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Dive into the unique properties of water and its significance for life on Earth. Understand the polarity, hydrogen bonding, surface tension, and capillary action of water. Discover why ice floats and the role of hydrogen bonds.
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Chapter 15: Water & Aqueous Systems Chemistry
15.1 - Water and Its Properties Introduction • Water covers about three quarters of Earth’s surface. • All life forms that are known to exist are made mostly of water. • You will learn about the properties of water and what makes this unique substance essential to life on Earth. • You could not live without water, nor could all the plants and animals on Earth.
1. Water in the Liquid State • Recall that water is a simple triatomic molecule. • The oxygen atom forms a covalent bond with each of the hydrogen atoms. • Because of its greater electronegativity, oxygen attracts the electron pair of the covalent O-H bond to a greater extent than hydrogen • Therefore oxygen has a partial negative charge and the hydrogen atoms acquire a partial positive charge. • Thus, the O-H bonds of a water molecule are highly polar.
Water in the Liquid State (cont.) • The shape of the molecule is the determining factor in how the polarities of the two O-H bonds affect the polarity of the molecule. • The bond angle of the water molecule is approximately 105⁰, which gives the molecule a bent shape. • The two O-H bond polarities do not cancel, so the water molecule as a whole is polar.
15.1 Water in the Liquid State (cont.) • Polar molecules are attracted to one another by dipole interactions. • The negative end of one molecule attracts the positive end of another molecule. • The intermolecular attraction among water molecules results in the formation of hydrogen bonds. • Many unique and important properties of water—including its high surface tension and low vapor pressure—result from hydrogen bonding.
15.1 Water in the Liquid State (cont.) • Surface Tension • The molecules within a body of water form hydrogen bonds with other molecules that surround them on all sides. • The attractive forces on each of these molecules are balanced. • However, water molecules at the surface experience an unbalanced attraction. • As a result, water molecules at the surface tend to be drawn inward. • The inward force, or pull, that tends to minimize the surface area of a liquid is called surface tension. Surface tension makes it possible for this water strider to walk on water.
Water in the Liquid State (cont.) • All liquids have a surface tension, but water’s surface tension is higher than most. • The surface tension of water tend to hold a drop of liquid in a spherical shape. • The water forms into round drops because spheres are the shape with the least amount of surface area for a given volume of liquid. • This is why, on some surfaces, water tends to bead up rather than spread out. • It is possible to decrease the surface tension of water by adding a surfactant. • A surfactant is any substance that interferes with the hydrogen bonding between water molecules and thereby reduces surface tension. • Soaps and detergents are surfactants.
Water in the Liquid State (cont.) • Capillary Action • Cohesion is the force of attraction between identical molecules. • Adhesion is the force of attraction between molecules that are different. • The surface of the water in a graduated cylinder is concave because the water molecules are more strongly attracted to the silicon dioxide in glass (adhesion) than to other water molecules (cohesion) • Capillary action is the upward movement of liquid into a narrow cylinder, or capillary tube.
15.1 Water in the Liquid State (cont.) • Vapor Pressure • Hydrogen bonding between water molecules also explains water’s unusually low vapor pressure. • Because hydrogen bonds hold water molecules to one another, the tendency of these molecules to escape is low, and evaporation is slow.
2. Water in the Solid State • Ice cubes float in water because solid water has a lower density that liquid water, which is not usual for liquids. • As a typical liquid cools, it begins to contract and its density increases gradually. • As water begins to cool, it behaves initially like a typical liquid. • It contracts slightly and its density gradually increases. • When the temperature of the water falls below 4˚C, the density of water starts to decrease. • Maximum density of water is 1.000 g/cm3
15.1 Water in the Solid State (cont.) • Hydrogen bonds hold the water molecules in place in the solid phase. • The structure of ice is a regular open framework of water molecules arranged like a honeycomb. • When ice melts, the framework collapses, and the water molecules pack closer together, making liquid water more dense than ice.
