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Introduction
Everything
around us has to be made from something. The different types
of stuff that make up the world around us are called "materials".
Materials come in three main forms - Gases, Solids and Liquids.
This is often refered to as the different states of matter.
There is also a fourth phase, plasma, but it exists at very
high temperatures, and molecules in that state can behave
very differently. Plasma will not be discussed here.
|
State
of Matter |
Description |
Features
|
Solids
|
Solids
are what we normally think of when we talk about materials.
The important thing about solids is that they have a
definite shape and they will keep that shape unless
we cut, bend or squash the material. In a solid the
tiny atoms or molecules that make up the material are
tightly packed together and hold each other in place. |
Volume:
Has a fixed volume
Shape:
Rigid; Has a fixed shape
Compressibility:
Incompressible |
Liquids
|
The
most common example of a liquid is water, but there
are plenty of other liquids around: oil,petrol,mercury,etc
The molecules in a liquid are also tightly packed together
but in a liquid they are not holding each other in place,
they can move around each other. This means that liquids
flow. They can be poured from one container to another.
It also means that you cant carve a shape out of a liquid.
The molecules that make up the liquid just slip and
slide past each other until your shape becomes a puddle!
If you want to carry a liquid around you will need to
put it into a container. |
Volume:
Has a fixed volume
Shape:
Has no fixed shape; conforms to the shape of the container
it occupies
Compressibility:
Incompressible (to any appreciable extent) |
Gases
|
People
often forget about gases altogether, that's because
most gases can't be seen and most of the time you
can't even feel them. But gases do exist. Air is a
mixture of the most common gases, and air certainly
exists or we would all stop breathing! You can soon
show that the space around you is full of air, get
a large sheet of cardboard and hold it with a friend.
Now try to run across the playground. If you hold
the card upright the air gets in the way and it is
difficult to run -you have to push the air out of
the way. If you hold the card flat then it cuts through
the air. There are lots of other simple experiments
that show that gases really exist. Blowing bubbles
in water, trying to squash air in a blocked syringe
or jumping out of an aeroplane with (or without) a
parachute.
The
molecules in a gas are well spread out, they do not
hold onto each other at all and will shoot off in
all directions. If you want to carry a gas around
you will need to make sure your container has a good
lid.
|
Volume:
Has no fixed volume; conforms
to the volume of the container it occupies
Shape:
Has no fixed shape; conforms to the shape of the container
it occupies
ompressibility:
Compressible |
Matter
can exist in more than one state under different conditions.
For
example, water can exist as ice (a solid), as water (a liquid)
or as steam (a gas).
Properties of Matter
Physical
properties: can be observed or measured without changing
the chemical identity (composition) of the substance.
For example,
physical state, colour, temperature, melting point, solubility,
odour etc.
Chemical
properties: describe how substances react or change
their chemical identity to form other substances.
For example,
flammability (e.g. hydrogen burns in the presence of oxygen
to produce water).
Physical and Chemical Changes of Matter
Physical
changes: involve a change in the physical appearance
of matter, without altering its chemical identity.
For example,
a change of state. Ice melts to form a liquid, however
its chemical identity is still H2O.
H2O(s)
H2O(l) H2O(g)
Note:
the use of the subscripts (s), (l), and (g) denote the solid,
liquid and gaseous states. The indicates the addition of
heat to the system.
Chemical
changes: involve a change in the chemical identity of
matter into other substances that are chemically different.
i.e., a chemical reaction has taken place. For
example, any chemical reaction.
2H2(g)
+ O2(g) 2H2O(l)
The Structure of the Atom - Protons, Neutrons adn Electrons
Here
is a summary of the modern view of an atoms structure.
- An
atom is the smallest sample of an element because breaking
it into its subatomic particles destroys its chemical
identity.
- An
atom consists of subatomic particles - protons, neutrons
and electrons.
- The
nucleus is in the centre of the atom.
- Protons
and neutrons are located in the nucleus.
- Protons
are positively charged (+1) and, therefore, the nucleus
has a positive charge.
- Neutrons
have no charge.
- The
nucleus occupies a very small volume.
- Most
of the mass of the atom is in the nucleus.
- Protons
and neutrons have nearly the same mass (and are about
1800 times more massive than an electron).
- Electrons
are located outside the nucleus.
- Electrons
are negatively charged (-1).
- The
largest volume is outside the nucleus in the region
where the electrons exist.
- In
a neutral atom, the number of electrons surrounding the
nucleus equals the number of protons in the nucleus.
- Electrons
are attracted to the protons in the nucleus by the law
of electrostatic attraction (particles of opposite charge
attract).
- All
atoms of the same element have the same number of protons
and electrons. Therefore,
atoms have no net charge.
- Atoms
are extremely small, and their diameters are measured
in angstroms (Å ). 1Å = 10-10
- Atoms
have extremely small masses, so atomic mass units (amu)
are used for convenience. 1 amu = 1.66054 x 10-24
g.
| Particle |
Location |
Releative
Mass |
Relative
Charge |
| Proton |
Nucleus |
1 |
+1 |
| Neutron |
Nucleus |
1 |
0 |
| Electrons |
Electron
Cloud |
1/1836 |
-1 |
Atomic Terms
Atomic
Number - The number of protons in the atom;
The number of electrons = the number of protons,
hence the overall charge in an atom is 0.
