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Elements and compounds are pure substances.
Their compositions are always the same, regardless of the source.
They also contain a fixed proportion of each composition.
Pure substances are rare though and we usually encounter mixtures of compounds or elements.
Unlike elements and compounds, mixtures can have variable compositions.
For example, a mixture of sugar and water can have a variable proportion to each other.
One can put more sugar or more water.
Mixtures can be homogeneous or heterogeneous.
Homogeneous mixtures have the same properties throughout the sample.
An example is a thoroughly stirred mixture of salt in water or sugar in water.
A homogeneous mixture can also be called solutions.
Solutions don't need to liquids.
Brass is a solid solution of copper and zinc.
The air we breath is a solution of many gases.
A heterogeneous solution consists of two or more regions called phases that differ in properties.
There are mainly two types--colloidal dispersions and suspensions.
The main difference is only the size of the items being mixed.
A mixture of sand and water is an example of a suspension.
They do not settle quickly.
The particles of sand are constantly being "bumped" by water molecules and continue to stay in suspension for a while.
A colloidal dispersion is a mixture in which the dispersed molecules are very small.
The dispersed particles in are small but they may sometimes join together
and finally separate on its own from the solvent.
Colloidal dispersions usually have an opaque or milky look.
If a beam of light is shined through a colloidal dispersion, it will be seen because the particles inside it reflect the light.
In solutions, it does not.
The light scattering is called the Tyndall effect.
Mixtures differ from compounds by the way they form.
For example, a mixture of powered sulfur in powder iron, can be mixed by stirring them together.
In the mixture, both elements retain their original properties.
It was created using a physical change, not a chemical change, because no new substances formed.
Mixtures can be separated into pure substances by physical methods.
To separate the mixture, a magnet could easily pull out the iron.
When iron and sulfur are put under a chemical reaction, a compound often called "fool's gold" is formed.
This compound no longer have the properties of the iron or the sulfur.
Water is another example; it is composed of only H2O, in which the hydrogen and oxygen atoms in a molecule of water always occur in the same ratio and cannot be separated by physical processes.
Salt water, on the other hand, is a solution of water containing lots of dissolved minerals.
Through a physical process, for example boiling, the water can be separated from those minerals.