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Q & A: mixtures and pure substances

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Most recent answer: 07/13/2010
Q:
What is the difference between Mixtures and Pure Substances? How can you tell when using only a graph?
- Anonymous
Colorado
A:
Usually by pure substances we mean either pure elements (all one type of atom), regular crystals (atoms arranged in a repeating pattern), or things made of only one type of molecule (a tightly bound structure of one or more types of atoms). So copper is a pure substance in any form (only copper atoms). Liquid water is a pure substance (all H2O molecules). Table salt is a pure substance (a regular crystal of Na and Cl atoms, or, to be picky, ions).

Things that have different types of atoms or molecules, not arranged in regular patterns, are typically called mixtures. Vodka, for example, has some ethanol molecules and some water molecules, mixed together in an irregular, changing pattern. It is at least a solution, with the molecules mixed on the scale of single molecules. Other things (like salami) are mixtures of different types of molecules (like fats and proteins) all clumped up on a much larger scale. Mixtures with large clumps of dissimilar materials are often called "suspensions". Air is a complicated mixture of nitrogen, oxygen, argon, water, carbon dioxide and other molecules, and usually has dust, pollen, soot, or other particles suspended in it. A mixture of different kinds of metal atoms is called an "alloy" if they are mixed on an atomic scale.

Mike W. (and Tom J.)

(published on 10/22/2007)

Follow-Up #1: are alloys mixtures?

Q:
Is alloy a pure substance or a mixture?
- lindsey
toronto
A:
According to what we wrote before, an alloy would be called a mixture. I have very little interest in whether that is correct, since it only concerns a name.

Here’s what a typical metal alloy really consists of. There are crystals (regular arrays of atoms) just like for a typical pure metal. However, there are several different types of atoms in those crystals. The positions of the different types are more or less random.
Some materials don’t really consist of different elements scrambled up on an atomic scale but instead of tiny crystals of different elements, all packed together. These would not usually be called ’alloys’.

Some alloys are not crystalline but ’amorphous’. In them, the atoms don’t line up in regular crystalline patterns. You can also have amorphous pure elements, but they tend to turn into crystals more readily than do some amorphous alloys.

That’s what I know about these things. Would the name ’pure substance’ or ’mixture’ give more information?

Mike W.

Lee H


(published on 10/22/2007)

Follow-Up #2: substance phases

Q:
do substances always have one phase?
- freddie r nufable (age 14)
batangas
A:
No. Water, for example, is a substance with three familiar phases: solid, liquid, gaseous.

Mike W.

(published on 07/13/2010)

Follow-up on this answer.