Properties of Water Chapter Notes

Cohesive and Adhesive Forces Cohesive forces are the forces of attraction between molecules of a single substance For water, the cohesive forces are hydrogen bonds Cohesive forces keep water molecules close to one another Adhesive forces are the forces of attraction between molecules of two different substances Adhesive forces are responsible for the attraction between water droplets and a glass surface

Surface Tension Surface tension is the elastic tendency found at the surface of a liquid Beneath the surface, each water molecule is attracted in every direction to neighboring water molecules because of hydrogen bonds There is no tendency to be pulled in any preferred direction—all equal However, a water molecule on the surface only has water molecules to either side and beneath it, but there are no water molecules above it; it feels a pull in all directions but there is no pull upwards Therefore, all of the molecules on the surface are pulled tightly and the surface behaves like a thin plastic film Lightweight objects that don’t pierce the surface will float

Surface Tension

Surface tension accounts for the spherical shape of raindrops, drops of oil, water droplets on a table top, etc. It is greater in water than in other liquids because of water’s hydrogen bonds—hydrogen bonds are relatively strong Surface tension is reduced by the addition of soap or detergent—the polar head of the soap molecule interferes with the hydrogen bonds of neighboring water molecules at the surface, therefore reducing the surface tension

Capillary Action Water is a polar molecule and therefore attracted to other polar molecules, like glass If you look at water in a graduated cylinder, it appears to “climb” up the sides—the curving of the water surface is called the meniscus Capillary action is the rise of liquid in a glass tube due to cohesive and adhesive forces

Capillary Action Adhesive forces initially cause a steep meniscus Cohesive forces respond by trying to minimize the surface area of the meniscus, and the water level rises Adhesive forces then cause another steep meniscus Cohesive forces then cause the meniscus to be “filled in” The cycle repeats itself until the upward adhesive force equals the weight of the raised water in the tube

Specific Heat Some foods stay hotter for much longer than others—for example, you may be able to eat a piece of toast as soon as it comes out of the toaster but you wait longer to eat a hot bowl of soup This is because different substances have different capacities for storing energy It takes joules of energy to raise the temperature of 1 gram of liquid water by 1°C—water has a very high specific heat (4.184 J/g·°C) Water’s high specific heat is caused by its hydrogen bonds It takes water a long time to heat up and cool down—think about when you go to the ocean in May, the air might feel warm, but the water is still cold

Specific Heat Substances with a low specific heat are able to heat up very quickly Metals, such as aluminum, iron, silver, and gold, have relatively low specific heats compared to water (0.90, 0.451, 0.24, and 0.13, respectively) Therefore we could heat up a block of gold more quickly than we could an equal volume of liquid water

Changing Phases At the surface of any body of water, water molecules are able to freely change between the liquid state and the gas state As molecules change from a liquid to a gas (evaporate), they take their kinetic energy with them—this causes the kinetic energy in the liquid to decrease and the liquid is cooled As water cools, the rate of evaporation slows down because fewer molecules have enough energy to escape the hydrogen bonds of the liquid phase Evaporation is a cooling process

Changing Phases At the surface of any body of water, condensation is also occurring (gas changing into a liquid) Condensation occurs as slow-moving water vapor molecules collide with and stick to the surface of a body of liquid water As gas molecules condense, energy is released as hydrogen bonds form The energy is absorbed by the liquid and the temperature increases Condensation is a warming process

Phase Changes Require Energy Changing from a solid to a liquid to a gas requires the input of energy The amount of heat energy needed to change a solid to a liquid is called the heat of melting The amount of heat energy needed to change a liquid to a gas is called the heat of vaporization Changing from a gas to a liquid to a solid results in a release of energy The amount of heat energy released when a liquid freezes is called the heat of freezing The amount of heat energy released when a gas condenses is called the heat of condensation

Pg , #11-16, 21-23