Chemistry Matter Notes

Name _____________________ Class ________ Date ______

I. Matter

Matter is defined as that which has mass and occupies space. This definition seems simple enough, yet there are profound issues which surround it. For example, there is currently no generally accepted scientific theory for why mass exists. It can be defined:

mass characterizes an object's resistance to a change in its motion.

But why it exists cannot yet be demonstrated. Mass just is, it just exists.

As we look into it more, definitions start to become circular, as in the definition of space:

space is that which is occupied by matter.

So, any definition of space exists only if there is matter. Take the matter away and space ceases to exist. Of course, saying "take the matter away" is very easy to say. It is quite another thing to specify what is meant by it.

As fun as these philosophical issues are, it's time to move on!!

II. Particles

All matter is particulate in nature. This basically means that between separate bits of matter there are spaces which contain no matter. In science it is called the "atomic nature of matter." It is generally agreed that the Greek philosopher Leucippus and his student Democritus were the first to propose this idea, about 440 B.C.. This "atomic theory" (to use a modern phrase) was opposed by Aristotle 100 years later, who taught that all space is filled with matter, that there are no empty spaces. Aristotle's ideas were accepted as correct by almost all educated people, until the early 1800's, when atomic ideas began to be more generally accepted as correct. There is a tutorial on this topic on this web site, titled The Greek Concept of   Atomos : The Indivisible Atom.

Today, we know that there are many different particles which make up matter. Some are long-lasting, such as the proton, while others are very, very short-lived, such as the

top quark. The primary "particle" in chemistry is the atom. However, you probably know that there is a substructure to an atom; that it is made of protons, neutrons and electrons. You may also know that protons and neutrons are each made of three quarks. There are many other particles beyond the proton and neutron, some containing two quarks and some containing three.

There are two other categories of particles which appear to NOT be made of quarks: electrons and neutrinos. As far as science is currently able to tell, there are three types of particles with no substructure that we can detect: quarks, electrons and neutrinos. It may be that someday we will learn the the electron, for example, is made of still smaller pieces like an atom is made of protons, neutrons and electrons. That would be pretty cool!!

There is also a fairly sophisticated concept called "virtual particles." While it is based on some concepts you have not yet learned, it is still fairly easy to describe in a general way. There is energy in the universe in addition to matter, we are just ignoring it for the time being in this tutorial. Some of the energy can spontaneously merge to form a particle of matter. (Einstein showed that matter and energy can be converted, one into another.) These "virtual particles" exist for very, very small amounts of time, so small that we cannot detect them before that pop back into being the energy they were made from. (These is another concept, called the Heisenberg Uncertainity Principle which affects the lifetime of virtual particles, but I will not go into here.)

Since the universe is flooded with energy, this means that, in every square centimeter of the universe, are trillions and trillions and trillions of virtual particles popping into existence every fraction of a second and then going back to energy. "Empty" space is actually a boiling cauldron of particles, popping into and out of existence. Even though individual particles cannot be detected, their effects can. If you want to look into this further, look up the "Casimir Effect" as well as learn what "polarizability of the vacuum" or "quantum fluctuation of the vacuum" mean. It may be that Aristotle was right all along.

A Classification Scheme for Matter

I would like to lead into discussing elements and compounds by first considering a general classification scheme for all matter.

The word "matter" describes everything that has physical existence. Remember, if something has physical existence, then it occupies space and has mass.

We can successively separate matter into categories by asking a sequence of "yes/no" questions.

Question #1: "Is only one chemical substance present in the sample being considered?"

YES - Pure SubstanceNO - Mixture

We can represent this question graphically:

Keep in mind that terms like "chemical substance" or "mixture" haven't really been defined yet. Hopefully, their definitions will be clearer as we go on.

All pure substances can be separated into two categories by asking the following question.

Question #2: "Can the sample be further decomposed by chemical means?" (Notice we are ignoring mixtures for the moment.)

YES - CompoundNO - Element

We can represent this question graphically:

The classification is developed in more detail in Mixtures and Pure Substances. The portion above is repeated with two additional questions concerning mixtures. The next

question in that tutorial will concern the difference between heterogeneous and homogeneous mixtures.

