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 (cP) 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 Stuff cP (J/gºC) Gold 0.129 Iron 0.450 Cement 0.88 Wood 1.76 Ethanol 2.46 Water 4.18 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 Hv! Hv 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 substancesConcentration 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 H2O 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 = 20.1825g 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) CaCl2 Ca+2 + 2Cl- (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 10-100 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