15.3 heterogeneous aqueous systems > 1 copyright © pearson education, inc., or its affiliates....

15.3 Heterogeneous Aqueous Systems >15.3 Heterogeneous Aqueous Systems >

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Chapter 15Water and Aqueous Systems

15.1 Water and Its Properties15.2 Homogeneous Aqueous Systems

15.3 Heterogeneous Aqueous Systems



CHEMISTRY & YOUCHEMISTRY & YOU

Why are some sunsets red?

The atmosphere contains particles of water and dust. As sunlight passes through the particles, it is scattered. However, not all wavelengths are scattered to the same extent.



SuspensionsSuspensions

Suspensions

What is the difference between a suspension and a solution?




Suspensions

What is the difference between a suspension and a solution?

• A 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.






• The particles in a typical suspension have an average diameter greater than 1000 nm.

• By contrast, the particle size in a solution is usually about 1 nm.




• A solution is a homogeneous mixture.

• Suspensions are heterogeneous because at least two substances can be clearly identified.




The difference between a solution and suspension is easily seen when the type of mixture is filtered.

The small size of the solute particles in a solution allows them to pass through filter paper.

The particles of a suspension can be removed by filtration.



Explain why a mixture of sand and water can be separated by filtration, but a mixture of salt and water cannot.



Explain why a mixture of sand and water can be separated by filtration, but a mixture of salt and water cannot.

A mixture of sand and water is a suspension, and a mixture of salt and water is a solution. The particles in the sand mixture are much larger than the ions in the salt mixture. The sand particles are too large to pass through filter paper; the ions are not.



ColloidsColloids

Colloids

What distinguishes a colloid from a suspension and a solution?



ColloidsColloids

A colloid is a heterogeneous mixture containing particles that range in size from 1 nm to 1000 nm.

• The particles are spread, or dispersed, throughout the dispersion medium, which can be a solid, liquid, or gas.



ColloidsColloids

The first substances to be identified as colloids were glues.

Some Colloidal SystemsSystem

Type ExampleDispersed phase

Dispersion medium

Gas Liquid Foam Whipped cream

Gas Solid Foam Marshmallow

Liquid Liquid Emulsion Milk, mayonnaise

Liquid Gas Aerosol Fog, aerosol

Solid Gas Smoke Dust in air

Solid Liquid Sols, gelsEgg white, jelly, paint, blood, starch in water, gelatin



ColloidsColloids

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



ColloidsColloids


• These intermediate-sized particles cannot be retained by filter paper as are the larger particles of a suspension.

• They do not settle out with time.



ColloidsColloids

The Tyndall Effect

You cannot see a beam of sunlight unless the light passes through particles of water (mist) or dust in the air.

• These particles scatter the sunlight.

• Similarly, a beam of light is visible as it passes through a colloid.



ColloidsColloids

The Tyndall Effect

The scattering of visible light by colloidal particles is called the Tyndall effect.

Flashlight

Solution Colloid Suspension



ColloidsColloids

• Suspensions also exhibit the Tyndall effect.

• The particles in solutions are too small to scatter light.

Flashlight





What would be the ideal conditions to see a red sunset?




What would be the ideal conditions to see a red sunset?

A misty or foggy evening would be ideal for seeing a red sunset. There would be a large number of particles to scatter the sunlight.



ColloidsColloids

Brownian Motion

Flashes of light, or scintillations, are seen when colloids are studied under a microscope.• Colloids scintillate because the particles

reflecting and scattering the light move erratically.



ColloidsColloids

Brownian Motion

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



ColloidsColloids

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

• These collisions help prevent the colloidal particles from setting.

Brownian Motion



ColloidsColloids

Coagulation

Colloidal particles also tend to stay suspended because they become charged by adsorbing ions from the dispersing medium onto their surface.

• Adsorption means to adhere to a surface.



ColloidsColloids

Coagulation

All the colloidal particles in a particular colloidal system will have the same charge, although the colloidal system is neutral.

• The repulsion between the like-charged particles prevents the particles from forming heavier aggregates that would have a greater tendency to settle out.



ColloidsColloids

Coagulation

A colloidal system can be destroyed or coagulated by the addition of electrolytes.

• The added ions neutralize the charged colloidal particles.

• The particles can clump together to form heavier aggregates and settle out from the dispersion.



ColloidsColloids

Emulsions

An emulsion is a colloidal dispersion of a liquid in a liquid.



ColloidsColloids

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.



ColloidsColloids

• Oils and greases are not soluble in water.

• However, oils and greases readily form a colloidal dispersion if soap or detergent is added to the water.

Emulsions



ColloidsColloids

• One end of a large soap or detergent molecule is polar and is attracted to water molecules.

• The other end of the soap or detergent molecule is nonpolar and is soluble in oil or grease.

• Soaps and other emulsifying agents thus allow the formation of colloidal dispersions between liquids that do not ordinarily mix.

Emulsions



ColloidsColloids

This table summarizes the properties of solutions, colloids, and suspensions.

Properties of Solutions, Colloids, and Suspensions

Property

System


Particle typeIons, atoms, small molecules

Large molecules or particles

Large particles or aggregates

Particle size 0.1–1 nm 1–1000 nm 1000 nm and larger

Effect of light No scattering Exhibits Tyndall effect Exhibits Tyndall effect

Effect of gravity

Stable, does not separate

Stable, does not separate

Unstable, sediment forms

FiltrationParticles not retained on filter

Particles not retained on filter

Particles retained on filter

Uniformity Homogeneous Heterogeneous Heterogeneous



Which of the following is a colloidal system?

A.mud

B.gasoline

C.blood

D.a mixture of sugar and water



Key Concepts Key Concepts





Glossary TermsGlossary Terms

• suspension: a mixture from which some of the particles settle out slowly upon standing

• colloid: a mixture whose particles are intermediate in size between those of a suspension and a solute solution

• Tyndall effect: scattering of light by particles in a colloid or suspension, which causes a beam of light to become visible



Glossary TermsGlossary Terms

• Brownian motion: the chaotic movement of colloidal particles, caused by collision with particles of the solvent in which they are dispersed

• emulsion: the colloidal dispersion of one liquid in another



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