I. Elements

An element is a pure substance which cannot be broken down by further chemical techniques. These include heating, cooling, electrolysis and reacting with other chemicals. (By the way, it is correct that an atom can be destroyed, but NOT by chemical means. You must use a more powerful reaction, called a nuclear reaction, to destroy or change atoms. That is a topic for a lesson in a different unit.)

A sample of an element contains only one kind of atom in the sample. Suppose you had a lump of copper in your hand. The ONLY type of atom in the lump is copper. In the lump there are trillions and trillions and trillions of copper atoms. NOTHING else. (I am ignoring impurities such as a tiny piece of rock occluded within the lump, some zinc atoms randomly trapped among the copper atoms, grease from your skin sticking to the surface of the lump or oxygen atoms from the atmosphere absorbed onto the surface of the copper.)

If you were to heat the lump of copper, it would melt and eventually vaporize. The smallest unit of the copper, called the atom, would remain unaffected by this. The atoms of copper would be in the solid state, the liquid state or the gaseous state, but they would be EXACTLY the same in each state.

The atom is the smallest subdivision of an element which still retains the properties of that element. In fact, a very good definition of an atom is:

the smallest part of an element that can enter into a chemical combination

There are around 118 elements known to man, of which 20-30 are really, really important. Almost every element that exists has some form of use. There are some which are so unstable they only last for seconds or even tiny fractions of a second and no use has yet been found for them. However, ya never know!

Elements have names and symbols. For example hydrogen has the symbol H and iron has the symbol Fe. Please note that Fe is one symbol, not two. Also, make sure to use lower case for the second letter. Writing BR for bromine is incorrect, writing it as Br is correct.

II. Compounds

A compound is a pure substance composed of two or more different atoms chemically bonded to one another. A compound can be destroyed by chemical means. It might be broken down into simpler compounds, into its elements or a combination of the two. The key distinction is that compounds break down whereas the SAME techniques do not cause an element to break down.

The molecule is smallest subdivision of a compound that still retains the properties of that compound. The parallel definition (to the element one above) for the molecule is:

the smallest part of a compound that can enter into a chemical combination

Another definition, equally good, is that a molecule is the smallest stable part of a compound.

Water is a typical example of a compound. One molecule of water is composed of two hydrogen atoms and one oxygen atom, chemically bonded together. It is identified with its formula: H2O.

If you were to heat water (let's start with ice), it would eventually melt, then vaporize. Each water molecule (each H2O) would act as an independent unit and zoom around in the gas sample. The three atoms making the water molecule would stay attached to each other. In addition, water would enter into a chemical reaction acting like a water molecule, NOT little separate atoms of hydrogen and oxygen.

An important point to remember is that the compound is going to have distinctly different properties than its elements. Hydrogen has a set of properties, as does oxygen. However, the set of properties that water has in no way like the two elements. For example, at room temperature (about 20-25 °C) water is a liquid while hydrogen and oxygen are gases.

Another classic example is sodium chloride (formula = NaCl). Sodium metal (Na) and chlorine gas (formula = Cl2) have very, very different properties from each other and neither one of the two is like sodium chloride at all.

Compounds have names and formulas. The formula is made from the symbols of the elements in the molecule and how many of each element there are. For example, glucose's formula is C6H12O6.

There are something over 12 million known chemical compounds. Well over 75% of them are mentioned in only one scientific article. Of the remaining bunch, there are several thousand of great interest and usefulness to science.

III. An Important Point about Compounds and Molecules

The difference between compound and molecule causes distress among students. I will try and explain it more.

The word compound is meant when you are making general reference to a chemical substance, as in "Go get a bottle of glucose from the storeroom" or "Glucose is the one of the end products of photosynthesis."

Inside the bottle is 500 grams of the compound glucose. Making up the 500 grams of glucose are trillions and trillions of individual glucose molecules, the formula of which is C6H12O6.

