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We can classify the things we work with in science into several broad catagories. First, we can seperate out pure substances from mixtures:
- Pure substances: these have a fixed composition and set of physical
properties.
- Mixtures: two or more pure substances mixed together.
We can further divide each of the catagories above into two subgroups.
Pure substances:
- Elements: any matter that cannot be broken down into further pure
substances. The elements are listed on the periodic table: we currently know
of about 115. Iron and uranium are elements
- Compounds: pure substances made up of one or more elements with a
fixed composition. Methane is a compound: it is made up of four hydrogen atoms
and 1 carbon atom. (CH4)
Mixtures:
- Homogeneous: a homogeneous mixture is one that has the same
composition throughout. Sea water is a homogeneous mixture of water, table
salt and a variety of other compounds, stainless steel is a homogeneous
mixture of iron, carbon, nickel and other elements.
- Heterogeneous: nonuniform mixtures. Dirt is a heterogeneous mixture
of soil, rocks, worms and other things.
Example: A banana split is a heterogeneous mixture of ice
cream, bananas, whipped cream, nuts, chocolate syrup and other things. The
chocolate syrup is a homogeneous mixture of water, cocoa powder, sugar
and other compounds. Water is a compound made up of the elements
hydrogen and oxygen in a 2-to-1 ratio.
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