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Name: Brett B.
Status: educator
Age: 30s

Location: N/A
Country: N/A
Date: 10/8/2004


Question:
My students are confused as to why brass is classified as a mixture than a compound. We discuss alloys and that Zn and Cu compose brass, but I do not know what else to say about it....


Replies:
I do not remember the exact compositions of Cu and Zn which compose brass, but that is just it. There are a range of compositions not a unique ratio, hence a mixtue not a compound.

Dr. Harold Myron


In brass the Zn and Cu are dissolved in one another to form a metallic solution. They do not form a compound in the normal sense that we use the term "compound" meaning a definite molecular composition. In the same sense we do not classify salt dissolved in water as a "compound" because over most of the concentration range the relative amounts of salt and water can be varied continuously. There are some compounds that do form specific compounds with water, and there are some metallic combinations that do have a fixed composition forming inter-metallic compounds.

Vince Calder


Hi Brett!

Brass is correctly classified as an alloy. But lets see with some detail: Alloys correspond to a combination of 2 or more metals.Several types of alloys depend on the nature of the interaction of the 2 or more metals in the alloys.Many combinations of metals form liquid solutions when fused at high temperatures.Once they are cooled reverting to the solid state they may form a polyphase system, or they may remain in solution and are said to be a solid solution.The metals likely to form solid-solution alloys are those most similar to each other in electronegativity, atomic radii, and chemical properties. The structure of a solid-solution alloy is between the 2 extremes of order and disorder. In the molten state a high degree of disorder prevails. Upon solidification the random arrangement may be preserved or different degrees of order can appear as result of atoms finding more stable positions in the lattice structure.

Now 2 metals with great differences in electronegativities and chemical properties tend to form alloys through compound formation called usually intermetallic compounds; in many of them there is an integral ratio between the sum of the number of valence electrons and the number of atoms.

In addition of these 2 types of alloys there is a third type involving the formation of a pure metal in the solid phase. In this type of alloy , the components do not form a solid solution or a compound. There the melting point of a pure metal is lowered by the presence of the second metal, leading to the formation of 2 solid phases in the resulting alloy.

Now and brass? It is an alloy of copper and zinc: alpha brasses with 37% of zinc, have a single metallographic phase beta-brasses have 40-45% zinc have 2 metallographic phases.

And thanks for asking NEWTON!

Mabel(Dr. Mabel Rodrigues)


Sometimes, I find that instead of introducing them to the idea of "alloys" I first make sure they understand the *concept* of "solutions" first (and, if necessary to drive the idea home, separation of solutions), from there it's only a small step to alloys. Hope it works for you.

Greg (Roberto Gregorius)


Brett,

True compounds (except for a rare group referred to as non-stoichiometric compounds) have a definite composition, That is, the relative numbers of atoms in the compound appear in ratios of small whole numbers. Thus, the formula of water is H2O rather that H2O2, <== the formula of hydrogen peroxide. A formula unit of aluminum sulfate is composed of two atoms of aluminum, three atoms of sulfur, and twelve atoms of oxygen. Thus, its formula is Al2(SO4)3. Sugar is C12H22O11. You will note that these formulas do not contain fractional numbers of atoms. For example, while indeed there is such a thing as NaCl (table salt), there is no such entity as NaCl3.4

Brass is an alloy -- a solution of metals that can form when the melted metals are mixed. Depending on temperature and relative component solubilities, alloys can be made with a variety of compositions -- in other words, a little of one, a lot of another. Alloys are sold by percent composition -- or like Karat gold is sold as parts per 24. Fourteen K (14 K) gold is 14/24 pure gold If the composition is 24/24 gold, its considered to be pure gold.

Regards,
ProfHoff 916


Brett,

Thanks for your question. Chemists (and people in general) have different ways to classify and group substances as a way of helping us understand the world around us. When these classifications are useful, they become accepted as a way to talk about things.

All matter can be classified as either a "pure substance" or a "mixture". Pure substances can be further classified as compounds or elements and mixtures can be further classified as homogeneous mixtures or heterogeneous mixtures.

To understand the difference between a pure substance and a mixture, it helps to be clear on the difference between a physical change and a chemical change. When ice melts and becomes liquid water, a physical change has occurred. Ice is made up of molecules of H2O and liquid water is made up of molecules of H2O also. Heating up the ice caused a phase change from solid water to liquid water, but there is no new substance created -- it is still H2O. You cannot write a meaningful chemical equation to describe this change (H20 ==> H20 ?????)

When sodium (Na) combines with chlorine (Cl) you get a new substance (table salt, NaCl) that is different from sodium or chloride. You can write a meaningful chemical equation for this chemical change: Na + Cl ==> NaCl

A mixture is something that you can use a PHYSICAL CHANGE to separate the components of the mixture. So for example, if you have a mixture of raisins and lettuce, you could physically pick out the raisins from the lettuce. If you have a mixture of sand and iron filings, you could use a magnet to separate the iron fillings from the sand. If you have a mixture of zinc and copper, you can heat it up until the Zinc melts (at 787 degrees F) and allow the liquid zinc to separate from the solid copper (which melts at a higher temperature of 1984 degrees F). Therefore (lettuce and raisins), (sand and iron fillings), and (copper and zinc) are all mixtures. For your information, mixtures of metals (such as brass and steel) are also called "alloys".

A pure substance is something that you cannot separate with a physical change. There is no magnet, no pair of tweezers, no boiling that will separate water into hydrogen and oxygen. Only a chemical change will separate the pure substance, water, into its component elements. And the chemical reaction for that is 2H2O ==> 2H2 + O2

If this helps, here are some examples of each of the four classifications of matter:

Pure Substances:

(1) Elements (as listed on the periodic table): Na, Cu, Zn, Au, etc.

