Water and Solutions A topic that’s “all wet”

Unique properties of water Polarity  O-H bond is polar – oxygen attracts electrons more strongly than hydrogen  Water molecule is asymmetric due to its bent shape, therefore the molecule is also polar

Unique properties of water Hydrogen bonds – strong attractions between the positive and negative ends of the water molecules Ice floats! Solid water is less dense than liquid water  Hydrogen bonds hold ice together in a crystal structure  In the crystal the water molecules are held farther apart than in liquid water Water is a liquid at RT!  Most other compounds of similar molar mass are gases Water has surface tension!  This causes water to bead up, to form a meniscus, and allows bugs to skate across the surface

Unique properties of water Water exhibits capillarity!  Capillarity is the tendency of a liquid to rise in a tube of narrow diameter Water has a high specific heat!  Specific heat (c P ) is the heat necessary to raise the temperature of one gram of stuff one degree Celsius.  Metals (conductors) have a low heat capacity (easy to heat up), water & other nonmetals have a high heat capacity (hard to heat)

Unique properties of water Stuffc P (J/gºC) Gold0.129 Iron0.450 Cement0.88 Wood1.76 Ethanol2.46 Water4.18 High specific heat is due to hydrogen bonds, which take a lot of energy to overcome

Unique properties of water Water acts as a heat buffer, which moderates temperature extremes on earth Water has a high  H v !   H v is the “heat of vaporization” – the heat needed to vaporize one gram of liquid at its boiling point Water – the universal solvent – it will dissolve a wide range of different materials

Solutions The dissolving process: Ionic substances Polar water molecules are attracted to the + and – ions in the crystal lattice

The dissolving process It takes several water molecules to break an ion free Free ions are solvated – surrounded by water molecules

Dissolving process The process of breaking up a salt crystal is called dissociation – chemical bonds are broken Polar covalent molecular substances  Dissolving does not require dissociation – the molecules remain intact glucose

Dissolving process The water molecules are attracted to the partially charged portions of the glucose molecule These interactions overcome the interactions of the crystal structure

Dissolving molecular substances

Dissolving molecular substances Concentration Nonpolar covalent substances are not soluble in water – weak interactions with water Solubility rule – like dissolves like Concentration Solutions consist of a solute and a solvent Concentrated solution – a lot of solute dissolved Dilute solution – little solute dissolved

Saturation Saturated solution – maximum amount of solute dissolved Unsaturated solution – less than the maximum amount dissolved Supersaturated solution – more than the maximum amount dissolved Made by slowly cooling a hot saturated solution Solute will crystallize if solution is disturbed

Solubility curve Temperature and solubility – generally, increasing temperature increases solubility – except for gases

Concentration Units of solubility – g/100g H 2 O Units of concentration – molarity Symbol – M M = moles solute liters solution Review – moles = mass/(molar mass), mass = molesx(molar mass)

Making solutions You need to make 2.3 liters of a 0.15 M solution of NaCl in water. How many g salt must you weigh? “Solution”: M = moles/liter 0.15M = xmol/2.3L x = 0.15x2.3 = 0.345mol molar mass of NaCl = 58.5g/mol mass = molesx(molar mass) 0.345mol x 58.5g/mol = g You would have to weigh about 20.2g NaCl, and dilute it to 2.3L with water.

Colligative properties Freezing point depression Freezing point of a solution is lower than that of the pure solvent Solute interferes with solvents’ ability to form crystals FP depression depends on concentration

Colligative properties If the solute is a salt, the effect is increased by the number of ions in the salt NaCl  Na + + Cl - (2 ions) CaCl 2  Ca Cl - (3 ions) Therefore calcium chloride causes the greater FP depression for each mole of salt. FP depression is the basis of de-icing compounds for aircraft, anti-freeze, and salts sold for melting ice on sidewalks.

Colligative properties Boiling point elevation Solutions have a higher boiling point than the pure solvent Solutes take up room at the surface of the solution, lowering the vapor pressure Lower vapor pressure means that more heat is needed to boil solution BP elevation is basis of anti-freeze and candy making

Colligative properties Osmosis Semi-permeable membrane – a thin film that allows only certain materials to pass (i.e. water but not salt) Water will move so as to dilute the solute

Colligative properties

Colloids Consist of particles permanently suspended in a medium Particles are times as large as those in a solution Tyndall effect – colloidal particles scatter light Examples – smoke in air, fog/smog, soapy water, blood, milk, toothpaste