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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.
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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.
