CHAPTER 9 Water and Solutions 9.1 Solutes, Solvents, and Water

What is a solution? Is vinegar a solution? Is milk a solution? Is marble a solution?

Copper sulfate solution A true solution is homogeneous on the molecular level Solid copper sulfate (CuSO4) Copper sulfate solution In solution the copper ions (Cu2+) dissociate from the sulfate ions (SO42–)

All solutions contain one solvent

All solutions contain one solvent and at least one solute dissolved: term used to describe when molecules of solute are completely separated from each other and dispersed into a solution.

Special case of water Most of your body is water! Water is an important solvent with many unusual properties

Special case of water Water is a small, polar molecule

Special case of water Water is a small, polar molecule There is a strong attraction among water molecules due to hydrogen bonding

Special case of water Hydrogen bonding leads to interesting properties in water Substances are generally denser in the solid phase than in the liquid phase. Water is different Why ice floats in water

Special case of water Hydrogen bonding leads to interesting properties in water Substances are generally denser in the solid phase than in the liquid phase. Water is different In ice, hydrogen bonds force water molecules to align in a crystal structure where molecules are farther apart than they are in a liquid. Why ice floats in water

Special case of water Hydrogen bonding leads to interesting properties in water Why ice floats in water

Why are the boiling points so different? Special case of water Water H2O 18 g/mole Methane CH4 16 g/mole Boils at +100oC Boils at –161oC Why are the boiling points so different?

Special case of water Hydrogen bonding leads to interesting properties in water Water H2O 18 g/mole Methane CH4 16 g/mole polar nonpolar Boils at +100oC Boils at –161oC

Special case of water Hydrogen bonding leads to interesting properties in water Water H2O 18 g/mole Methane CH4 16 g/mole polar nonpolar Boils at +100oC Boils at –161oC Hydrogen bonds keep water molecules together, preventing them from separating easily. Without these strong intermolecular forces, methane is a gas at room temperature

H-bonds keep the water molecules together Special case of water Why a drop of water doesn’t “lie flat” on a hard surface: H-bonds keep the water molecules together In reality water molecules are much, much smaller than on this drawing! surface tension: a force acting to pull a liquid surface into the smallest possible area.

Special case of water Hydrogen bonding leads to interesting properties in water Surface tension can make a paper clip float on water

Dissolves many molecular compounds Water as a solvent Water is often called the “universal solvent” Dissolves ionic compounds Dissolves many molecular compounds

Water as a solvent Not chemically bonded hydration: the process of molecules with any charge separation to collect water molecules around them.

Water as a solvent A glucose molecule has a total charge of zero, but it has regions of positive and negative charge separation. Not chemically bonded

Are there different “kinds” of water? Distilled water Deionized water Tap water

Tap water contains dissolved salts and minerals. Distilled water and deionized water have been processed to remove dissolved salts and minerals. Deionization is a specific filtration process to remove all ions. Distillation boils water to steam which is then condensed back to liquid water

How do we know that tap water contains ions and distilled water does not? Light bulb goes off!

Reactions in solids Chemical reactions in solids do occur, but they are slow It takes thousands of years for limestone to become marble, under heat and pressure in the Earth High density Low mobility

Reactions in gases Chemical reactions in gases do occur, and happen quickly Low density High mobility

Reactions in liquids Chemical reactions in liquids occur easily High density High mobility Chemical reactions in liquids occur easily

Reactions in liquids Life involves many complex chemical reactions that only occur in aqueous solutions! A step in the Krebs cycle – this is how energy is extracted from glucose

Not everything dissolves in water. Why not?

Not everything dissolves in water. Why not? In general, “like” dissolves “like” Polar solvents dissolve polar solutes Nonpolar solvents dissolve nonpolar solutes

Polar solvents dissolve polar solutes Hexane can dissolve oil paints because both are nonpolar. In general, “like” dissolves “like” Polar solvents dissolve polar solutes Nonpolar solvents dissolve nonpolar solutes

Water is often called the “universal solvent”: Dissolving ionic salts Dissolving molecular compounds In general, “like” dissolves “like” Polar solvents dissolve polar solutes Nonpolar solvents dissolve nonpolar solutes