The plant makes the chemical compound called glucose. A sample of the chemical compound glucose is made up of many glucose molecules, all having the formula C6H12O6.

I hope that helps a little bit.

IV. An Important Point about Elements and Molecules

At room temperature and pressure, there are nine elements which exist as molecules.

(1) These are the seven diatomic molecules: H2, N2, O2, F2, Cl2, Br2, I2

(2) P4 and S8 also exist.

At elevated temperatures, these molecules will break down into single atoms, but even when in the molecular state, they are considered to be elements. The atoms are chemically bonded to each other, so they are considered to be molecules, but they are not considered to be compounds. They are elements.

All the other elements are considered to be mono-atomic.

These are the six mono-atomic gases: He, Ne, Ar, Kr, Xe, Rn.

There is one mono-atomic liquid: Hg (by the way, bromine is also a liquid at room temperature and pressure).

At room temperature and room pressure, all other elements exist as solid aggregates of atoms.

I wish to stress that these aggregrates (as well as liquid mercury) are not thought of as molecules. The elements in question are considered to exist as single atoms.

With the diatomics, an area of confusion sometimes arises: how do I know if you are discussing a molecule (for example: a molecule of oxygen) or individual atoms of the element? Good question.

If I am discussing the element oxygen with no context, I mean O2. If I am discussing the element sulfur with no context, I mean S8. If I wished to discuss oxygen atoms or sulfur atoms, I would have to say it explicitly or the context would clearly have to demand that individual atoms of the element are being discussed.

This area of confusion is often dealt with by saying 'dioxygen' or 'dichlorine.' When you say this, you clearly mean O2 or Cl2.

Having said the above, I think that the problem arises because the speaker (often the teacher) is more well-versed in the chemistry that the listener (you, the student). What happens is that the contect is clear to the speaker, but not clear to the student. In that case, you really have only one recourse: ask the person if they mean molecules or atoms.

There are three states of matter which exist in the environment of the Earth and there are two more states which exist in temperature extremes.

I. The Three States of Matter

The three states of matter most important to chemistry are solid, liquid and gas. Chemistry deals more-or-less in the temperatures that these phases exist in. The definitions of each:

solid: definite shape, definite volume.liquid: indefinite shape, definite volume.gas: indefinite shape, indefinite volume.

Indefinite shape means that the sample in question takes on the shape of the container. If some water was poured from a round container into a square container, the shape of the sample would change.

Indefinite volume means the sample would expand to fill the entire container. Only gases do this. If I had a sample of gas in a 5-liter container and moved the gas sample into a 10-liter container, the gas molecules would move farther apart from each other and fill the entire 10-liter volume. Note that, in doing this, the gas would become less dense.

Definite (for both shape and volume) means that the container makes no difference whatsoever. If 5-liters of liquid water is poured into a 10-liter container, the liquid would occupy 5-liters of the container and the other 5-liters would be empty. Suppose some water was frozen in the shape of a sphere and then put into a larger cubical shaped container. The spherical ball of ice would retain its spherical shape as well as retaining its volume even though it had been put into a container that was both larger and of a different shape.

II. Plasma and Bose-Einstein condensate: the other two states of matter

These two states of matter exist at temperature extremes. They ae interesting and useful to study, but are considered to be part of physics, not chemistry. This is because chemistry studys tend to be more-or-less close to room temperature and room pressure. These two states of matter might be considered, to use a favorite word of this era, extreme.

A plasma is created at several million degrees Celsius. In a plasma, all the electrons have been stripped from the atom and are free to move about. For example, each gold atom normally has 79 electrons. Forming a plasma with a sample of gold atoms means each atom has lost all 79 electrons. So a plasma has two things in it: free electrons and the bare nuclei of the atoms.

The high temperature is required for the existence of the plasma. As the temperature falls below the several million degrees required, the electrons begin to return to the atom and take up their usual places within the atom. Plasmas can be made on Earth using the proper (very expensive!!) equipment, but only in very small amounts for short periods of time. Plasmas were first successfully prepared sometime in the 1950's (I think) and have been a subject of study since then.

