Water and Its Properties Prentice-Hall Chapter 15.1 Dr. Yager

Objectives Identify how properties of water are related to the structure of the water molecule. Explain how hydrogen bonds determine the high surface tension and low vapor pressure of water . Identify the factors that determine the structure of water in the solid state (ice).

Water in the Liquid State 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.

Water is a Polar Molecule

Polar molecules are attracted to one another by dipole interactions 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 attractions among water molecules results in the formation of hydrogen bonds.

Key Idea: Many unique and important properties of water – including high surface tension and low vapor pressure – result from hydrogen bonding.

Surface Tension Surface tension is the inward force, or pull, that tends to minimize the surface area of a liquid. All liquids have a surface tension, but water’s surface tension is higher than most.

Surface tension makes it possible for this water strider to walk on water.

Surfactant A surfactant is any substance that interferes with the hydrogen bonding between water molecules and thereby reduces surface tension. Soap is an example of a surfactant.

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.

Water in the Solid State 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.

Hydrogen bonds hold the water molecules in place in the solid phase.

Water and Ice

1. Many of the unusual properties of water are the result of hydrogen bonding. non-polar molecules. low molar mass. dispersion forces.

1. Many of the unusual properties of water are the result of hydrogen bonding. non-polar molecules. low molar mass. dispersion forces.

2. A surfactant causes water to spread out over a surface 2. A surfactant causes water to spread out over a surface. This spreading occurs because the surfactant attaches to the surface. interferes with hydrogen bonding. lowers the vapor pressure. lowers the density of water.

2. A surfactant causes water to spread out over a surface 2. A surfactant causes water to spread out over a surface. This spreading occurs because the surfactant attaches to the surface. interferes with hydrogen bonding. lowers the vapor pressure. lowers the density of water.

3. Ice is less dense than liquid water because in ice the water molecules expand because of weakened covalent bonds. have a regular open honeycomb framework. expand because of weakened hydrogen bonds. have a more disorderly arrangement with lower density.

3. Ice is less dense than liquid water because in ice the water molecules expand because of weakened covalent bonds. have a regular open honeycomb framework. expand because of weakened hydrogen bonds. have a more disorderly arrangement with lower density.