Chapter 12 Water

Water Molecules Water’s bent shape and ability to hydrogen bond gives water many special properties. Water molecules are attracted to one another. This gives water high surface tension, low vapor pressure, high specific heat, high heat of vaporization, and high boiling point

Surface Tension liquid water acts like it has a skin. Water forms round drops. All because water hydrogen bonds.

Surface Tension d- d+ d+ d- d+ One water molecule H bonds to another. Can H bond to molecules all around. H O d+ d+ d- H O d+

Surface Tension A water molecule in the middle of solution is pulled in all directions.

Surface Tension Not true at the surface. Only pulled down and to each side. Holds the molecules together. Causes surface tension.

Surface Tension Water drops are round because all the molecules on the edge are pulled to the middle.

Surface Tension Glass has polar molecules. Glass can hydrogen bond. Attracts the water molecules. Some of the pull is up.

Meniscus Water curves up along the side. This makes the meniscus.

Meniscus In Plastic In Glass

Heat Capacity Water has a high heat capacity (same as specific heat). It absorbs 1 cal/gºC while iron absorbs only 0.107 cal/gºC. Remember SH = heat Mass x DT Calculate the heat need to raise the temperature of both iron and water by 75ºC.

Heat of vaporization Because of the strong hydrogen bonds it takes a large amount of energy to change water from a liquid to a solid. 540 cal/g is the heat of vaporization. It takes this much energy to boil water. You get this much energy back when it condenses. Steam burns, but heats things well.

Ice Most liquids contract (get smaller) as they are cooled. They get more dense. When they change to solid they are more dense than the liquid. Solid metals sink in liquid metal. Ice floats in water. Why?

Ice Water becomes more dense as it cools until it reaches 4ºC. Then it becomes less dense. As the molecules slow down they arrange themselves into honeycomb shaped crystals. These are held together by H-bonds.

Liquid H O H O H O H O H O H O H O Solid H O H O H O H O H O

Ice 10% less dense than water. Water freezes from the top down. It takes a great deal of energy to turn solid water to liquid water. Heat of fusion is 80 cal/g.

Aqueous Solutions Solution - a homogenous mixture mixed molecule by molecule. Solvent - the stuff that does the dissolving. Solute -the stuff that is dissolved. Aqueous solution- a solution with water as the solvent.

Aqueous Solutions Water dissolves ionic solids and polar covalent solids best. The rule is “like dissolves like” Polar dissolves polar. Nonpolar dissolves nonpolar. Oil is non polar. Oil and water don’t mix. Salt is ionic- salt water.

How Ionic solids dissolve Called solvation. Water breaks the + and - charged pieces apart and surround them.

How Ionic solids dissolve

Solids will dissolve if the attractive force of the water molecules is stronger than the attractive force of the crystal. If not the solids are insoluble. Water doesn’t dissolve nonpolar molecules because it the water molecules can’t hold onto them. The water molecule hold onto each other and separate from the polar molecules.

Hydration Water molecules chemically bonded to solid salt molecules. These compounds have fixed amounts of water. The water can be driven off by heating. CuSO4.5H2O CuSO4 + 5H2O Called copper(II)sulfate pentahydrate. - heat + heat

Electrolytes Substances that conduct electricity when dissolved in water. Must have charged particles that can move. Ionic compounds break into charged ions. NaCl ® Na+ + Cl- These ions can conduct electricity.

Non electrolytes do not conduct electricity when dissolved in water. Polar covalent molecules such as methanol CH3OH don’t fall apart into ions when they dissolve. Weak electrolytes don’t fall completely apart into ions. Strong electrolytes do ionizes completely.

Mixtures that are NOT Solutions Suspensions are mixtures that slowly settle upon standing. Particles of a suspension are more than 100 times bigger. Can be separated by filtering. Colloids particles are between the size of a solid and that of a liquid. Emulsions are colloids of liquids in liquids.