Chemical properties

Mixtures and pure substances

- Pure substances are matter made up of only one type of atom or molecule

- Mixtures are a collection of 2 or more pure substances physically mixed
together that cannot be represented by a chemical formula.

- A mixtures properties may vary, because the proportions of the substances
in them may vary. Gold for example is too soft to maintain it\'s shape in jewelry
so copper silver, or nickel is usually added to create more hardness in the god
this solid mixture is known as an alloy.

- Pure substances have specific chemical and physical properties. Pure
substances are often hard to find so if a substance is mixed with small amounts
of impurities the substance is still considered to be pure.

- Mixtures may be classified as either homogeneous or heterogeneous.
Homogeneous mixtures are ones where substances are uniformly distributed
throughout. Gas, syrup, and saltwater are homogeneous mixtures. Heterogeneous
mixtures contain substances which are not evenly distributed throughout. A
salad, cereal, and Snickers bars are heterogeneous mixtures.

- Mixtures contain phases in which part of a system has uniform composition
and properties.

Elements

- Elements are the simplest pure substances, since they contain only one kind
of atom.

- Every element has a set of chemical and physical properties, purified gold
discovered today cannot be distinguished from gold found years ago because pure
gold from any source is the same substance, an unreactive soft metal which melts
at 1064ºC.

- There are over 109 elements but only about 12 make up things we see
everyday. Hydrogen is the most common of all it makes up more than 90% of the
atoms of the known universe. Oxygen and silicon make up over 70% of the Earth\'s
crust.

- Living things a mainly made up of four elements, those being carbon,
oxygen, hydrogen, and nitrogen, these combine to create the molecules needed for
life.

- Allotropes are different molecular forms of an element in the same physical
state. Oxygen is an allotrope, ozone and diatomic oxygen are allotropes of
oxygen. Allotropes may have properties that vary widely, diatomic oxygen is
colorless, odorless, and essential to life, whereas ozone is blue, toxic, and
have a very sharp odor.

Compounds

- Compounds are pure substances composed of two or more different elements.

- Molecular compounds are composed of molecules. Ionic compounds such as
table salt, are made of at least 2 different elements that are chemically
joined, but the atoms are not made of discrete molecules but instead are
distributed throughout a continuous network.

- Compounds have unique sets of properties, these properties may vary greatly
from those of the elements used to create them.

- There are 3 major differences between compounds and mixtures.

1. Mixtures are never made of only a single compound.

2. A mixtures properties reflect the substances used to create it, whereas a
compound often bears no resemblance to the properties of the elements used to
make it.

3. Compounds have a definite composition by mass of their combining elements.
Pyrite for example always exists with a mass of 46.55% iron 53.45% sulfur. While
substances exist in any mass ratio.

Methods of separating mixtures

- Filtering, evaporating, centrifuging, or decanting a mixture brings about
only physical changes. The substances gained from these processes are not
chemically changed.

- Chromatography can be used to separate the components of a solution so they
may be identified. It works because different substances are attracted to
different solvents and other media.

- Distillation is a method of separating substances that have different
boiling points

- In some spots around the world, people use distillation to obtain drinking
water from sea water. But this process requires much energy and is very
expensive, so other ways of getting drinking water are preferred. Distillation
is used in the petroleum industry to separate crude oil. The separated
components of oil have many uses, gas, heating oil, lubricants, and materials
from which plastics are made all come from crude oil being distilled.

- A relationship between 2 quantities can be shown on a graph.

- Density is the measure of how tightly matter is packed. Osmium is the
densest substance on the earth a piece the size of a football is too heavy to
lift.

- Density may be used to identify substances. Ice floats in water because it
has a lower density than water.

Category: Science