5.3 Compounds and Elements
Matter
Mixtures
physical
Pure Substances
change
Mechanical Mixtures Solutions
Ordinary
Mechanical
Mixtures
Suspensions
Compounds
Colloids
Elements
chemical
change
Shifting Views of the Elements
Ancient Greek Science: One of the earliest attempts to
explain the properties of matter was proposed by ancient
Greek Philosophers. The Greek Philosopher Empedocles
thought that all substances consisted of varying amounts of
four elements: Air, Fire, Earth, and Water.
Theory…….
E.g,
A warm breeze, would consist of air and fire and a
cool breeze, air and less fire.
E.g., Lava, would contain earth and much fire.
The philosophers’view of the elements was not the only
view, however. Hands-on investigations of matter were
carried out for many centuries by people known as
alchemists. Part pharmacist, part mystic, alchemists
practised their craft all over Europe and the Middle East.
Alchemists:
These were the first experimenters. They had three main
goals:
1. To turn base metals into gold.
2. To find the “elixir of life” that would give them eternal
life.
3. To produce the “universal solvent” that would dissolve
all substances.
Shifting Views of the Elements
• Sir Francis Bacon (1561-1626) – used scientific method to investigate
the physical world. Published a book arguing that science should be
built on the basis of experimental evidence rather than philosophical
experimentation.
• Robert Boyle(1627-1691) – Expressed skepticism about the 4-element
theory of the ancient philosophers. Also, recognized that elements
could be combined to form compounds, which laid the foundation for
our modern definition.
Bacon’s and Boyle’s ideas about elements led to
the widespread search for elements using this new
approach to investigating matter.
The Development of a Modern
View of the Elements
• Antoine Lavoisier (1743- 1794)-Defined elements as pure substances
that cannot be decomposed (broken down) into simpler substances by
means of a chemical change. Earliest chemists to use “balanced” view
of chemical changes. He identified 23 elements.
• Allesandro Volta(1745-1827)- invented the”voltaic pile” – a device
now called the battery. Used this new tool to pass electricity through
water and realized that it could be decomposed into hydrogen and
oxygen.
• Humphry Davy (1778-1829)- Experimented with substances to see if
they could be decomposed by using electric currents.
• Michael Faraday(1791-1867)- named the process of decomposing
substances electrolysis.
• Joseph Proust(1754-1826)- Performed experiments to decompose
compounds into elements and to measure the mass of elements.
Noticed a pattern, now called the law of definite proportions.
DEMOnstration
• Electrolysis:
The process of decomposing a chemical compound
by passing electric current through it.
• Law of Definite Proportions:
A law stating that compounds are pure substances
that contain two or more elements combined in
fixed (or definite) proportions.
• Law of Conservation of Mass:
In a chemical change the total mass of the new
substances is always the same as the total mass of
the original substances.
Elements
A type of pure substance that can not be broken down into
simpler parts by ordinary chemical means.
E.g.,
Oxygen O2 : a gas needed for
objects to burn
Hydrogen H2: a very volatile
(explosive) gas
E.g., Sodium (Na): a very reactive metal
Chlorine (Cl): a toxic yellow-green gas
Compounds
A compound is a pure substance in which two or more
elements are chemically combined in definite proportions.
E.g.,
Water is a combination of one oxygen atom and two
hydrogen atoms (H2O). A liquid that puts flames out.
E.g.,
Table salt, or sodium chloride (NaCl) is a
combination of one atom of sodium and one atom of
chlorine.
The properties of a compound are different from the
properties of the elements which are combined to make the
compound.
Testing For Elements and
Compounds
Imagine that you are a technician in a chemical lab.
While cleaning up after a flood in the laboratory, you
discover that the labels of three gas cylinders have been
damaged. You suspect that the cylinders contain carbon
dioxide, oxygen and hydrogen.
•
How do you find out which cylinder contains which
gas?
•
Whether the cylinders have been contaminated by the
flood water?
•
Do they now contain water vapour?
There are four common tests used to identify four common gases:
Oxygen- The ‘Glowing Splint’
Test
•
•
•
•
A common chemical reaction
is combustion (burning).
Oxygen must be present for
combustion to take place.
Substances such as wood and
oil burn readily in air, which
is about 20% oxygen. In pure
oxygen they burn much more
intensely.
This chemical propertysupporting combustionallows you to identify it.
1.
2.
3.
4.
Light a wooden splint.
Blow out the flame but
leave the splint glowing.
Hold the glowing splint
in a small amount of the
unknown gas.
If the splint bursts in to a
flame, the gas is oxygen.
Hydrogen-The ‘Burning Splint’
Test
Hydrogen is known to be
explosive in air
1. Light wooden splint
2. Hold the burning
splint in a small
amount of the
unknown gas.
3. If you hear a loud
“pop”, the gas is
hydrogen.
Carbon Dioxide- the Limewater
Test
• Carbon dioxide does not burn
and does not allow other
materials to burn. (If you put a
burning splint into carbon
dioxide , the flame will go out.
• The chemical test for carbon
dioxide uses a liquid called
limewater, a clear colourless
solution of calcium hydroxide
in water.
• Carbon dioxide reacts with the
dissolved calcium hydroxide
producing a precipitate.
1. Bubble the gas or
add a few drops of
limewater to the gas
and swirl it around.
2. If the limewater turns
cloudy or looks
milky, the gas is
carbon dioxide.
Water Vapour- The Chloride Test
• Water is a liquid at
room temperature, but
many chemical
reactions produce
water as a product.
• When water vapour
touches a cold surface,
it condenses to liquid
water.
1.
2.
3.
Hold cold surface water
near the suspected water
vapour.
Touch a piece of blue
cobalt chloride paper to
any liquid that
condenses.
If the paper changes
from blue to pink, water
is present.