Ch. 15: Water and Aqueous Systems15.1 Water and Its Properties

surface tension, vapor pressure, hydrogen bonding15.2 Homogenous Aqueous Systems

aqueous solution, solvent, solvation, electrolyte, hydrate15.3 Heterogenous Aqueous Systems

suspension, colloid, Tyndall effect, (Brownian motion, coagulation, emulsion)

Water covers about three quarters of Earth’s surface. All life forms that are known to exist are made mostly of water. You will learn about the properties of water and what makes this unique substance essential to life on Earth.

15.1 Water and Its Properties

• Water in the Liquid State

How can you account for the high surface tension and low vapor pressure of water?

15.1

You could not live without water, nor could all the plants and animals on Earth.

Many unique and important properties of water—including its high surface tension and low vapor pressure—result from hydrogen bonding.

Water in the Liquid State

• A water molecule is polar.

15.1

Water in the Liquid StatePolar molecules are attracted to one another by dipole interactions. The negative end of one molecule attracts the positive end of another molecule.

15.1

The intermolecular attraction among water molecules results in the formation of hydrogen bonds.

Water in the Liquid State

Surface Tension– The inward force, or pull, that tends to minimize the

surface area of a liquid is called surface tension.

– All liquids have a surface tension, but water’s surface tension is higher than most.

15.1

Surface tension makes it possible for this water strider to walk on water.

Water in the Liquid State

• A surfactant is any substance that interferes with the hydrogen bonding between water molecules and thereby reduces surface tension. (examples are soaps and detergents)

Vapor Pressure• Hydrogen bonding between water molecules also explains

water’s unusually low vapor pressure. Because hydrogen bonds hold water molecules to one another, the tendency of these molecules to escape is low, and evaporation is slow.

15.1

Water in the Solid State

• Water in the Solid State

15.1

As water begins to cool, it behaves initially like a typical liquid. It contracts slightly and its density gradually increases. When the temperature of the water falls below 4˚C, the density of water starts to decrease.

Water in the Solid StateHydrogen bonds hold the water molecules in place in the solid phase.

15.1

The structure of ice is a regular open framework of water molecules arranged like a honeycomb.

When ice melts, the framework collapses, and the water molecules pack closer together, making liquid water more dense than ice.

1.Many of the unusual properties of water are the result of

a) hydrogen bonding.

b) nonpolar molecules.

c) low molar mass.

d) dispersion forces.

15.1 Section Quiz.

2. A surfactant causes water to spread out over a surface. This spreading occurs because the surfactant

a) attaches to the surface.

b) interferes with hydrogen bonding.

c) lowers the vapor pressure.

d) lowers the density of water.

15.1 Section Quiz.

3. Ice is less dense than liquid water because in ice the water molecules

a) expand because of weakened covalent bonds.

b) have a regular open honeycomb framework.

c) expand because of weakened hydrogen bonds.

d) have a more disorderly arrangement with lower density.

15.1 Section Quiz.

15.2 Homogeneous Aqueous Systems

An ordinary dill pickle from the deli can be a source of light! Iron or copper electrodes are inserted into the ends of the pickle and connected to a source of alternating electric current. You will learn what kind of solution conducts electricity.

15.2

Solvents and SolutesAn aqueous solution is water that contains dissolved

substances.In a solution, the dissolving medium is the solvent.In a solution, the dissolved particles are the solute.

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If you filter a solution through filter paper, both the solute and the solvent pass through the filter.

Water is the universal solvent!

The Solution ProcessAs individual solute ions break away from the crystal, the negatively and positively charged ions become surrounded by solvent molecules, and the ionic crystal dissolves.

The process by which the positive and negative ions of an ionic solid become surrounded by solvent molecules is called solvation.

Polar solvents such as water dissolve ionic compounds and polar compounds. Nonpolar solvents such as gasoline dissolve nonpolar compounds.

“Like dissolves like” —water is polar!

15.2

The Solution Process

Solvation of an Ionic Solid

15.2

Oil (nonpolar) and water (polar) do not mix.

Electrolytes and Nonelectrolytes

An electrolyte is a compound that conducts an electric current when it is in an aqueous solution or in the molten state.

All ionic compounds are electrolytesbecause they dissociate into ions.

15.2

The bright glow shows that sodium chloride is a strong electrolyte because nearly all the dissolved sodium chloride exists as separate Na+ and Cl– ions.

Electrolytes and Nonelectrolytes

A weak electrolyte conducts electricity poorly because only a fraction of the solute in the solution exists as ions.

15.2

A compound that does not conduct an electric current in either aqueous solution or the molten state is called a nonelectrolyte.

A compound that contains water of hydration is called a hydrate.

– In writing the formula of a hydrate, use a dot to connect the formula of the compound and the number of water molecules per formula unit.

Hydrates15.2

A sample of blue CuSO4·5H2O is heated.

Blue crystals of CuSO4·5H2O crumble to a white anhydrous powder that has the

formula CuSO4.

HydratesThe deliquescent (removes sufficient water from the air to dissolve completely and form solutions) substance, sodium hydroxide, absorbs moisture from air.

