ELEMENTS, COMPOUNDS, MIXTURES
1. Element- is a pure substance (homogeneous material) that broken down further
(decomposed) by ordinary chemical means. (An element is made of only one kind of atom).
Example: aluminum, hydrogen, calcium, mercury.
Elements are listed on the periodic table. Scientists have found 90 elements in nature, and
about 20 more have been produced in the laboratory.
2. Compound- two or more elements chemically combined. (Note: if a sample of matter is
made of atoms of two or more elements joined together, always in the same ratio, then that
matter is a compound.)
Example: alcohol (C2H5OH), water (H2O), salt (NaCl), sugar C12H22O11) ammonia (NH3).
NOTE: When scientists refer to substances, they mean elements or compounds. A substance
always has a definite composition.
Physical and Chemical Properties
I. Physical Properties
A physical property of a pure substance is anything that can be observed without
changing the identity of the substance. The observations usually consist of some type of
numerical measurement, although sometimes there is a more qualitative (nonnumerical) description of the property. There are many physical properties and each
textbook will have a different list of examples. Here are some of the more common ones:
melting point electrical conductivity color density
boiling point
thermal conductivity
odor
hardness
There are others which are not mentioned as often. Examples include:
refractive index
atomic radius ductility
ionization energy allotropes
malleability
II. Chemical Properties
This one is more difficult. Here is one way to define "chemical property:" characteristics
which are exhibited as one substance is chemically transformed into another. Or one
could say that “a chemical property describes the way a substance may change or react
to form other substances”.
Here are some examples.
(1) iron rusting. When iron (an element, symbol = Fe) rusts, it combines in a complex
fashion with oxygen to form a reddish-colored compound called ferric oxide (formula =
Fe2O3). Not all substances rust.
(2) glucose, mixed with yeast, ferments to make alcohol. Glucose (C6H12O6) is a
chemical compound which enzymes in yeast can use to make ethyl alcohol (C2H5OH).
Not all substances ferment.
(3) trinitrotoluene (TNT) reacts very, very fast when it is ignited. Among other
products, it makes LOTS of nitrogen gas and LOTS of heat. Inside the proper
container, it can cause an explosion. Not all substances can make an explosion.
I. Physical Changes
A physical change is any change NOT involving a change in the substance's chemical
identity. Here are some examples:
(1) any phase change. Moving between solid, liquid and gas involves only the amount of
energy in the sample (this amount is the subject of future lessons). There is no effect on
the chemical identity of the substance. For example, water remains water, no matter if it
solid, liquid or gas.
(2) grinding something into a powder. Or the reverse process of making a bigger lump of
stuff, say by melting lots of small pellets of copper into one big piece.
(3) iron (and other metals) can be made to be magnetic. This change in no way affects
the chemical identity of the element. Iron that is magnetized rusts just as easily as iron
that is not magnetized.
Now would be a good time as any to list the names of the various phase changes:
Change
Name of change
Solid to liquid melting, fusion
Liquid to gas
boiling, evaporation
Solid to gas
sublimation
Gas to solid
deposition
Gas to liquid
condensation, liquefaction
Liquid to solid freezing, solidification
II. Chemical Changes
A "chemical change" means that the reacting substances(s) are changed into new
substances. The actual atoms involved remain, they are simply rearranged. The
rearrangement is called a chemical reaction. For example:
2H2O ---> 2H2 + O2
is a chemical reaction in which water is broken down into the hydrogen and oxygen
which make it up. Notice how the amounts of hydrogen atoms (four) and oxygen atoms
(two) do not change from one side of the arrow to the other. However, the arrangement
of the atoms is different. Some chemical bonds (the one involved in the water) have been
broken and some new chemical bonds (the one in hydrogen and oxygen) have been
formed.
This is another way to define "chemical change:"
A process in which chemical bonds are broken and new ones are made.
A process like grinding some salt crystals into a fine powder does not involve the
breaking of chemical bonds and the formation of new ones, so it is a physical change.
Mixture- two or more elements or compounds that are blended without combining
chemically. Each part of the matter in a mixture has its own identity (properties).
Mixtures can be separated using physical or mechanical means.
Example: fabrics, vinegar, soil, rocks, rocks, milk, lemonade
Mixtures can be heterogeneous mixtures or homogeneous mixtures
Heterogeneous mixtures- the substances in the kind of a mixture are not spread out
evenly. Example: a bottle of liquid salad dressing.
Homogeneous mixtures- the substances are spread evenly throughout, a
homogeneous mixture is called a solution. Example: vinegar (water and acetic acid are
mixed evenly throughout). Other examples: sea water, soft drinks, glass
Solution- a homogeneous mixture in which one substance (the solute) is dissolved in
another substance (the solvent). Example: salt water (Water, the solvent, plus salt, the
solute, produces the solution of salty water.)
Suspension- a heterogeneous mixture in which the particles are large enough to be seen by a
microscope or the unaided eye (eventually, they settle out of the mixture). Example: stirring
a teaspoon of dirt in a glass of water.
Colloid- a mixture where the size of particles in the mixture are between those
of a solution and a suspension. NOTE: The particles appear evenly
distributed. Examples: fog, cheese, butter, jellies, whipped cream.