Water

Aqueous Systems and Solutions

Water Simple, triatomic molecule Polar covalent bonds Polar molecule Hydrogen bond(F-H, O-H, N-H)

– High surface tension– Low vapor pressure– High specific heat– High heat of vaporization– High boiling point

Surface Properties High surface tension Cohesive forces caused by hydrogen bonding The inward force or pull which tends to

minimize the surface area of a liquid is surface tension.

Allow some objects to float(needles,bugs) May be decreased by use of a wetting agent

such as soap or detergent(surfactant)

Aqueous Solutions Water is not found pure because it is one of

the best solvents known. In a solution the dissolving medium is the

solvent and the dissolved particles are the solute.

True solutions have several properties:– Homogeneous– Do not settle– Particles are ionic or molecular– They cannot be filtered.

Solvation

In some ionic compounds the attractive forces within the crystals are stronger than the attractive forces exerted by the solvent. That kind of solid does not dissolve well. (insoluble)

The general rule is like dissolves like but IMF play a role also.

Which of the following will dissolve in water?

Hydrochloric acid Sodium iodide Ammonia Magnesium sulfate Methane Gasoline Grease

Water of Hydration The water in a crystal is called the water of

hydration or water of crystallization. Copper (II) sulfate pentahydrate is an

example. Water can be driven from the crystals by

gentle heating. If the hydrate has a vapor pressure higher

than atmospheric pressure then the crystal will effloresce or lose its water of hydration.

If the compound has a very low vapor pressure then it will remove moisture from the air. It is deliquescent.

Water of Hydration

If they only draw small quantities of water from the air, they are hygroscopic.

Hygroscopic substances are used to dry materials and are called dessicants.

Suspensions and Colloids Suspensions are mixtures from which some

particles will settle upon standing. – Its particles are larger(>100 nm) than those

of a true solution(<1 nm)– Its particles can be trapped by filtration.

Colloids are mixtures containing particles (1 nm-100 nm) that are intermediate in size. – They are usually milky or cloudy.– They cannot be filtered nor will they settle.– They exhibit the Tyndall Effect, which is the

scattering of visible light in all directions.

Suspensions and Colloids

The chaotic movement of colloidal particles is called Brownian motion.

For a summary of solutions, colloids, and suspensions see page 363.

Emulsions are colloidal dispersions of liquids in liquids. Soaps and detergents are emulsifying agents.

Electrolytes and Nonelectrolytes

Electrolytes are substances which conduct electric current.

These substances may be aqueous or molten.

Electrolytes may be strong or weak. A strong electrolyte has many ions in solution.

Factors Affecting Rate of Dissolution

Surface area of soluteAgitationHeating

Solubility

Defined as the number of grams of solute per 100g of solvent at a given temperature

Saturated solution – a solution that contains as much solute as possible at that temperature.

Unsaturated solution – a solution that contains less solute than the maximum at that temperature.

Supersaturated solution – a solution that contains more than the maximum at that temperature.

Heat Capacity of Water

1 calorie per gram per oC. Quantity of energy needed to raise 1.00

g of a substance 1 oC Cp

q = mCpt

Vaporization of Water

Heat of vaporization of water = 540 cal/g. Quantity of energy needed to vaporize 1.00

gram of substance at its normal boiling point. Compare water(540cal/g, gfm = 18, 100oC),

ammonia(327 cal/g, gfm = 17,

-33oC), and methane(122 cal/g, gfm = 16, bp)

Ice

Water has its greatest density at 4oC. Phase diagrams show transitions of

substances very concisely.