(2) Compounds: H20, NaCl, CH4, NaOH, etc.

Mixtures:

(1) Homogeneous mixtures: saltwater, Kool-Aid and water, brass, steel, air, etc.

(2) Heterogeneous mixtures: tossed salad, iron fillings and sand, etc.

Regards,

Todd Clark, Office of Science
U.S. Department of Energy


Brett-

That is a simple question that leads you to a broad field of science.

Brass is mostly a solid solution of zinc, dissolved into the element copper. It happens that up to 30% Zn atoms can fit in amongst copper atoms without disrupting the crystal arrangement of the copper atoms. The average spacing, mass-density, and color changes gradually, but the shape of the lattice, the 3-D pattern of the atoms, stays the same.

In solid solutions, sometimes "alien" atoms fit in between the atoms of the"normal" crystal structure, but in this case I am sure the zinc atoms replace and pretend to be copper atoms, and are part of the crystal lattice that copper likes to form.

Add more than 35% Zn, and seeds of a different-structured crystal with 45-50% Zn in Cu, a new "phase", a new compound, must start sprouting throughout the bulk of the soft metal. The old crystals of mostly copper still co-exist beside this new phase. The old ones no longer bother to get any richer in zinc. Adding yet a little more Zinc merely makes the new crystals bigger or more numerous compared with the old.

These are all called "alloys", whether all one element, all one compound, or one homogeneous solid solution, or variable mixes of micro-crystals of many compounds. To our common sense, metals seem relatively good at acting like all one substance even when there are many jumbled together inside. The word "alloy" is our way of using that human-perceived unity. Each specific alloy is defined only by a non-integer composition list (percentages with many decimal points), and a mechanical/thermal treatment history which affects details like the hardness and size of the grains of the mix.

"Compound", on the other hand, is a relatively specific term, the scientific term meaning "a single substance". Compounds usually have "integer stoichiometry" (ratio between atom-counts of the elements), and one favorite crystal structure for each temperature. Each solid "phase" in a phase diagram is a new compound. some have a relatively wide stoichiometric range. The phase Beta-Cu/Zn might be considered the compound Cu1Zn1, even though it can be stretched to have only 47% Zinc atoms at room temperature.

Atoms can bond together into compounds in about three ways: covalent, ionic, and metallic. Covalent is the only one that really corresponds to atom-and-stick models, where there is a discrete link between each two adjacent atoms. Each link is a pair of electrons shared only between those two atoms. This enforces the integer ratios. Metallic is a much broader sharing of electrons. Each metal atom gives up at least one electron to the "common pool" of free electrons they all bathe in. The electron bath pulls all the metal atoms together as tightly as there is room for, like balls in a vacuum-packed bag, and densest packing defines the crystal structures of the metals. The crystal packing, in turn, favors certain integer ratios, making the metallic compounds. But the stoichiometric range of the metallic compounds can be rather broad.

Alloys can be any kind of mix: compounds, or amorphous solid solutions, or mixed clumps of crystals of different compounds. The word is usually applied to mixed metals, rather than mixtures of non-metallic substances. Metallic bonding bridges fairly easily from crystals of one compound to crystals of another. An alloy is often more than one "substance", but because metals melt together and solidify with microscopic crystals, and because the solid behaves malleably (like a really stiff clay), it seems like one substance to us.

I have gone around in circles about twice. Unfortunately none of these terms means anything very clear until you are familiar with examples of all of them. Sometimes it takes a while to get used to materials-science terms. I recommend you look at these composition references for Brass, and at phase diagrams for Cu + Zn.

In the phase diagrams, the zones with a Greek letter (alpha, beta, gamma, etc.) in them are a phase/(element or compound)/substance. So are the zones which reach the left or right edge of the diagram, because those edges represent 100% element A or 100% element B. The un-marked regions in between phases are "solubility gaps", compositions which cannot make a stable substance, which if artificially created will spontaneously reorganize into a mix of the two phases to left & right of that spot in the diagram. Towards the top there are the horizontal lines and arching curves which represent partial and total melting points.

http://www.key-to-metals.com/Article98.htm

- table of alloy compositions

Google- brass "phase diagram"

http://www.mmat.ubc.ca/courses/mmat380/lectures/2004/Lecture%2020%20-Copper (Complete).pdf

- table of compositions of various kinds of brass: pg12

- Cu/Zn phase diagrams: pg13

http://www.mmat.ubc.ca/courses/apsc278/lectures/Phase%20Diagrams%20II/Phase %20Diagrams%20II(fill%20in%20the%20blanks).pdf

James S. Swenson


Brett,

Just some information regarding alloys and brass. For starters, an alloy is a substance composed of two or more metals. Alloys, like pure metals, possess metallic luster and conduct heat and electricity well, although not generally as well as do pure metals of which they are formed. The properties of alloys are frequently far different from those of their constituent elements, and such properties as strength and corrosion resistance may be considerably greater for an alloy than for any of the separate metals. For this reason, alloys are more widely used than pure metals. Steel is stronger and harder than wrought iron, which is approximately pure iron.

As you already know, brass is an alloy of copper and zinc. It is harder than copper, it is ductile and can be hammered into thin leaves. Formerly any alloy of copper, especially one with tin, was called brass, and it is probable that the "brass" of ancient times was of copper and tin. The modern alloy came into use about the 16th century. I hope that this helps.

The above information was excerpted from ENCARTA 2005, DVD.

Sincerely,

Bob Trach



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