Bose-Einstein condensates are the reverse of plasmas. These exist only at very, very low temperatures, close to absolute zero. The existence of this state of matter was proposed by Albert Einstein in 1924, having been inspired by a 1924 paper by the relatively unknown Satyendra Bose. However, the technical demands of preparing a Bose-Einstein condensate was so great that it was not until 1995 that success was obtained.

Imagine 2000 people milling about in a field. From above, we look out and say "Oh yes, there are 2000 people. I can see each one and distinguish each one from another. I'll take a moment and count them." Of course, we can do this because people are different from each other.

Now imagine 2000 atoms of rubidium together in a small space and at a very, very low temperature. Keep in mind that atoms ARE NOT people. Each atom is a "separate" entity, but they are identical in many respects also. Same number of protons, some distribution of electrons, governed by the same laws of nature. In many important ways, one rubidium atom is completely indistinguishable from another.

This identical nature of the atoms results in a remarkable behavior of the 2000 rubdium atoms at something around 200 billionths of a degree above absolute zero. The 2000 atoms merge into one "superatom" of rubdium. This is the Bose-Einstein condensate. It is NOT 2000 little pieces stuck together. It is one entity and it is the element rubidium.

This first Bose-Einstein condensate lasted for 10 seconds, several people have won Nobel Prizes for work in this area and research continues today.

. Physical Properties

A physical property of a pure substance is anything that can be observed without changing the identity (that is, the chemical nature) of the substance. The obervations usually consist of some type of numerical measurement, although sometimes there is a more qualitative (non-numerical) description of the property. There are many physical propertiesand each textbook will have a different list of examples. Here are some of the more common ones:

melting point electrical conductivity color density

boiling point thermal conductivity odor hardness

There are others which are not mentioned as often. Examples include:

refractive index atomic radius ductility

ionization energy allotropes malleability

There are more which have not be mentioned. There is no single, definitive list of physical properties. A few example properties are cited, there is some discussion and the author moves on.

Groups of similar elements or compounds can be characterized by commonality in their physical properties. Metals have a whole bunch of physical properties that are similar. For example, metals are very ductile and very malleable. All easily conduct electricity and heat and all have a bright luster. These all reflect a commonality of structure.

However, the similarities in a group do not extend to every property. Both tantalum and sodium are metals. Tantalum's melting and boiling points are 2996 °C and 5425 °C. Sodium? 98 °C and 883 °C. However, they are both considered metals and no one in the scientific world disputes this. The reason is that both exhibit the characteristic arrangement of atoms and electrons all metals have. (This arrangement will be taught later in the course.) The wide disparity in the melting and boiling points between tantalum and sodium simply highlight the wide range that exists within the common structure all metals have.

I guess human beings would be a good example for the above point. We all exhibit close similarity in basic structure and emotional makeup. However, we range across a wide spectrum in many areas. Mozart was composing music by the age of four and, more than 200 years later, people are still listening to his music. His father took him and his sister all over Europe, showing off their musical skills. You and me, when we play an instrument, dogs begin to howl, the sky clouds over and mothers rush to bring their little children in the house. When you and I try to play an instrument, SWAT teams from 4 counties show up and the guy on the bullhorn says "Come out slowly. Show your hands. NOW!! Lay on the ground."

II. Chemical Properties

This one is more difficult. Here is one way to define "chemical property:"

characteristics which are exhibited as one substance is chemically transformed into another.

Here are some examples.

(1) iron rusting. When iron (an element, symbol = Fe) rusts, it combines in a complex fashion with oxygen to form a reddish-colored compound called ferric oxide (formula = Fe2O3). Not all substances rust.

(2) glucose, mixed with yeast, ferments to make alcohol. Glucose (C6H12O6) is a chemical compound which enzymes in yeast can use to make ethyl alcohol (C2H5OH). Not all substances ferment.

(3) trinitrotoluene (TNT) reacts very, very fast when it is ignited. Among other products, it makes LOTS of nitrogen gas and LOTS of heat. Inside the proper container, it can cause an explosion. Not all substances can make an explosion.

