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Q & A: Compounds and Mixtures

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Most recent answer: 10/22/2007
Q:
I just wanted to know something. Can you answer this question because it’s really puzzling me! Different samples of a compound always have the same properties but different samples of a mixture may have different properties. Explain this statement, using the kinetic molecular theory in the explanation. Thanks a lot guys!!!!!!!!!!
- Brittany (age 17)
Princess margarat secondary, surrey,british columbia
A:
Brittany,

The difference between mixtures and compounds is whether there is more than one kind of molecule or not. A sample of something that is made up of one kind of molecule is called a "pure substance", and all other collections of different kinds of molecules are called mixtures. Mixtures may be held together by weak bonding forces between the molecules, such as surface tension, electrostatic forces, or just plain gravity. Some mixtures are better mixed than others.

An example of a mixture from which you can draw samples of varying properties is ice cream with chocolate chips embedded in it. We call this a "suspension". If you use a small scoop, you may or may not get a chocolate chip, and the ice cream you get will taste different depending on what came up. On a smaller scale, picking tiny things out of the ice cream may give you an ice crystal or a blob of congealed cream. On smaller scales yet, you might get a long oil molecule, a sugar molecule, or a water molecule, each of which has very different properties.

In a compound, the atoms are bonded to each other to form molecules, and all molecules are the same as each other. For example, water is a compound where the simplest component is made of two hydrogens bonded to one oxygen. In any sample of water (assuming it is pure water), I will always have exactly the same ratio of hydrogen to oxygen (2:1). If the proportion is different, then it isn't really a water compound.

In compounds, the ratio of the components never changes (water is always 2 hydrogens to 1 oxygen), and the simplest component is always two hydrogens bonded to one oxygen (not 4 hydrogens bonded to 2 oxygens in one big molecule) so all samples will have the same properties.

Here's a special case (it's really not all that special) which breaks the rules you assume in your question: Suppose your mixture is icewater. It's made entirely of water molecules, but the properties of a sample of it will depend on how much ice you pick up. It is therefore possible to make a mixture out of just one compound.

math dan and Tom

(published on 10/22/2007)

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