bonds with one another. Cohesive forces are responsible for surface tension, a phenomenon that results in the tendency of a liquid’s surface to resist rupture when placed under tension or stress. Water molecules at the surface (at the water-air interface) will form hydrogen bonds with their neighbor water molecules. A simple example of cohesion in action comes from the water strider (below), an insect that relies on surface tension to stay afloat on the surface of water. Adhesion of water Water likes to stick to itself, but under certain circumstances, it prefers to stick to other types of molecules. Adhesion is the attraction of molecules of one kind for molecules of a different kind, and it can be quite strong for water, especially with other polar molecules bearing positive or negative charges. For instance, adhesion enables water to “climb” upwards through thin glass tubes (called capillary tubes) placed in a beaker of water. This upward motion against gravity, known as capillary action, depends on the attraction between water molecules and the glass walls of the tube (adhesion), as well as on interactions between water molecules (cohesion). Why are cohesive and adhesive forces important for life? They play a role in many water-based processes in biology, including the movement of water to the tops of trees and the drainage of tears from tear ducts in the corners of your eyes. Solvent properties of water Thanks to its ability to dissolve a wide range of solutes, water is sometimes called the "universal solvent." Generally speaking, water is good at dissolving ions and polar molecules, but poor at dissolving nonpolar molecules. Water interacts differently with charged and polar substances than with nonpolar substances because of the polarity of its own molecules. Water molecules are polar, with partial positive charges on the hydrogens, a partial negative charge on the oxygen, and a bent overall structure. The unequal charge distribution in a water molecule reflects the greater electronegativity, or electron-greediness, of oxygen relative to hydrogen: the shared electrons of the O-H bonds spend more time with the O atom than with the Hs. In the image below, the partial positive and partial negative charges on a water molecule are represented by the symbols δ\[^+\] and δ\[^-\], respectively. Because of its polarity, water can form electrostatic interactions (charge-based attractions) with other polar molecules and ions. The polar molecules and ions interact with the partially positive and partially negative ends of water, with positive charges attracting negative charges (just like the + and - ends of magnets). When there are many water molecules relative to solute molecules, as in an aqueous solution, these interactions lead to the formation of a three- dimensional sphere of water molecules, or hydration shell, around the solute. Hydration shells allow particles to be dispersed (spread out) evenly in water. Water molecules forming hydration shells around Na+ and Cl- ions. The partially positive ends of the water molecules are attracted to the negative Cl- ion, while the partially negative ends of the water molecules are attracted to the positive Na+ ion. Nonpolar molecules, like fats and oils, don't interact with water or form hydration shells. These molecules don't have regions of partial positive or partial negative charge, so they aren't electrostatically attracted to water molecules. Thus, rather than dissolving, nonpolar substances (such as oils) stay separate and form layers or droplets when added to water. Density of ice and water Water’s lower density in its solid form is due to the way hydrogen bonds are oriented as it freezes. Specifically, in ice, the water molecules are pushed farther apart than they are in liquid water. That means water expands when it freezes. Water is an anomaly (that is, a weird standout) in its lower density as a solid. (Left) Crystal structure of ice, with water molecules held in a regular 3D structure by hydrogen bonds. (Right) Image of icebergs floating on the surface of the ocean. Image: modified from OpenStax Biology. Modifications of work by Jane Whitney (left), image created using Visual Molecular Dynamics (VMD) software (Humphrey, 1996), and by Carlos Ponte (right). Because it is less dense, ice floats on the surface of liquid water, as we see for an iceberg or the ice cubes in a glass of iced tea. In lakes and ponds, a layer of ice forms on top of the liquid water, creating an insulating barrier that protects the animals and plant life in the pond below from freezing. Heat capacity of water It takes a lot of heat to increase the temperature of liquid water because some of the heat must be used to break hydrogen bonds between the molecules. In other words, water has a high specific heat capacity, which is defined as the amount of heat needed to raise the temperature of one gram of a substance by one degree Celsius. The amount of heat needed to raise the temperature of 1 g water by 1 °C is has its own name, the calorie. Because of its high heat capacity, water can minimize changes in temperature. For instance, the specific heat capacity of water is about five times greater than that of sand. The land cools faster than the sea once the sun goes down, and the slow-cooling water can release heat to nearby land during the night. Heat of vaporization of water Just as it takes a lot of heat to increase the temperature of liquid water, it also takes an unusual amount of heat to vaporize a given amount of water, because hydrogen bonds must be broken in order for the molecules to fly off as gas. That is, water has a high heat of vaporization, the amount of energy needed to change one gram of a liquid substance to a gas at constant temperature. Water’s heat of vaporization is around 540 cal/g at 100 °C, water's boiling point. Note that some molecules of water – ones that happen to have high kinetic energy – will escape from the surface of the water even at lower temperatures. As water molecules evaporate, the surface they evaporate from gets cooler, a process called evaporative cooling.. In humans and other organisms, the evaporation of sweat, which is about 99% water, cools the body to maintain a steady temperature.