Mass
Number - The number of protons in the nucleus
+ the number of neutrons in the nucleus, ie the
total number of particles in the nucleus.
Element
- Consists of atoms all having the same atomic numbers.
Isotopes
- Atoms which have the same atomic number but different
mass numbers; ie, belong to the same element but have different
number of neutrons. For
example, there are three kinds of carbon atom 12C,
13C and 14C. They all have the same
number of protons, but the number of neutrons varies.
| isotope |
protons |
neutrons |
mass
number |
| carbon-12 |
6 |
6 |
12 |
| carbon-13 |
6 |
7 |
13 |
| carbon-14 |
6 |
8 |
14 |
These
different atoms of carbon are called isotopes.
The fact that they have varying numbers of neutrons makes
no difference whatsoever to the chemical reactions of the
carbon.
Elements, Compounds and Mixtures
All
substances have mass and therefore must be composed of atoms.
These atoms and how they assemble themselves in the substance
determines their chemical and physical properties. Substances
can be classified according to how these atoms are assembled
and is known as Classification of Matter: All matter falls
into one of three categroies: elements, compounds or mixtures.
Furthermore, mixtures can be classified as homogeneous or
inhomogeneous.
Elements
- Cannot
be decomposed into simpler substances (by any chemical
reaction).
- Are
made up of only one kind of atom (i.e. atoms having the
same atomic number).
- A
molecule consists of two or more atoms of the same element,
or different elements, that are chemically bound together.
Note that the two nitrogen atoms which comprise a nitrogen
molecule move as a unit.
Compounds
- Are
made up of two or more elements
- Have
a fixed composition, e.g., water (H2O) has
fixed proportions, by mass, of hydrogen (H) and oxygen
(O).
i.e. 2 H atoms for every one oxygen atom.
- The
physical and chemical properties of compounds are different
to those of the elements that make them, e.g. sodium chloride
(NaCl) is common table salt and is relatively harmless
and stable. The elements that combine to make NaCl are
sodium (Na) and chlorine (Cl). The former is violently
reactive, while the latter is a poisonous gas.
Mixtures
- Are
combinations of two or more substances (elements or compounds).
- Mixtures
can be homogeneous or heterogeneous (see below).
- Mixtures
have variable composition. i.e. they may be mixed
in any proportion.
- Mixtures
can be separated into substances by physical processes.
e.g.
filtration separates substances according to particle size.
- Substances
in the mixture retain their individual chemical and physical
properties.
There
are two types of mixtures:
- Homogeneous
mixtures
- Are
also known as solutions
- Are
uniform throughout. i.e. if you take a sub-sample,
it is representative of the whole mixture.
- Consist
of one phase.
- Examples
include: sodium chloride (table salt) dissolved in
water.
- Heterogeneous
mixtures
- Are
not uniform throughout. i.e. if you take a sub-sample,
it is not representative of the whole mixture.
- Consist
of more than one phase
- 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.
- Examples
include: beach sand, milk, granite
Separating the Components of a Mixture
Most
laboratory work in biology requires the use of techniques
to separate the components of mixtures. This is done by
exploiting some property that distinguishes the components,
such as their relative
- size
- density
- solubility
- electrical
charge
| Technique |
Basis
for separation |
Apply
this technique to: |
| adsorption
/ desorption |
phase
transfer to a solid surface |
liquid
or gaseous mixtures that contain at least one component
that adsorbs |
| chromatography |
phase
transfer from a mobile mixture to a stationary phase |
liquid
or gaseous solutions that contain several components
with differing affinities for the stationary phase |
| condensation |
phase
separation by condensing gases in the mixture to liquids |
gaseous
mixtures containing at least one gas with a much higher
boiling point than the others |
| dialysis |
phase
transfer through a porous membrane that allows some
molecules to pass through, but not others |
solutions
containing small molecules mixed with very large molecules |
| effusion |
gases
with faster molecules flow through tiny pinholes faster
than gases with slow molecules |
gaseous
mixtures containing gases with different molecular weights |
| dissolution
(washing, solvent extraction) |
soluble
components can be washed away, leaving behind insoluble
components (phase transfer to a washing solvent) |
mixtures
of solids with different solubilities |
| electrorefining |
separate
a metal from impurities by dissolving it and then plating
it onto an electrode |
solid
mixtures with a metal as one component |
| filtration |
collect
solid particles on a filter |
heterogeneous
mixture containing a solid phase |
| floatation |
dense
components sink, and lighter ones float |
heterogeneous
mixture with phases with different densities |
| ion
exchange |
ions
in the mixture bind to surfaces with oppositely charged
sites (phase transfer to an ion exchange resin) |
solutions
containing ions |
| precipitation |
convert
solutes to an easily separated solid form |
solutions
containing a solute that can be precipitated |
| scrubbing |
bubble
mixture through a solution that selectively absorbs
a component (phase transfer from gas to solution) |
gaseous
mixtures containing a solute that can be selectively
absorbed by a scrubbing solution |
| stripping |
a
gas bubbled through the mixture carries off the most
volatile components (phase
transfer from solution to gas) |
a
liquid mixture containing at least one volatile component |
| volatilization
(drying,
distillation, sublimation) |
components
with widely differing volatility can be driven out of
the mixture by heating (phase change from solid or liquid
to gas) |
a
mixture containing components with differing volatility |
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