Properties of Water

Properties of Water Looking back at Earth from space, an astronaut called it “the blue planet,” referring to the oceans of water that cover nearly three fourths of Earth’s surface. The very presence of liquid water tells a scientist that life may also be present on such a planet. Why should life itself be connected so strongly to something so ordinary that we often take it for granted? There is something very special about water and the role it plays in living things. The Water Molecule Water is one of the few compounds found in a liquid state over most of Earth’s surface. Like other molecules, water (𝐻2 𝑂) is neutral. The positive charges on its 10 protons balance out the negative charges on its 10 electrons. Polarity Because of the angles of its chemical bonds, the oxygen atom is on one end of the molecule and the hydrogen atoms are on the other. With 8 protons in its nucleus, an oxygen atom has a much stronger attraction for electrons than does a hydrogen atom with its single proton. There is a greater probability of finding the shared electrons in water close to its oxygen atom than near its hydrogen atoms. As a result, the oxygen end of the molecule has a slight negative charge and the hydrogen end of the molecule has a slight positive charge. A molecule in which the charges are unevenly distributed is said to be “polar,” because the molecule is a bit like a magnet with two poles. The charges on a polar molecule are written in parentheses, (–) or (+), to show that they are weaker than the charges on ions such as 𝑁𝑎+ and 𝐶𝑙 – Hydrogen Bonding Because of their partial positive and negative charges, polar molecules such as water can attract each other. The attraction between a hydrogen atom on one water molecule and the oxygen atom on another is known as a hydrogen bond. Water is able to form multiple hydrogen bonds, which account for many of its special properties. Hydrogen bonds are not as strong as covalent or ionic bonds, and they can form in other compounds besides water. Cohesion Cohesion is an attraction between molecules of the same substance. Because a single water molecule may be involved in as many as four hydrogen bonds at the same time, water is extremely cohesive. Cohesion causes water molecules to be drawn together, which is why drops of water form beads on a smooth surface. Cohesion also produces surface tension, explaining why some insects and spiders can walk on a pond’s surface. Adhesion Adhesion is an attraction between molecules of different substances. The surface of water in a graduated cylinder dips slightly in the center, forming a curve called a meniscus, because the adhesion between water molecules and glass molecules is stronger than the cohesion between water molecules. Adhesion between water and glass also causes water to rise in a narrow tube against the force of gravity. This effect is called capillary action. Capillary action is one of the forces that draws water out of the roots of a plant and up into its stems and leaves. Cohesion holds the column of water together as it rises. Heat Capacity Because of the multiple hydrogen bonds between water molecules, it takes a large amount of heat energy to cause those molecules to move faster and raise the temperature of the water. Water’s heat capacity, the amount of heat energy required to increase its temperature, is relatively high. Large bodies of water, such as oceans and lakes, can absorb large amounts of heat with only small changes in temperature. This protects organisms living within from drastic changes in temperature. At the cellular level, water absorbs the heat produced by cell processes, regulating the temperature of the cell. Solutions and Suspensions • • • How does water’s polarity influence its properties as a solvent? How does water’s polarity influence its properties as a solvent? Water’s polarity gives it the ability to dissolve both ionic compounds and other polar molecules. Water is not always pure; it is often found as part of a mixture. A mixture is a material composed of two or more elements or compounds that are physically mixed together but not chemically combined. Living things are in part composed of mixtures involving water. Two types of mixtures that can be made with water are solutions and suspensions. Solutions If a crystal of table salt is placed in water, sodium and chloride ions on the surface of the crystal are attracted to the polar water molecules. Ions break away from the crystal and are surrounded by water molecules. The ions gradually become dispersed in the water, forming a type of mixture called a solution. All the components of a solution are evenly distributed throughout the solution. In a saltwater solution, table salt is the solute—the substance that is dissolved. Water is the solvent—the substance in which the solute dissolves. Water’s polarity gives it the ability to dissolve both ionic compounds and other polar molecules. Water easily dissolves salts, sugars, minerals, gases, and even other solvents such as alcohol. When a given amount of water has dissolved all of the solute it can, the solution is said to be saturated. Suspensions Some materials do not dissolve when placed in water, but separate into pieces so small that they do not settle out. Such mixtures of water and no dissolved material are known as suspensions. Some of the most important biological fluids are both solutions and suspensions. Blood is mostly water. It contains many dissolved compounds, but also cells and other undissolved particles that remain in suspension as the blood moves through the body. Acids, Bases, and pH • • Why is it important for cells to buffer solutions against rapid changes in pH? Buffers dissolved in life’s fluids play an important role in maintaining homeostasis in organisms. Water molecules sometimes split apart to form hydrogen ions and hydroxide ions. This reaction can be summarized by a chemical equation in which double arrows are used to show that the reaction can occur in either direction. In pure water, about 1 water molecule in 550 million splits to form ions in this way. Because the number of positive hydrogen ions produced is equal to the number of negative hydroxide ions produced, pure water is neutral. The pH Scale Chemists devised a measurement system called the pH scale to indicate the concentration of H+ ions in solution. The pH scale ranges from 0 to 14. At a pH of 7, the concentration of H+ ions and OH– ions is equal. Pure water has a pH of 7. Solutions with a pH below 7 are called acidic because they have more 𝑂𝐻 + ions than 𝑂𝐻 – ions. The lower the pH, the greater the acidity. Solutions with a pH above 7 are called basic because they have more OH– ions than H+ ions. The higher the pH, the more basic the solution. Each step on the pH scale represents a factor of 10. For example, a liter of a solution with a pH of 4 has 10 times as many H+ ions as a liter of a solution with a pH of 5. Acids An acid is any compound that forms H+ ions in solution. Acidic solutions contain higher concentrations of H+ ions than pure water and have pH values below 7. Strong acids tend to have pH values that range from 1 to 3. Hydrochloric acid (HCl) is a strong acid produced by the stomach to help digest food. Bases A base is a compound that produces hydroxide (OH–) ions in solution. Basic, or alkaline, solutions contain lower concentrations of H+ ions than pure water and have pH values above 7. Strong bases, such as the lye (commonly NaOH) used in soapmaking, tend to have pH values ranging from 11 to 14. Buffers The pH of the fluids within most cells in the human body must generally be kept between 6.5 and 7.5 in order to maintain homeostasis. If the pH is lower or higher, it will affect the chemical reactions that take place within the cells. One of the ways that organisms control pH is through dissolved compounds called buffers, which are weak acids or bases that can react with strong acids or bases to prevent sharp, sudden changes in pH. Adding acid to an unbuffered solution causes the pH of the unbuffered solution to drop. If the solution contains a buffer, however, adding the acid will cause only a slight change in pH.