There really isn't a set of chemical properties in the same way there is, more or less, a set of physical properties. That's because the chemical properties are tied to the change, whereas a given substance has a property (such as melting point) all to itself.

Another textbook I consulted defined "chemical property" this way:

chemical properties describe the way a substance may change or react to form other substances.

One example was given: flammability - the ability of a substance to burn in the presence of oxygen. Some substances (wood, alcohol) are very flammable, others are not. Iron (see above) reacts with oxygen, but so slowly we do not say the iron burns, but that it rusts.

Generally speaking, information about physical properties is clearly laid out and chemical properties is harder to pin down. That's just the way it is sometimes.

Physical Changes

A physical change is any change NOT involving a change in the substance's chemical identity. Here's another way to say it:

a change that alters the physical form of a substance without changing its chemical identity.

Here are some examples:

(1) any phase change. Moving between solid, liquid and gas involves only the amount of energy in the sample (this amount is the subject of future lessons). There is no effect on the chemical identity of the substance. For example, water remains water, no matter if it solid, liquid or gas.

By the way, all phase changes include sublimation (solid to gas) and deposition (gas to solid). Since they are fairly unusual phase changes, teachers like to use them for test questions.

(2) grinding something into a powder. Or the reverse process of making a bigger lump of stuff, say by melting lots of small pellets of copper into one big piece.

(3) iron (and many other metals) can be made to be magnetic. This change in no way affects the chemicalidentity of the element. Iron that is magnetized rusts just as easily

as iron that is not magnetized. (Yes, rusting is a chemical change. Rust is chemically different from iron.)

Now would be a good time as any to list the names of the various phase changes:

Change Name of changeSolid to liquid melting, fusionLiquid to gas boiling, evaporationSolid to gas sublimationGas to solid depositionGas to liquid condensation, liquefactionLiquid to solid freezing, solidification

An example of sublimation is dry ice. It is solid carbon dioxide and goes directly from the solid state to gas in the open atmosphere. You can make liquid carbon dioxide, but it must be done under about 5 atmospheres of pressure.

Deposition is a bit of a non-standard word, but it fits better than using sublimation or condensation again. Ice cubes in the freezer undergo sublimation to water vapor, even with the ice is cold. The vater vapor deposits back onto the solid ice without even going through the liquid phase. By the way, this is how ice cubes become "welded" together if they sit undisturbed in the freezer.

Here is a great example of deposition. First a fact: solid water exists in nine different solid forms (at various combinations of temperature and pressure), called ice I to ice IX. (The one we use in our sodas is ice I.) There is a tenth solid form which is only obtained when water vapor is deposited onto a solid surface which is below -120 °C. At -80°C it spontaneously changes to ice I, however it cannot be obtained by cooling ice I.

II. Chemical Changes

A "chemical change" means that the reacting compound(s) are changed into new compounds. The actual atoms involved remain, they are simply rearranged into the new compounds. The rearrangement is called a chemical reaction. For example:

2H2O ---> 2H2 + O2

is a chemical reaction in which water is broken down into the hydrogen and oxygen which make it up. Notice how the amounts of hydrogen atoms (four) and oxygen atoms (two) do not change from one side of the arrow to the other. However, the

arrangements of the atoms is different. Some chemical bonds (the one involved in the water) have been broken and some new chemical bonds (the one in hydrogen and oxygen) have been formed.

This is another way to define "chemical change:"

A process in which chemical bonds are broken and new ones are made.

A process like grinding some salt crystals into a fine powder does not involve the breaking of chemical bonds and the formation of new ones, so it is a physical change.

A chemical change always involves a change in the chemical relationship between the various substances involved. This change is seen in the fact that some chemical bonds are broken and some bonds are newly made.

Here is another example of a chemical change:

N2(g) + 3H2(g) ---> 2NH3(g)

While all three substances are gases, the two reacting substances are quite different chemically from the product. This is because the bonds between the nitrogen atoms have been broken, as well as between the hydrogen atoms. In the place of these broken bonds has come something not present in the reactants, bonding between a nitrogen atom and some hydrogen atoms.