15.2

Example: Trial by Fire Lab

1. Choose the correct words for the spaces. In any solution, the material that is dissolved is called the _______ and the material in which the substance is dissolved is called the _______ .

a) solid, liquid

b) solvent, solute

c) solute, solvent

d) crystal, disperser

15.2 Section Quiz.

2. In the solution process, the a) solvent molecules are surrounded by positive and

negative ions.

b) solvent molecules become dispersed in an ionic crystal.

c) ions become emulsified in the solvent.

d) positive and negative ions are surrounded by solvent molecules.

15.2 Section Quiz.

3. All ionic compounds are electrolytes because they a) conduct electricity in the solid state.

b) dissociate into ions in the aqueous and/or the molten state.

c) remain as positive-negative pairs even when dissolved.

d) only conduct electricity in the molten state.

15.2 Section Quiz.

4. The formula for iron(II) sulfate heptahydrate isa) FeSO4(H2O)7

b) FeSO4•(H2O)7

c) Fe•2SO4•7H2O

d) FeSO4•7H2O

15.2 Section Quiz.

15.3

15.3 Heterogeneous Aqueous Systems

In 1996, American astronaut Shannon Lucid shared a gelatin dessert with her Russian crewmates. Gelatin is a heterogeneous mixture called a colloid. You will learn more about the characteristics of colloids and a related mixture called a suspension.

Suspensions and ColloidsA suspension is a mixture from which particles settle out upon standing.

– A suspension differs from a solution because the particles of a suspension are much larger and do not stay suspended indefinitely.

A colloid is a heterogeneous mixture containing particles that range in size from 1 nm to 1000 nm. The particles in a colloid are spread throughout the dispersion medium.

– Colloids have particles smaller than those in suspensions and larger than those in solutions.

15.3

Suspensions and Colloids15.3Suspensions are heterogenous because at least two substances

can be clearly identified.

Mayonnaise is heterogeneous mixture of oil and vinegar. Such a mixture would quickly separate without the presence of egg yolk, which is the emulsifying agent.

ColloidsThe Tyndall Effect--The scattering of visible light by colloidal particles

15.3

Particles in colloids and suspensions reflect or scatter light in all directions. Solutions do not scatter light.

ColloidsBrownian Motion

The chaotic movement of colloidal particles, which was first observed by the Scottish botanist Robert Brown (1773–1858), is called Brownian motion.

Brownian motion is caused by collisions of the molecules of the dispersion medium with the small, dispersed colloidal particles.

CoagulationA colloidal system can be destroyed, or coagulated, by the addition of ions

having a charge opposite to that of the colloidal particles. The added ions neutralize the charged colloidal particles. The particles can

clump together to form heavier aggregates and precipitate from the dispersion.

Emulsions An emulsion is a colloidal dispersion of a liquid in a liquid. An emulsifying agent is essential for the formation of an emulsion and for maintaining the emulsion’s stability.

15.3

Colloids15.3

1. One characteristic that is unique to a suspension is that the particles in it

a) settle due to gravity.

b) are too small to be seen.

c) cannot be filtered.

d) produce the Tyndall effect.

15.3 Section Quiz.

2. Homogenized milk contains finely divided particles that do not settle out rapidly and are not readily filtered. Thus, homogenized milk is a

a) colloid.

b) substance.

c) suspension.

d) solution.

15.3 Section Quiz.

3.The scattering of light known as the Tyndall effect can be seen in

a) emulsions and solutions.

b) colloids and suspensions.

c) solutions and suspensions.

d) colloids and solutions.

15.3 Section Quiz.

Properties of Water Lab

Capillary Action– water moves along because it clings to itself and other things

Wikipedia: Capillary action, capillarity, capillary motion, or wicking is the ability of a substance to draw another substance into it. The standard reference is to a tube in plants but can be seen readily with porous paper. It occurs when the adhesive intermolecular forces between the liquid and a substance are stronger than the cohesive intermolecular forces inside the liquid. The effect causes a concave meniscus to form where the substance is touching a vertical surface. The same effect is what causes porous materials such as sponges to soak up liquids.

Cohesion and Adhesion—water likes to stick to itself; water likes to stick to other things

Water Video• Human body is at least 60% water• Only 1% of water on Earth is fresh water• Water is polar (polar likes polar)• Water is the universal solvent• Chemical composition of water says it should be a gas

at room temperature, but because of hydrogen bonding, water is a liquid in a wide range of conditions

• Ice floats; other substances are more dense in their solid form

• What else did I miss?….

Ch. 15--What you need to knowWater and Its Properties• Know why a water molecule is polar (bond

angle 105º)

• Know how and why hydrogen bonding occurs

• Know properties affected by hydrogen bonding

• Know surface tension (draw picture on p. 447)

• Know surfactant and use

• Know why water’s vapor pressure is low

• Know density of water at 4ºC and at 0ºC and why ice floats

Homogeneous Aqueous Systems

• Know aqueous solution

• Know solute and solvent

• Know that polar likes polar and non-polar likes non-polar

• Know electrolyte and that all ionic compounds are electrolytes because they dissociate into ions

• Know nonelectrolyte and that molecular

compounds, carbon compounds, and alcohol are

generally nonelectrolytes

• Know hydrate

• Know deliquescent compounds

Heterogeneous Aqueous Systems• Know suspension

• Know colloid

 

From notes given in kmt/water power point

• Know adhesion, cohesion, capillary action

• Know six physical states (melting, vaporization,

sublimation, condensation, freezing, deposition)

• Know water freezes at 0ºC and boils at 100ºC

• Know phase diagram