Chapter 13.1 Objectives Demonstrate the uniqueness of water as a chemical substance. Demonstrate the uniqueness of water as a chemical substance. Model the 3 dimensional geometry of a water molecule. Model the 3 dimensional geometry of a water molecule. Relate the physical properties of water to the molecular level. Relate the physical properties of water to the molecular level.

Chapter 13.1: Uniquely Water Water may be a common substance on Earth, but its properties are anything but common. Those properties make water essential for life. Water may be a common substance on Earth, but its properties are anything but common. Those properties make water essential for life.

The Structure of Water A molecule of H 2 O (water) is just 2 hydrogen atoms and 1 oxygen atom linked by covalent bonds. A molecule of H 2 O (water) is just 2 hydrogen atoms and 1 oxygen atom linked by covalent bonds. The electron locations and 3-D shape of the water molecule are the source of water’s unique properties. The electron locations and 3-D shape of the water molecule are the source of water’s unique properties.

Recall the Lewis structure of water. Recall the Lewis structure of water. There is a large electro-negativity difference between the covalently bonded hydrogen and oxygen. There is a large electro-negativity difference between the covalently bonded hydrogen and oxygen. Thus the electron pair between them is shared unequally, with the electrons spending more time around the oxygen atom. Thus the electron pair between them is shared unequally, with the electrons spending more time around the oxygen atom. This makes water a This makes water a polar molecule. H – O - H Water’s 3-d shape is referred to as “bent”.

Polarity Polarity One end of a polar molecule has a + charge and the other end has a – charge. For water, the H ends are + and the oxygen ends are -. One end of a polar molecule has a + charge and the other end has a – charge. For water, the H ends are + and the oxygen ends are -. This means there are intermolecular forces – like attraction and repulsion – between water molecules. This means there are intermolecular forces – like attraction and repulsion – between water molecules. The attraction between the H of one molecule and The attraction between the H of one molecule and an O of another molecule is called a hydrogen an O of another molecule is called a hydrogen bond. bond.

Polarity Because of hydrogen bonding, water is a liquid at room temperature. Similarly-sized nonpolar molecules are gases. Because of hydrogen bonding, water is a liquid at room temperature. Similarly-sized nonpolar molecules are gases. Water has a high boiling point and high melting point because of the attraction between water molecules (H bonding). Water has a high boiling point and high melting point because of the attraction between water molecules (H bonding). Many of waters physical characteristics are due to the hydrogen bonding that occurs between water molecules. Many of waters physical characteristics are due to the hydrogen bonding that occurs between water molecules.

Other Properties: Density Water is less dense as a solid than as a liquid. That is why ice floats rather than sinking. Water is less dense as a solid than as a liquid. That is why ice floats rather than sinking. As water cools its volume decreases (like most substances) because the molecules draw closer together. As water cools its volume decreases (like most substances) because the molecules draw closer together. However... However...

Ice Floats However, at about 4 0 However, at about 4 0 C, the water molecules move apart due to H bonding and form an organized, stable arrangement called a crystal. The density of solid water, because the molecules have moved apart somewhat, is therefore LESS than that of liquid water.

Since liquid water expands when it freezes, frozen water pipes can break. Since liquid water expands when it freezes, frozen water pipes can break. The forces involved in the expansion of freezing water are surprisingly great. The forces involved in the expansion of freezing water are surprisingly great. The processes of freezing and thawing can eventually break up big boulders, contributing to soil formation and erosion on the surface of the Earth. The processes of freezing and thawing can eventually break up big boulders, contributing to soil formation and erosion on the surface of the Earth.

Surface Tension Have you ever watched water drip from a faucet? Each drop is composed of an enormous number of water molecules, roughly 2 x Have you ever watched water drip from a faucet? Each drop is composed of an enormous number of water molecules, roughly 2 x The observation that this large number of molecules can hold together as a single drop is more evidence of H bonding. The observation that this large number of molecules can hold together as a single drop is more evidence of H bonding.

Water molecules form drops because of surface tension. Water molecules form drops because of surface tension. Surface tension is the force needed to overcome intermolecular attractions and break through the surface of a liquid or spread the liquid out. Surface tension is the force needed to overcome intermolecular attractions and break through the surface of a liquid or spread the liquid out. Water has a HIGH surface tension so it is resistant to having its surface broken. Water has a HIGH surface tension so it is resistant to having its surface broken. Surface Tension

In a drop of water, water molecules at the surface are not surrounded on top by other molecules so there is a net force pulling inward on these surface molecules. In a drop of water, water molecules at the surface are not surrounded on top by other molecules so there is a net force pulling inward on these surface molecules. This inward pull makes the water’s surface behave like a kind of skin. It is “tough” to break through. This inward pull makes the water’s surface behave like a kind of skin. It is “tough” to break through.

Surface Tension in Everyday Life

Cohesion So, due to H bonding, water molecules will essentially “stick” to other water molecules. So, due to H bonding, water molecules will essentially “stick” to other water molecules. This property is called cohesion. This property is called cohesion.

Adhesion Water will also “stick” to surfaces. This is because of the attractive forces between the water molecules and the surface. Water will also “stick” to surfaces. This is because of the attractive forces between the water molecules and the surface. This property is called adhesion. This property is called adhesion.

Capillarity Adhesion explains capillarity. Adhesion explains capillarity. Capillarity or capillary action is the movement of water through a narrow tube against the force of gravity. Capillarity or capillary action is the movement of water through a narrow tube against the force of gravity. Water molecules in the tube form H bonds with the O atoms in the SiO 2 of the glass. Water molecules in the tube form H bonds with the O atoms in the SiO 2 of the glass. This draws the water up the sides of the tube. This draws the water up the sides of the tube.

Specific Heat Water has a very high specific heat. This means A LOT of energy must be added to a sample of water before the temperature of the water will rise. Water has a very high specific heat. This means A LOT of energy must be added to a sample of water before the temperature of the water will rise. At the beginning of the summer, the ocean water temperature is low. It takes a lot of sunlight to raise the temperature. At the beginning of the summer, the ocean water temperature is low. It takes a lot of sunlight to raise the temperature. Once warmed, however, it will cool off very slowly when the outdoor temperature once again drops. This is because the water has a great deal of stored energy. Once warmed, however, it will cool off very slowly when the outdoor temperature once again drops. This is because the water has a great deal of stored energy.

Heat of Vaporization The heat of vaporization of water is the amount of energy that must be added to cause the water to vaporize (change to gas). The heat of vaporization of water is the amount of energy that must be added to cause the water to vaporize (change to gas). Because of H bonding, water has a high specific heat and a high heat of vaporization. Because of H bonding, water has a high specific heat and a high heat of vaporization.

Heat of Vaporization This property of water is extremely important in maintenance of body temperature. This property of water is extremely important in maintenance of body temperature. The vaporization of water from our body surface requires heat. The heat is removed from the surface of the skin when our sweat evaporates. The vaporization of water from our body surface requires heat. The heat is removed from the surface of the skin when our sweat evaporates. Perspiration cools us down. Perspiration cools us down.

The Universal Solvent Water is an excellent solvent. Water is an excellent solvent. The + and – charges on the surface of the water molecule attract other polar molecules and “pull” them into solution. Polar substances dissolve readily in water. The + and – charges on the surface of the water molecule attract other polar molecules and “pull” them into solution. Polar substances dissolve readily in water. Almost all of the reactions necessary to maintain life take place in aqueous solutions, solutions with water as the solvent. Almost all of the reactions necessary to maintain life take place in aqueous solutions, solutions with water as the solvent.