Obviously, chemistry is made up of many chemical reactions (which are the cause of chemical changes). One interesting one is the ion-exchange column, of which one type is used to remove certain chemicals from drinking water.

Remember, chemical change always alters the chemical characteristics of a given substance. Chemical change always results in new chemical substance(s) being produced and the substance(s) that were present at the start of the change are always consumed.

A Classification Scheme for Matter

The word "matter" describes everything that has physical existence.

We can successively separate matter into categories by asking a sequence of "yes/no" questions. The classification scheme that follows is only one possible sequence. There are other ways to ask questions and in different orders. For example, a possible first

question might be "Is it uniform throughout?" in which case, the initial division is into heterogeneous and homogeneous categories.

I. Matter

Question #1: All matter can be separated into two categories by asking the question "Is only one chemical substance present in the sample being considered?"

YES - Pure SubstanceNO - Mixture

A mixture is one in which two or more pure substances retain their chemical identity. For example, if you dissolve some sugar into water, the sugar molecules and water molecules remain as sugar and water, it is just that the two are now dispersed in each other. Another definition of mixture: a dispersion of two or more pure substances that can be separated using physical means only.

All mixtures have two parts, the "dispersing medium" and the "dispersed phase." Generally speaking, the dispersed phase is in the smaller amount and is spread throughout the dispersing medium. In most cases, the dispersed phase is quite small in amount compared to the amount of the dispersing medium. Only sometimes, in our studies in this class, will the two amounts become near to equal.

IIa. Pure Substances

Question #2: All pure substances can be separated into two categories by asking the question "Can the sample be further decomposed by chemical means?"

YES - CompoundNO - Element

The definitions of element and compound, as well as examples, are found in Elements and Compounds.

First Historical Note: our concept of an element is due to Robert Boyle (1627-1691). His definition was experimentally-based: an element could not be broken down into simpler substances. This meant that all element identifications were tenative, since better techniques meant that a compound, mistakenly thought to be an element, might be shown to be an element.

Misindentifications of compounds as elements continued to be problems through the 1800's. Not only problems associated with methods, but with getting everybody to agree to use the same definitions. In our era, these problems have been resolved.

One story: hydrogen is the lightest element known. Up until 1913, it was a possibility that there could be lighter elements and there was even an occasional announcement of such a discovery. However, in 1913, work done by Henry Moseley (which you will learn about later) forever shut the door to the possibilty of elements lighter than hydrogen.

Second Historical Note: another important idea at that time was the immutability of atoms. An atom of copper has always been copper and always will be copper. Nothing can change it. This idea has been shown to be wrong by the modern discoveries of radioactivity, fission and fusion. These topics overlap between physics and chemistry and will be studied later in the school year.

IIb. Mixtures

Question #3: All mixtures can be separated into two categories based on the question "Is the sample of constant composition?"

YES - Homogeneous mixtureNO - Heterogeneous mixture

Constant composition means that all parts of the mixture are the same. For example, dissolve sugar in water and mix it completely. Now take several samples from random areas. They will be the same, therefore this is a homogeneous mixture. Take some sand and some water and mix it up well. Take some samples and MAYBE they are the same. Allow the water to stand undisturbed and then sample it. One portion will be more sand than water and another will be more water than sand. This is heterogeneous.

Generally speaking, heterogeneous mixtures can be separated by allowing them to stand undisturbed, letting the "formed portion" (the solids) to settle out. However, filtering or centrifuging may be required. For example, not all the solid components of blood will settle out simply by standing. The blood sample must be placed in a centrifuge and spun at several times the force of gravity. The ribosomes in a cell can be separated from the cell in an ultracentrifuge, a device which can produce 50,000 to 100,000 times the force of gravity.

The technical name for a heterogeneous mixture is 'suspension.' The solid pieces which are dispersed in the suspension are sometimes able to be seen with the naked

eye and can definitly be seen under a light microscope. (Homogeneous mixtures will be divided into two types -- solution and colloid. In both cases the dispersed phase cannot be seen under a microscope.)

In chemistry, homogeneous mixtures are much more common than heterogeneous, so we will pretty much end our heterogeneous mixture discussion at this point.

III. Homogeneous Mixtures

Homogeneous mixtures do not settle out upon standing undisturbed and they cannot be separated by filtering or centrifuging. There are two broad categories of homogeneous mixtures.

Question #4: All homogeneous mixtures can be separated into two categories based on the question "Are the constituents of the sample at a molecular or ionic level?"

YES - SolutionNO - Colloid

Solutions: these are, by far, the most important homogeneous mixture in chemistry. Only in more advanced classes will you start to study the characteristics of colloids.

Solutions are made up of a solute and a solvent. The solvent (usually liquid water) is the dispersing medium (component present in greater amount) and the solute (usually, but not always, a solid) is the dispersed phase (component present in the lesser amount). In solutions, the solute is present either as individual ions or individual molecules. There is no "clumping" into pieces made of many ions or molecules.

The word homogeneous is important: the solue is dispersed in an equal manner throughout the solvent. If you sampled two equal-sized regions of the solution, they would contain identical amounts of solute.

There is no standard way to classify solutions. Here are some possible ways solutions could be sub-divided into different types:

acid, basic, neutral (based on pH of the solution) saturated, unsaturated, supersaturated (based on amount of solute) solid, liquid, gas (based on physical state of the solution)

These types of categories are usually discussed during a specific teaching unit. For example, acid base chemistry is a teaching unit found in most chemistry classes. The

classification of acid, base, and neutral solutions would most often be discussed during that unit.

Colloids: this is a state intermediate between solutions and suspensions. The dispersed phase IS NOT at the molecular level nor is it of such a size to be visible under the microscope. Generally speaking, the dispersed colloidal particles are on the order of nanometers (10¯9 meters), anywhere from about 1 nm to about 100 nm. They are sometimes called colloidal suspensions.

Types of Colloids

 Dispersing Medium

Gas Liquid SolidGas   Foam FoamLiquid Aerosol Emulsion Gel

Solid Aerosol Sol Solid Sol

There is no gas-gas colloid. Why? Go to the answer.

Colloids which are transparent are characterized by something called the "Tyndall Effect." When the sun sometimes rises or sets with all the brilliant reds and oranges; the colors come about due to the Tyndall Effect. When you see "rays of sunlight," like on a misty day or in the forest, this is caused by the Tyndall Effect. Mist is tiny drops of water suspended in air and, in the forest, dust plays the some role as the mist.

Examples of Colloids

 Dispersing Medium

Gas Liquid Solid

Gas   shaving cream,whipped cream

foam rubber,sponge,pumice

Liquid fogs, clouds,aerosol can spray

mayonnaise,milk, face cream,

hair gel

jelly, cheese,butter

Solid smoke,car exhaust,

Gold in water,milk of magnesia,

alloys of metals(steel, brass)

airborne viruses river silt

The "gold in water" refers to a famous colloid made by Michael Faraday. It still exists (in the collection of the Royal Institution in London) and it HAS NOT stttled out. By the way, colloids as a group were first recognized to exist by Thomas Graham around 1860. He found that substances like starch or gelatin diffused very slowly though water as compared to sugar or salt. Also, the former DID NOT diffuse through membranes that sugar and salt could. Graham also found he could make crystals of salt or sugar, but not of starch or gelatin. He coined the word "colloid" (from the Greek for "glue") to describe this new category of mixtures so different from suspensions or solutions.

Some colloidal substances have now been crystallized, but only with great difficulty. This points out the difficulty of drawing a sharp dividing line between solutions and colloids. However, it remains true that the particles of a colloidal suspension are relatively large compared to the molecules making up a solution. Since colloids are made up of finely divided particles, possessing LOTS of surface area, many of the properties of colloids are based on the properties of surfaces. This point will not be explored further.

Colloids with water as the dispersing medium can be classified as "hydrophobic" or "hydrophilic," but we will leave that discussion for another chemistry class.